Fruit trees chill hours

Cold weather is not everyone’s favorite, but chilly winters are crucial to the life cycle of California’s temperate fruit (and nut) trees, whose health and productivity affect farmers and backyard orchardists alike. The harvest of these trees are mainstays of the Central Valley’s economy and kitchen tables all over the United States.

The term “temperate fruit tree” refers to trees that go dormant in winter, preferring moderately cold winters without the killing freezes of the coldest zones. Dormancy begins in late fall and lasts into early winter, initiated by lengthening days and cooler temperatures; it is chemically wrought by hormones suppressing buds for next season’s foliage and flowers until conditions are right for tender new growth.

What is critical is how the tree breaks dormancy. Trees listen for the signal that yes, winter has arrived. The necessary signal strength varies between species, but is officially referred to as “chill hours”, or vernalization, when the temperature stays between 32°-45°F. The hormone responsible for dormancy breaks down in this range, allowing buds to develop into flowers or foliage when the weather warms up in late winter. Interestingly, temperatures below 32°F are ineffective and do not count; hours when temperatures exceed 60°F are actually subtracted from the accumulated chill hours.

When a tree does not receive the necessary signal to break dormancy, the buds tend to leaf out later, and the flower buds may appear irregularly, which can result in a longer bloom period. This may seem beneficial, but flowers are delicate things, and the longer the bloom period, the more likely they will be exposed to diseases such as fire blight and brown rot, meaning a harvest of fewer and deformed fruits.

Why all this talk about tree signals and deformed fruit? Climate change is making warm winters more frequent, resulting in poor harvests for certain fruit trees. As stated above, some require more chill hours than others, depending on species and varieties within species. Choosing “low chill” (requiring less than 300 hours at 32°-45°F) will hedge your bets, with the added benefit of getting more for your water and compost inputs when chill-needy trees have a low yield.

Figs, olives, and quince have the lowest natural chill requirements, followed by persimmons, pomegranates, almonds, and chestnuts. The more commonly grown fruits such as cherries, apples, peaches, and plums required breeding to develop low-chill varieties for folks living in areas like southern California where chill hours are minimal. Fortunately, us valley-dwellers can benefit from this horticultural achievement as well.

For a list of low-chill fruit tree varieties, I recommended visiting the UC’s California Backyard Orchard webpage:

Select a fruit and click on “varieties for planting in the home garden” and scroll down to the list of low-chill varieties, if applicable (naturally low-chill species such as fig and persimmon will not have a separate list).

If you have a gardening related question you can contact the UC Master Gardeners at 209-953-6112. More information can be found on our website.

Chill hours are the number of cold hours or days that a deciduous fruit tree (or nut tree) requires for flowering and fruit production each year. Every fruit tree variety has its own number of hours of chill needed for fruit production. Some fruit trees need as few as 100 chill hours, others need as many as 1,000 chill hours or more.

Apples, peaches, and apricots are fruit trees most affected by chill hours. Apples have the highest chilling requirements of all fruit trees, followed by apricots and, then, peaches.

‘Fuji’ and ‘Gala’ apple varieties require 900 chill hours; ‘Dorset Golden’ apple requires just 100 chill hours. ‘Dorset Golden’ can be grown in southern Florida but ‘Fuji’ and ‘Gala’ cannot—the winters are too mild.

Figs, olives, and quince have the lowest chill requirements, followed by persimmons, pomegranates, almonds, and chestnuts. Apples, apricots, cherries, peaches, and plums have higher chill hour requirements.

When choosing a fruit tree, it is important to choose a tree that can grow fruit where you live. You must be sure to choose a tree with a chill hours requirement that matches the number of hours or days of chill or cold where you live.

A fruit tree may grow well where winters are mild, but if there are not enough chill hours over the course of winter, the tree will not fruit properly. If a tree bears fruit after a cold winter but then produces little or nothing after a mild winter, the tree is likely not a good match for the local climate. You can do everything right for a fruit tree—the right water and fertilizer, but if the climate and chill hours are not a good match, the tree will not produce.

The optimum chill gathering temperature ranges from 34° to 48°F. Fruit trees can gain chill hours when the temperature is continuously between 34° and 48°F, but a tree can lose chill hours when the weather warms and the temperature rises above 48°F. If the temperature stays cold chill hours accumulate. If the weather vacillates between cold and warm, chill hours will not accumulate.

Check the chill hours of the fruit tree you want to grow. For example, ‘Desert Gold’ peach tree needs 300 chill hours each year; ‘Red Haven’ peach tree needs 950 chill hours each year. Make sure there are enough chill hours where you live to grow the variety you want to grow.

To know the average number of chill hours in winter where you live contact the nearby Cooperative Extension Service or a Master Gardener group. You can also find chill hours for many locations online. Chill hour estimates for locations are averages; some winters can be colder or warmer than others, but the number of chill hours a particular fruit variety needs will be constant.

The number of chill hours a fruit tree needs often appears in nursery catalogs online.

Apple espalier dormant in winter

Chill Hours Explained

Fruit trees develop next year’s flower buds in the summer (flowers that are pollinated become fruit). When autumn comes buds become dormant as days lengthen and temperatures cool. Healthy dormancy is triggered by a certain minimum exposure to cool and cold temperatures—chilling hours. If the number of chill hours is not accumulated in winter, bud break, flowering, and fruiting can be delayed or interrupted the next spring. (Cold and warm temperatures affect hormones and chemicals in the plant that regulate the length of dormancy and the time and fullness of bud break in spring; dormancy is needed by plants to survive winter cold and conserve energy for the next growing season. The hormone that regulates dormancy is regulated by chill hours and warming temperatures in spring.)

There are several models for determining chilling hours, but the simplest is: one chilling unit (CU) is equal to one hour’s exposure to the chilling temperature; these units are added up and totaled for a whole season. Advanced models assign different weights to different temperature bands, for example, for any hour colder than 34.5 degrees there is no chilling; for any hour between 34.6F and 36.4F there is a half-hour of chill; for any hour between 36.5F and 48.4F there an hour of chill is added; for any hour between 61.0Fand 64.4F, there is a half-hour of chill hour loss.

Once dormancy is reached in autumn or early winter it cannot be reversed by short-term warm temperatures, but if warm temperatures persist, dormancy can be broken; if that happens without sufficient accumulated chill hours, bud break may be delayed, buds may not break or maybe damaged which in turn will affect fruit set and production.

How to Figure the Monthly Average or Mean Temperature and Chill Hours

Here’s how the average or mean temperatures is calculated each month:

  1. The maximum temperatures each day of the month are added together and divided by the number of days in the month to get the average maximum temperature for the month.
  2. The low temperatures for each day of the month are added together and divided by the number of days in the month to get the average minimum temperature for the month.
  3. The average monthly maximum and minimum temperatures are added together and divided by two; the result is the monthly average or mean temperature.

Here are the hours of chill for several monthly average or mean temperatures: 46°F, 988 chill hours; 48°F, 883 chill hours; 50°F, 779 chill hours; 52°F, 675 chill hours; 54°F, 575 chill hours; 56°F, 475 chill hours; 58°F, 355 chill hours, 60°F, 288 chill hours; 62°F, 200 chill hours; 64°F, 118 chill hours; 66°F, 58 chill hours; 68°F, 0 chill hours. Chill hours are not accumulated when the temperature is less than 35°F or greater than 68°F.

Low Chill Hours Fruit Trees

If you are unsure of the chill hours where you live, plant a tree with a low chill hours requirement.

Here are some fruit trees that need low chill hours:

Also of interest:

How to Grow Apples

How to Grow Peaches and Nectarines

How to Grow Apricots

How to Grow Cherries

How to Grow Plums

Chill hours guide for the home gardener

What are chill units?

Firstly, both chill units and chill hours refer to the same thing – the total amount of time a fruit tree needs to be exposed to effective winter temperatures to help them break dormancy and flower and set fruit normally.

The exposure time to these particular temperatures is what is referred to as chill units or hours.

Another way that people refer to chill units with regard to fruit trees is to ask whether a fruit tree needs high, medium or low chill conditions in winter.

A good example of this is stone fruit such as peaches and nectarines. There are low chill stone fruit varieties which can be grown in the subtropics such as Queensland and there are high chill stone fruit varieties which can be grown in cold climate areas such as Tasmania and Victoria.

How are chill units measured?

It must be noted that there is no single scientific way to measure chill units and horticulture scientists use different practices to measure chill units.

We are not here to explain the different sciences of chill units, but merely to give you a gardener’s guide to help you understand which fruit trees are right for you.

Chill units are measurements of temperatures within a certain temperature range measured during the 3 main months of winter (June, July and August).

Before explaining more about the methods of measuring chilling it must be noted that there are many factors that can affect the amount of chill units measured.

All deciduous fruit trees have limiting factors that affect the way they grow and fruit. For example, sunlight, aspect, soil type, shade, wind and even the fruit variety itself, can all influence how your fruit tree will grow. Some of these factors can influence the amount of chill units received by your fruit tree.

The simplest chilling model to look at it in this way is: How many hours during winter is your fruit tree exposed to temperatures below 7°C? The number of chilling minimum or nightly temperatures can be affected by hot days. This will offset the number of hours of night or minimum chilling.

How can I determine the chill hours where I live?

There is a simple way to determine your suburb/area’s chill units.

First you need to determine the average temperature of the coldest month (eg July) for your suburb/area. The Elders weather website can help you to establish this: http://www.eldersweather.com.au

Select your suburb or town in the Local Weather search button at the top right of the webpage and once your local weather is displayed, look for the Climatology button on the bottom right hand of the screen displaying your local data.

Once the ‘Long Term’ averages are displayed, find July and add the maximum and minimum temperatures together and divide this total by 2 to give you the average monthly temperature for July. It’s that easy!

Once you have determined the average temperature for the coldest month you can use the chart or table below to determine the approximate amount of chill units your suburb/area receives. You can then use this as a basis in determining the best varieties that will grow and fruit in your region…we have included some examples to help you.

Average Temperature Winter Chill Units
0 1250
7.9 1100
10.2 800
12.7 600
13.1 520
13.6 500
14 450
15.3 330
15.5 300
19.7 0
City Average Temperature of coldest month Approximate Winter Chill Units High, Medium or Low Chill?
Sydney 12.2 640 Medium
Newcastle 12.6 600 Medium
Wollongong 13.3 510 Medium
Port Macquarie 12.5 600 Medium
Coffs Harbour 13.1 520 Medium
Melbourne 9.7 830 High
Bendigo 7.5 1120 High
Ballarat 6.6 1180 High
Mildura 9.8 810 High
Wodonga 8 1100 High
Perth 13.1 520 Medium
Bunbury 12.3 620 Medium
Albany 11.9 650 Medium – High
Kalgoorlie 10.9 700 High
Geraldton 14.5 400 Low
Adelaide 11.4 730 High
Murray Bridge 10.8 700 High
Mount Gambier 9.1 900 High
Port Augusta 11.1 710 High
Brisbane 14.2 300 Low
Gold Coast 16.6 <300 Low
Toowoomba 11.5 730 High
Rockhampton 16.3 <300 Low
Bundaberg 16.1 <300 Low
Hobart 8.1 1100 High
Launceston 7.4 1100+ High
Devonport 8.7 1050 High
Canberra 5.6 1200+ High

Low, medium and high chill fruit varieties and a bit of common sense….

There are generally three types of chill groups used to find stone fruit varieties you can grow in your suburb/area, but note that these examples are very general.

  • Low chill areas = up to 450 chill units. You can only grow low chill varieties in low chill areas.
  • Medium chill areas = 450 – 650 chill units. You can generally grow all low and medium chill fruit varieties providing low chill plants are protected from late spring frosts.
  • High chill areas = >650 chill units. You can generally grow all low, medium and high chill fruit varieties providing low and medium chill plants are protected from late spring frosts.

Generally speaking, you don’t need to be too concerned about winter chilling and chill units if you follow a guide as below:

  • Tropical and sub-tropical areas will generally be low chill areas – for example coastal areas from the east coast near Port Macquarie north to Queensland are generally sub-tropical becoming more tropical as you move further north.
  • Temperate regions are generally medium chill areas – such as on the east coast from Port Macquarie south to Victoria. Many fruit varieties fall into this category which means most Australians living in these areas can enjoy growing peaches and nectarines in their back yard.
  • Cool to cold regions are generally medium to high chill areas – this includes an area that runs from Toowoomba down to Armidale, Tamworth, Bathurst, Canberra, Shepparton, Melbourne and right down to Tasmania.

See the map above (need a better map if we are going to refer people to it) and examples from the Elders website to get detailed information on the east coast regions of Australia.

This completes your crash course in CHILL UNITS! Happy gardening!

For further information on where PlantNet fruit trees and ornamentals grow see the growing regions map on every product page.

Chill Hours Calculator

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Minimum chill requirement for apple trees

Apples, pears, plums, cherries and other stone fruit such as apricots and peaches evolved in central Asia, which has a “continental” climate with hot summers and very cold winters. Although apple-growing in particular is now widespread in many climate zones, nearly all cultivars still require an annual cycle of cold winter weather in order to set blossom and produce fruit each year.

This winter chilling requirement, or minimum chill requirement, is usually defined as the number of hours per year where the temperature should be below about 45F / 7C, but above freezing. Periods when the temperature is substantially below freezing are not thought to be as useful for counting towards chill hours as the period when the temperature is just above freezing.

Most apple varieties have a chill requirement of about 1,000 hours or more, which is readily achieved in the temperate apple-growing regions of the USA, South America, South Africa, Australia, New Zealand, and Europe. Some of the best commercial apple-growing areas are in regions where winter temperatures hover in, rather than below, the chill range – such as Kent, the main apple-growing region of England. If you are in any of these temperate areas then chilling hours are of little relevance to the varieties you can choose to grow.

However if you live in a climate where winter temperatures are not often below this chill threshold, such as southern California or Florida, then you might need to consider growing low-chill varieties. For convenience these are usually classified as needing less than 700 chill hours – and they often seem to prefer hotter summers than other varieties. Medium-chill varieties are those in the range 700 – 1,000 hours.

Note that low-chill and medium-chill varieties can usually be grown successfully in high-chill climates – but high-chill varieties might not do so well in low-chill climates. A good example of this is Gala, which is a medium / low-chill variety, but grows very well in high-chill areas.

Another feature of some low-chill apple varieties is that even in colder climates they have a tendency to retain their leaves long after other varieties have lost theirs.

Israel is at the forefront of the development of apple varieties which have little or no winter chill requirement. Anna, a Golden Delicious style apple, and Ein Shemer, a yellow/green variety, both tolerate climates with 300-400 chilling hours. Dorsett Golden, which was found in the Bahamas, needs less than 100 hours.

As a final word it is worth noting that little research has been done on this subject, and it is perhaps best not to get too precise about the exact number of chill hours, but simply categorise varieties as high, medium, or low chill. In addition, chilling hours is not the only factor in determining whether a particular apple variety will do well in hot climates, but is certainly worth taking into consideration.

The apple is a hardy, deciduous woody perennial tree that grows in all temperate zones. Apples grow best where there is cold in winter, moderate summer temperatures, and medium to high humidity.

There are apples for fresh eating, some for cooking, and some for preserving. Some apples are sweet and some are tart. Some apples come to harvest in summer, some in autumn.

Apples can grow from 10 to 30 feet tall and nearly as wide. They are moderately fast-growing, but growth slows with age. Apple trees can live for 100 years or more.

Apple trees bloom in the spring, set fruit, and take from 100 to 200 days to reach harvest depending upon the variety.

Best Climate and Site for Growing Apples

  • Apples grow in Zones 3 to 9. Some can tolerate winter temperatures as low as -40°F. Choose an apple tree suited for winter temperatures where you live. See Chilling Hours below.
  • Apples generally do not grow well close to the ocean where temperatures remain moderate most of the year.
  • Apples grow best in full sun. An apple tree planted in partial sunlight will not bear as many fruits like an apple planted in full sun.
  • Apples grow best in well-drained loamy soil, although they will grow in more sandy soil or in soil with some clay.
  • Apples grow best in a neutral soil pH of 6.0 to 7.0.
  • Plant apples sheltered from a prevailing wind or breeze. Avoid planting apples in a low spot where cold air or frost can settle.
  • Late spring frosts can kill apple flowers. Apples bloom in late spring after peaches, cherries, and almonds. Early fall frosts can damage the fruit. Choose a variety suited to your growing region.
  • Avoid planting in the same spot where apple trees have previously grown. Pests and diseases that attack apple trees may still live in the soil.

Choosing the Right Apple for Your Garden

  • There are nearly 10,000 different varieties or cultivars of apples. About 7,000 varieties or cultivars grow in North America. Only about 1,000 are grown commercially or in home gardens. Contact the nearby Cooperative Extension Service or a nearby garden center to learn which varieties grow well in your area.
  • When choosing an apple or apples to plant consider how you want to eat your apples; some cultivars are for fresh eating, some for cooking, and some for preserving.
  • When choosing an apple or apples consider when the fruit will come to harvest; some apples ripen midsummer, some in late summer, and some in autumn. If you have room, you may want to plant one of each to extend the harvest.
  • Check to see if the apple you want to grow needs a pollinator; many cultivars require a pollinator. You may need to plant two or more trees to get fruit.
  • Check the rootstock of the apples you want to grow; some rootstock are suited for very cold regions, some tolerate drought, some tolerate wet soil, some are dwarfing or semi-dwarfing. The nursery or grower can tell you if the rootstock is suitable for your garden and needs.
  • Some apple varieties are “sports” or accidental mutations of another variety and others are bred–meaning they are created by apple breeders through cross selection. Not every sport is productive and worthy of growing.

See also: How to Choose an Apple Tree

Apple fruit spur

Spur-Type and Branching-Type Apple Trees

  • Apples can be divided into spur-type or branch-fruiting type trees.
  • Spur-type trees bear fruit on short twigs called spurs.
  • Branch-bearing trees bear fruit along branches.
  • Spurs tend to grow close together and so spur-type cultivars bear more fruit than the branch-bearing or non-spur trees. Spur varieties tend to bear fruit earlier in life than branching varieties.
  • Individual spurs may bear fruit for ten years or more. Standard non-spur trees bear on twigs that tend to be short-lived.
  • Spur-type apples are pruned differently than branch-fruiting apples.

Apple Chilling Requirements

  • Apples have chilling requirements or chilling hour requirements. This is the number of hours at 45°F (7°C) or less than the cultivar or variety requires each winter in order to flower and leaf out in the in spring. Chilling hours can vary from 1,000 or more to as few as 400 hours.
  • It is important to choose an apple variety with chilling hours suited to your climate and winter temperatures.

Apple Pollination

  • Most apple varieties have flowers that contain male and female parts and so are self-pollinating. These trees will set fruit without cross-pollination. However, there are some varieties that are self-infertile and require a pollenizer.
  • Even trees that are self-pollinating may have a better fruit-set if there is cross-pollination.
  • Apples can be pollinated by bees and insects or by pollen that floats on the wind.
  • Plant your apple tree within 40 to 50 feet of another apple tree that blooms at the same time or graft a branch from a suitable pollinator onto your tree.
  • Flowers that are only partially pollinated will tend to bear fruit that prematurely drops.

Apple Pollination Groups

Not all apple trees flower at the same time; some flower early in spring, some in early middle spring, some in late middle spring, and others in late spring. Apples are divided into three flowering groups–A, B, C; the apples in each group flower at the same time. To ensure pollination—even if an apple is self-fertile—plant two or more apples in the same group; some may flower at the same time as an apple in an adjoining pollination group.

  • Group A flowers in early spring and includes the varieties: ‘Gravenstein’, ‘Egremont Russet’, ‘Idared’.
  • Group B flowers in mid-spring and includes the varieties: ‘Cortland’, ‘Cox’, ‘Granny Smith’, ‘Honeycrisp’, ‘Jonagold’, ‘Baldwin’, ‘Braeburn’, ‘Fuji’, ‘Gala’, ‘Golden Delicious’, ‘Rome Beauty’.
  • Group C flowers in late spring and includes the varieties: ‘Court Pendu Plat’, ‘Edward VII’, ‘Mother’, ‘Sheepnose’.

Apple Rootstock and Tree Size

  • An apple tree can be a standard or full-sized tree which grows to 30 feet tall; standard trees can take up to 6 years to bear first fruit.
  • An apple tree can be a dwarf or semi-dwarf tree that grows less than half the size of a standard. A dwarf will grow 6 to 10 feet tall; a semi-dwarf will grow 12 to 20 feet tall. Most dwarf and semi-dwarf trees are grafted onto a rootstock which keeps them small. Dwarf and semi-dwarf trees produce full-sized apples in about three years.
  • A grafted apple tree has a root system that is different from the fruit-producing portion of the tree. Some apple trees have more than one graft; they will produce more than one variety of fruit on the same tree.
  • Apples are dwarfed as a result of the type of root system onto which they are grafted. Many apple varieties can be purchased as either standards or dwarfs or semi-dwarfs depending upon the rootstock. Root systems are identified by their growth at maturity and planting needs. Here are some, but not all, examples:
    • ‘Seedling’: this is a full-growth tree with strong roots.
    • ‘M.27’: this tree makes 15 percent of full growth and is good for containers.
    • ‘M.9’: this tree makes 25 to 35 percent full growth; plant in moist, well-drained soil.
    • ‘M.26’: this tree makes 30 to 40 percent full growth; plant in well-drained, dry soil.
    • ‘MARK’: this tree makes 30 to 40 percent full growth and is very cold hardy and resistant to fireblight.
    • ‘M-7’: this tree makes 40 to 60 percent full growth and can be grown in wet soil.
    • ‘MM.106’: this tree makes 45 to 65 percent full growth; it can be planted in wet soil but may be susceptible to root rot.
    • ‘MM.111’: this tree reaches 65 to 85 percent full size, tolerates a wide range of soil and is drought resistant and fireblight resistant.
  • Many apple dwarfing rootstocks were originated in England at the Malling Research Station; this accounts for the “M” in their names.

Apple Yield

  • An apple tree can yield from 75 to more than 130 pounds of fruit each year.

Spacing Apples

  • Space apple trees according to their height.
  • Standard apples that can grow to 20 or 30 feet tall; they should be spaced 25 to 30 feet apart.
  • Semi-dwarf trees that can grow to 12 to 15 feet tall; they should be spaced 15 feet apart.
  • Dwarf trees that can grow 6 to 10 feet tall; they should be spaced 8 to 10 feet apart.
  • Allow enough room for sunlight and air circulation to reach all parts of the tree.
  • If you are short on space, plant dwarf trees.

Gardener planting an apple tree in to a prepared hole – variety is Bramley

Planting Apples

  • Apple trees can be purchased either bare-root, balled-and-burlapped, or in a container.
  • Bareroot trees are available in the winter and early spring when the trees are dormant and without leaves. Plant bare-root trees in spring as soon as the soil can be worked and before the trees begin to significantly leaf out. Bareroot trees are commonly grafted and without branches, and so are called whips. Make the planting hole large enough that the roots can be spread out fully. Look for the soil line on the tree and plant the tree at that level or an inch or two deeper. If the tree is grafted, set it in the hole so that the graft is visible when planted, an inch or so higher than the surrounding soil.
  • A balled-and-burlapped tree is a tree whose roots are in soil; the roots are enclosed in burlap. Balled-and-burlapped trees are commonly available in spring; however, they may be found later in the year. Plant a ball-and-burlapped tree by positioning the tree in the planting hole at the same depth that it was growing at the nursery. After positioning the root ball into the hole, remove all twine or rope used to hold the burlap and ball together. Then open the top of the burlap and slide it out of the hole. Lightly tamp in soil around the root ball; see General Planting Instructions below.
  • A container-grown tree can be planted at any time during the growing season. Remove the container carefully and plant the root ball at the same depth as in the container.
  • Avoid planting apple trees in hot, dry weather.

Planting a young apple whip

General Planting Instructions

  • Prepare a planting site in full sun that is sheltered from a prevailing breeze or wind.
  • Work well-rotted compost or manure into the soil and add a cupful of all-purpose fertilizer to the bottom of the hole.
  • Dig a hole half again as deep and twice as wide as the tree’s roots.
  • Put a tree stake in place before planting. Drive the stake into the ground to the side of the hole to at least 2 feet deep.
  • Set the tree in the hole so that the soil mark on the stem is at the surface level of the surrounding soil. Remove all twine and burlap from balled and burlapped trees. Spread the roots out in all directions.
  • Re-fill the hole with half native soil and half aged compost or commercial organic planting mix; firm in the soil so that there are no air pockets among the roots. Water in the soil and create a modest soil basin around the trunk to hold water at watering time.
  • Secure the tree to the stake with tree ties.
  • After planting, water each tree thoroughly and fertilize with a high-phosphorus liquid starter fertilizer.

Container Growing Apples

  • Dwarf apple trees can be grown in containers.
  • Choose a large pot or tub at least 18 inches wide and deep that is well-drained.
  • Plant trees in a commercial organic potting mix.
  • Keep the soil evenly moist but not wet.
  • Feed apples growing in containers with an all-purpose fertilizer that is slightly higher in potassium.
  • Repot the tree after two years into a container that is 24 inches wide and deep.

Apple Care, Nutrients, and Water

  • Newly planted apple trees require moderate watering weekly. Set the water on low and allow it to seep into the soil; roots will follow deep watering and become well established.
  • An established apple tree requires only infrequent watering but be sure to water all trees during prolonged dry periods.
  • Feed apples with a mulch of aged compost applied liberally around the base of the tree once or twice a year, in spring or in late fall after leaves have dropped.
  • Feed an apple tree a half-pound of balanced 10-10-10 fertilizer for each year the tree has been alive to a maximum of 10 pounds per tree per year.
  • Low levels of potassium, calcium, or boron can reduce growth and fruit quality. Test the soil for its nutrient content. Spread gypsum on the soil to raise the calcium level.
  • Yields can be improved with a foliar feeding of seaweed extract when buds begin to show color, again after petals fall, and once again when fruits are less than 1 inch in diameter.
  • A young apple tree will grow 12 to 24 inches in a year. A mature, fruit-bearing apple tree will grow 8 to 12 inches each year.

Care of Young Apple Trees

  • Allow the roots of a young apple tree to become well-established before allowing the tree to fruit.
  • The first two years handpick off flowers and young fruit not allowing them to develop; this will give the tree increased energy to establish its roots.
  • The third year allow the tree to bear a small crop. Do not allow a limb to become so burdened with fruit that it will bend or break.

Young tree with leader and three lateral branches

Training Apple Trees

  • Freestanding apple trees can be trained in three ways: (1) central leader, (2) modified central leader, and (3) open center. Apple trees tend to be naturally vase-shaped having no central leader or a weak central leader but several potential scaffold branches. Training an apple tree should begin soon after planting.
  • Central leader: A mature central-leader tree has a somewhat conical shape. The main stem is the central leader; from the central leader even spaced lateral branches are selected to grow as the tree’s scaffold branches. At planting a one- or two-year-old whip is cut off at about 30 inches above the ground; four even spaced lateral branches are selected to become the scaffold branches; all others are removed. In the second year, even spaced sub-lateral branches are selected to grow on; other sub-laterals are removed. Each year the central leader is shortened by one-third of the previous summer’s growth until the conical shape of the tree is established.
  • Modified central leader: A modified central leader tree does not have a central main stem or trunk; the main stem is shortened in the second or third year and lateral-scaffold branches are encouraged to grow. Follow the training directions for a central leader form tree; once 4 or 5 strong scaffold branches have formed, cut back the central leader to just above the topmost scaffold branch. Sub-laterals will grow from the scaffold branches; prune these to keep the form of the tree and remove any vertical sub-laterals.
  • Open center, also called multi-leader: A mature open-center tree has a vase-like shape. At planting time, the top of the whip is cut off at about 30 inches above the ground. In the first year select four even spaced lateral branches; these should be spaced along the trunk about 4 to 8 inches apart and should be growing in different directions from the central stem/trunk (these will become the main scaffold branches); cut off all other small branches. At the end of the second season, cut off the main trunk or leader just above the top lateral branches—above the branches you have selected to become the scaffold branches; you have just created an open center. At the same time, shorten the laterals by one-third to one-half to encourage sub-lateral branching; cut all other small branches back to four or five buds. In the next two years, prune back the laterals and sub-laterals by one-quarter to encourage strong growth. Allow even spaced smaller side branches (sub-sub-laterals or side shoots) to grow even spaced; prune the sub-laterals and their side shoots to two or three buds. In the following years as the tree begins to fruit, pruning can be lighter.

Pruning Mature Apple Trees

  • An apple tree that has been trained (see above) will be near maturity in the fourth and fifth years. Then training pruning gives way to maintenance pruning.
  • Mature apple trees, like most trees, will benefit from pruning. Pruning will allow the tree to produce quality fruit.
  • Prune an apple tree so that plenty of sunlight and air can penetrate into the center of the tree. One guideline is to prune so that a bird can fly directly through the tree without touching its feathers on a branch. That means pruning out dense, crossed branches.

Maintenance Pruning Step-by-Step

  1. Remove all diseased, dead, or broken branches.
  2. Remove all water sprouts. Water sprouts are fast-growing vertical branches that usually have no side branches.
  3. Remove all suckers. Suckers are the fast-growing shoots that grow out of the soil from the roots below the soil surface.
  4. Remove a branch that creates a tight V-branch crotch, a crotchless than 45 degrees. These branches will not support the weight of a full crop of fruit.
  5. Remove crossing or rubbing branches. If two branches cross and rub against each other they can cause a wound that may allow insects or fungal disease to attack the tree. Remove the least desirable branch.
  6. Never prune away more than one-third of the total tree in a single growing season.
  7. Always prune to a growth bud or flush to a main branch or trunk. Remember that spur-bearing apple trees produce fruit on the same spurs several years in a row.
  8. Tip-bearing apples bear fruit on last year’s growth, so be careful not to remove too much recent growth that will bear fruit next season; lightly tip-prune the leaders of the main branches; cut back sub-laterals to a strong bud but not more than 12 inches; do not prune any sub-laterals shorter than 12 inches.
  9. Spur-bearing apples bear fruit on the same spurs for years and years. Be careful not to remove or damage fruiting spurs unless you mean to. Prune new side shoots to encourage the growth of new spurs; cut back shoots to buds facing the direction laterals and fruits should grow.
  10. Prune every year. Once a tree has been well pruned, it will need less annual pruning; only the removal of crossing branches and twiggy growth.
  11. Prune in late winter when the tree is dormant and before buds appear. A light maintenance pruning can be done in summer working around the fruit set.

See also: Apple Pruning

Thinning Apples

  • Thinning fruit will ensure the quality and size of the crop. Thinning will also reduce the tendency of some apple varieties to alternate-bear that is bear fruit every other year. When a tree bears a heavy crop one year, it will produce a much, much smaller crop the next year; this is called alternate bearing.
  • A few weeks after fruit sets, some fruit on the tree will naturally drop off. This is called “June drop”; it is nature’s way of thinning the crop. Apple trees produce more blossoms and fruit than is necessary for a full crop.
  • Additional thinning will benefit the tree. The rule of thinning fruit is to allow plenty of room for fruit to develop. Look for clusters of fruit and remove smaller apples in each cluster before the fruit reaches one inch in diameter. On larger trees, you can leave two fruits on each spur and on dwarf trees leave one fruit on each spur. One thinning method is to remove all the fruit on every other spur. It is probably best to reduce fruit clusters leaving just a single fruit. Fruit that touches another fruit can be susceptible to disease or pest attack.

See also: Thinning Apples

Harvest and Storing Apples

  • Dwarf cultivars begin to bear fruit in one to three years.
  • Standard cultivars begin to fruit in five to ten years.
  • Apples come to harvest from midsummer through late fall; fruit ripens 100 to 200 days after fruit set depending on the variety.
  • The best way to know if apples are ready for harvest is to taste them; select one and try it. Also, consider skin color and fruit drop. Apples are usually ready for harvest when they reach full color; full color may vary according to the variety.
  • A mature apple will come away from the tree easily; lift the apple up and twist in a rotating motion. It should not be necessary to cut an apple from the stem.
  • Late ripening apples usually come to harvest more quickly than long-maturing early and mid-season varieties.
  • Storing and preserving. Apples will keep for 6 to 8 weeks in a cool place; a refrigerator just above 32°F is best. Late-maturing apples are better keepers than summer apples. Commercial growers often place apples in cold storage for 6 to 12 months.
  • Apples kept in storage should not be diseased or damaged or other fruit may be affected. Apples are often wrapped individually in paper to avoid spoilage.

See also: Apple Harvest Time by Variety

Also of interest: Apples: Kitchen Basics

Propagating Apples

  • Propagate apple trees by grafting scions (fruiting wood) onto rootstocks.

Apple Pest and Disease Control

  • The best preventative approach to apple diseases is to choose varieties that are resistant to the diseases in your region.
  • Apart from disease-resistant cultivars, prune trees regularly to allow for ample sun and air penetration into the crown of the tree and prune out any diseased branches, leaves, or fruit.
  • Keep the garden or orchard clean of dead leaves and branches and plant debris.
  • Pests that sometimes attack apples include: aphids, apple tree borers, apple fruitworms, apple leafminers, apple maggots, birds, Codling moths, European apple sawflies, European red mites, flathead borers, roundhead borers, leafrollers, oriental fruit moths, plant bugs, plum curculio, scale, spider mites, tarnished plant bugs, tortrix moths, wasps, white apple leafhoppers, winter moths.
  • Diseases that sometimes attack apples include apple canker, apple scab, bitter pit, black rot, blossom blight, brown rot, cedar apple rust, powdery mildew, fireblight, fly speck, sooty blotch.

Apple Spraying Schedule

  • Apples are commonly sprayed to combat diseases and pests.
  • Apply a dormant oil spray before buds open and when the temperature has been above 33°F for 48 hours. Dormant spray oils help kill overwintering pests and diseases.
  • Apply a multipurpose fruit tree spray when the buds begin to break. Multipurpose fruit tree sprays help control both pests and diseases during the growing season.
  • Do not spray when the tree is in bloom.
  • When nearly all the flower petals have fallen begin applying a multipurpose fruit tree spray every 10 to 14 days.
  • Discontinue all spraying two weeks before harvest.

Fall and Winter Apple Care

  • Prune trees in winter; the best time is just before buds break in late winter or early spring.
  • Fruiting spurs can be thinned; remove crowded spurs and also remove unproductive spurs.
  • Place hardware cloth around tree trunks to protect them from rodents and rabbits.
  • Paint trunks with diluted latex to protect the bark from sunscald.
  • Hang deer deterrents from tree branches; bars of deodorant soap will repel deer.

Apple Varieties to Grow

  • There are hundreds of apple varieties to choose from. Consider first your region and the number of chilling hours. Next, consider the space you have to grow an apple tree. Then consider how you plan to use the fruit and when you want the fruit to come to harvest, early, mid-season or late.
  • Choose two apples from the same pollination group to ensure the best fruit yield. See Pollination Groups above.
  • Very good pollinators: ‘Golden Delicious’ pollinates almost every other variety and is self-fruitful. ‘Winter Banana’ good pollinator, especially for ‘Red Delicious’.
  • Varieties that are not self-fruitful (they need a pollinizer): ‘Red Delicious’, the ‘Winesaps’, ‘Red Gravenstein’, ‘Rhode Island Greening’, and triploid crosses including ‘Mutsu’, ‘Spigold’.
  • Varieties for cold northern cold regions: ‘Duchess’, ‘Gravenstein’, ‘Haralson’, ‘McIntosh’, ‘Cortland’, ‘Macoun’, ‘Spartan’, ‘Empire’, ‘Wealthy’.
  • Varieties for all regions except southernmost areas and the coldest regions: ‘Baldwin’, ‘Golden Delicious’, ‘Grimes Golden’, ‘Jonathan’, ‘Jonagold’, ‘Liberty’, ‘Lodi’, ‘Mutsu’, ‘Northern Spy’, ‘Paulared’, ‘Prima’, ‘Red Delicious’, ‘Rhode Island Greening’, ‘Rome’, ‘Rome Beauty’, ‘Sir Prize’, ‘Winesap’, ‘Yellow Newton’, ‘Yellow Transparent’, ‘York’.
  • Varieties that will grow southern regions: ‘Granny Smith’.

See also: Winter and Late Season Apple Varieties

Botanical name. Malus pumila

Origin. Southwestern Asia

Apple Chilling Info: How Many Chill Hours Do Apples Need

If you grow apple trees, then you are no doubt familiar with the chill hours for apple trees. For those of us who are new to cultivating apples, what exactly are apple chill hours? How many chill hours do apples need? Why do apple trees need chilling? It all seems a bit confusing, but the following article contains all the apple chilling info you are likely to need.

Apple Chilling Info

So you’re immersed in choosing bare root apple trees from a catalogue for your particular USDA zone and notice that not only is the hardiness zone listed but another number as well. In the case of apples, these are the number of apple chill hours needed for the tree. Okay, but what the heck are chill hours for apple trees?

Chill hours or chill units (CU) are the number of hours when temperatures stay at 32-45 F. (0-7 C.). These chill hours are prompted by longer nights and lower temperatures in the fall and early winter. This period of time is critical for apple trees and is when the hormone responsible for dormancy breaks down. This allows buds to develop into flowers as the weather warms up.

Why Do Apple Trees Need Chilling?

If an apple tree doesn’t get enough chill hours, the flower buds may not open at all or they may open late in the spring. Leaf production may also be delayed. Blossoms may also bloom at irregular intervals and, although this might seem beneficial, the longer the bloom time, the increased likelihood that the tree will be exposed to disease. As you may expect then, a lack of chill hours will affect fruit production as well.

So, it is important to not only match your USDA zone with your choice of apple variety but also the chilling hours the tree needs. If you buy, for instance, a low chill tree and you live in a high chill area, the tree will break dormancy too early and be damaged or even die from the cold temperatures.

How Many Chill Hours Do Apples Need?

This really depends on the cultivar. There are over 8,000 apple varieties worldwide and more being introduced annually. Most apple varieties need 500-1,000 chill hours or temps below 45 F. (7 C.) but there are some low chill varieties available that need no more than 300 chill hours.

Low chill varieties need less than 700 chill hours and can withstand hotter summers than other varieties. Medium chill varieties are apples that need chill hours of between 700-1,000 chill hours and high chill apples are those that require more than 1,000 chill hours. Low chill and medium chill apples can generally be grown in high chill regions, but high chill apples will not thrive in low chill climes.

Although most apples need high chill hours, there are still plenty of medium to low chill cultivars.

  • Fuji, Gala, Imperial Gala, Crispin, and Royal Gala all require chill times of at least 600 hours.
  • Pink Lady apples need between 500-600 chill hours.
  • Mollie’s Delicious requires 450-500 chill hours.
  • Anna, a golden delicious type of apple, and Ein Shemer, a yellow/green cultivar, tolerate areas with 300-400 chill hours.
  • A truly low chill apple, Dorsett Golden, found in the Bahamas, requires less than 100 hours.

APPLE Fuji

SOIL

Soil preference depends on the rootstock that the variety is grafted onto. Dwarf rootstocks require fertile, well draining soils. M116 & M793 types are suitable for heavy clay soils.

PLANTING SITE

Sunny sheltered sites are best.

CLIMATE

Apples are best in a temperate environment. Some varieties with ‘low chill requirements’ can be grown in warmer northern areas.

WATERING

Water well during the early stages, during long dry periods and when the fruit is developing.

PESTS & DISEASES

Some varieties require a spray program to control pests and diseases that affect the tree, leaves and fruit. The Rezista range of varieties are resistant to some diseases.

PLANTING IDEAS

Apple trees can be espalier trained against a wall or fence. Ballerina® columnar varieties are fantastic planted in rows or in pots as garden or entry features.

FRUIT

Harvest time is from February to April. Trees on dwarf rootstocks will produce fruit within 2 years, others may take 3-4 years to produce significant numbers of fruit.

POLLINATION

In most residential areas Apples will often be pollinated by bees from nearby Apple and Crabapple trees. Some varieties are self fertile.

TREE SIZE

The ultimate size of the tree depends on the rootstock the variety is grafted onto, the site, pruning and training of the tree. Dwarf trees can be kept to 1.5m, while more vigorous trees can grow to 5-6m.

General Apple tree information:

Apple trees will start to produce fruit between its second to fifth year depending on rootstock. (Please refer to rootstock tab).

Fruit will produce on either tips or spurs. Some varieties produces fruit on both. (Please refer to description above).

Fruit is ripe when the shaded side turns from green to a greeny/yellow colour.

The later the Apple trees fruit ripens, the longer the storage life.

Planting:

Apple trees perform best in temperate areas. Most varieties need plenty of winter chill, but there are low chill varieties available for warmer climates. (Check out description above).

Apple trees like planting sites that are sunny, and sheltered. Soil preferences depends on rootstocks. (Please refer to rootstock tab).

General Care:

Water Apples well during early planting stages, long dry periods and when fruit is developing.

Some Apple varieties require a spray program to control pests and diseases that affect the tree, leaves and fruit.

To ensure large apples each year and prevent biennial bearing, thin apple bunches by half.

Pruning and Training:

Best to prune Apple trees in late winter so cuts heal faster with spring flush. To minimise disease entering tree, paint cuts with pruning paste.

  • Spur bearing: Reduce growth made the year before, by a third. Cut just after a good strong bud, that points in the direction you want the new branch to go.
  • Tip bearing: All of last years growth on main shoots should be pruned off to the first/second strong and healthy bud. (Unless new shoots are less than 12 inches long).

Prune into a modified central leader or vase shaped tree, or espailer against a wall or fence in triple horizontal cordon, candelarra or double-U shape.

General Apple tree information:

Apple trees will start to produce fruit between its second to fifth year depending on rootstock. (Please refer to rootstock tab).

Fruit will produce on either tips or spurs. Some varieties produces fruit on both. (Please refer to description above).

Fruit is ripe when the shaded side turns from green to a greeny/yellow colour.

The later the Apple trees fruit ripens, the longer the storage life.

Planting:

Apple trees perform best in temperate areas. Most varieties need plenty of winter chill, but there are low chill varieties available for warmer climates. (Check out description above).

Apple trees like planting sites that are sunny, and sheltered. Soil preferences depends on rootstocks. (Please refer to rootstock tab).

General Care:

Water Apples well during early planting stages, long dry periods and when fruit is developing.

Some Apple varieties require a spray program to control pests and diseases that affect the tree, leaves and fruit.

To ensure large apples each year and prevent biennial bearing, thin apple bunches by half.

Pruning and Training:

Best to prune Apple trees in late winter so cuts heal faster with spring flush. To minimise disease entering tree, paint cuts with pruning paste.

  • Spur bearing: Reduce growth made the year before, by a third. Cut just after a good strong bud, that points in the direction you want the new branch to go.
  • Tip bearing: All of last years growth on main shoots should be pruned off to the first/second strong and healthy bud. (Unless new shoots are less than 12 inches long).

Prune into a modified central leader or vase shaped tree, or espailer against a wall or fence in triple horizontal cordon, candelarra or double-U shape.

Chill Units or hours refers to the total amount of time a fruit tree needs to be exposed to effective winter temperatures to help them break dormancy so they will flower and set fruit normally.
The time exposed to these particular temperatures is often referred to as chill units or hours
Another way that people refer to chill units with regard to fruit trees is to ask whether a fruit tree needs high, medium or low chill conditions in winter.
Chill units are measurements of temperatures within a certain temperature range measured during the 3 main months of winter (June, July and August).

A simple way to calculate chill units

The available chill units is the number of hours during winter the temperature, during the day and night, is below 7°C. The total amount of chilling can be affected by hot days which offset the number of hours of night or minimum chilling.
First you need to determine the average temperature of the coldest month (July) for your suburb/area. The Elders weather website can help you to establish this: http://www.eldersweather.com.au/
Choose your suburb or town in the Local Weather search button at the top left of the webpage and once your local weather is on the screen, look for the Climatology button on the bottom right hand of the webpage.
Once the Long Term averages are displayed, find July and add the Maximum and Minimum temperatures together and divide the total by 2 to give you the average Monthly temperature for July.

Taking Ballarat, for example:

Mean Max + Mean Min = 13.2 2 = 6.6° for July

Therefore ave. monthly temperature for July is 6.6°

Determining the Chill Units for your area

Once you have determined the average temperature for the coldest month you can use the chart or table below to determine approximately the amount of chill units your suburb/area receives and you will then be able to determine what varieties will grow and fruit in your region…we have included a few examples to help you.

There are generally three chilling categories used when considering suitable varieties for your locality – and these examples are very general:

Low chill areas =
You can only grow low chill varieties in low chill areas.

Medium chill areas = 450 – 650 chill units
You can generally grow all low and medium chill fruit varieties providing low chill plants are protected from late spring frosts.

High chill areas = >650+ chill units
You can generally grow all low, medium and high chill fruit varieties providing low and medium chill plants are protected from late spring frosts.

Guidelines to Chill Units and how they affect the blossom

• Select varieties that have a chilling requirement at least 20% less than local averages.
• Selecting a low chill variety in a cold area will result in trees flowering too early and being damaged by late frosts.
• Selecting a high chill variety in warm areas will result in little or no fruit production.
• Early flowering varieties are best in warm climates, late flowering varieties are best in cooler areas.
• Early ripening varieties are best in areas with intense summers, late ripening varieties are best in cooler summers.
• Climate extremes may eliminate certain varieties that would otherwise meet the chilling requirements.
• Terrain can affect the chilling hours received. Open slopes may receive more chilling hours than sheltered areas next to warm buildings.
• Various sellers of fruit trees publish significantly varying chilling hour requirements for the same variety. It is difficult to know the exact requirements. Experiment and ask around for promising local cultivar success stories.

The effect of the Chill Units to the fruit tree buds

The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom.

Deciduous fruit and nut trees develop their vegetative and fruiting buds in the summer. As winter approaches, the already developed buds go dormant in response to both shorter day lengths and low temperatures. This dormancy or sleeping stage protects buds from the effects of cold weather. Once buds have started dormancy, they will be tolerant to temperatures much below freezing and will not grow in response to mid-winter warm spells.

Buds remain dormant until they have accumulated sufficient chilling units (CU) of cold weather. A chill unit is allocated when temperatures occur within certain parameters (chill accumulation models). When enough chilling accumulates, the buds are ready to grow in response to high temperatures. As long as enough CU have been accumulated the flower and leaf buds develop normally. If the buds do not receive sufficient CU during winter to completely release dormancy, trees may have uneven flowering, poor fruit set and shoot dieback.

Fruit and nut species and cultivars have different requirements for CU and the selection needs to be matched to the climate of the area to be planted. When selecting cultivars consideration of the future effects of climate change and likely reductions in CU for the production area.

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