Cold tolerant citrus trees

Understand How Cold Temperatures Affect Citrus Trees

By Dr. William Johnson, Extension Horticulturist, Galveston County Texas AgriLife Extension Service

Many gardeners have inquired about the susceptibility of citrus to cold temperatures. The winter season has been tough on citrus plants.

It is important to understand how cold temperatures affect citrus trees. Among the citrus types most easily killed or damaged by freezing weather are citrons, lemons and limes. Temperatures in the high 20s will kill or severely damage these plants.


Some gardeners who protected their citrus trees during
the recent cold snap were surprised to see their plants
setting flower buds as temperatures started to warm.

Sweet oranges and grapefruits are somewhat more cold-hardy and usually require temperatures in the mid 20s before incurring major damage to large branches.

Tangerines and mandarins are quite cold-hardy, usually withstanding temperatures as low as the low 20s without significant wood damage.

But, among the edible types of sweet citrus, the satsuma and kumquats have the greatest degree of cold hardiness. Properly hardened bearing trees will withstand temperatures as low as 20 degrees Fahrenheit without appreciable wood damage.

Temperatures at ground level can be several degrees lower than temperatures around the canopy of the tree, especially if there is no wind.

Keep in mind the temperature ranges given above only refer to leaf or wood damage. Citrus fruits easily freeze at 26 to 28 degrees when these temperatures occur for several hours.

A longer duration of freezing temperatures is required to freeze grapefruit compared to sweet oranges.

The particular temperature at which tissue of a given plant will freeze and the degree of the damage sustained are functions of a number of factors in addition to the species and variety involved.

Some of the more important are:

  • The freezing temperature reached;
  • The duration of the minimal temperature;
  • How well the plant became hardened or conditioned before freezing temperatures occurred (the freezing point of tissue of a hardened citrus plant might be 5 to 6 degrees lower than an unhardened plant);
  • Age of plant (a young plant cannot withstand as much cold as a more mature tree); and
  • Healthy trees are hardier than diseased trees.

Another complicating factor contributing to observations by some that citrus plants seem to freeze at higher temperatures in some years is the difference between air (ambient) temperatures and leaf (tissue) temperature.

On a windy night with clear or cloudy skies, leaf temperature will be about the same as air temperature. On a cold, clear night with little or no wind movement, however, leaf temperature can easily drop several degrees (3 to 4 degrees) below the air temperature because of supercooling caused by frost.

Thus, under the latter circumstances, while the minimum air temperature on a given night may have only been 25 degrees, actual leaf temperature of the plants may have reached 21 to 22 degrees.

The critical temperature is that of the leaf or fruit and not the ambient air temperature.

Trees with a good fruit crop are less hardy than those with no fruit.

Research data provided by Louisiana State University indicated trees growing on bare ground have a higher probability of survival than trees growing in turf areas.

The heat from the ground can radiate upward into the canopy of trees. The difference in the canopy of the tree can be up to 5 degrees.

In general, it is recommended citrus trees be protected when the temperatures is expected to go below 27 degrees for an extended period.

The good news is before the cold snap, temperatures had been on the cool side for a while and citrus trees had hardened off and were fairly dormant.

Citrus trees can better withstand cold weather when they are dormant.

No immediate action is needed when freeze injury is suspected. There is no benefit to pruning the plant until spring growth commences, and the full extent of injury is manifested. Pruning might actually be counterproductive by stimulating faster bud activity before the danger of additional frost/freeze events has truly passed.

Dr. William Johnson is a horticulturist with the Galveston County Office of Texas AgriLife Extension Service, The Texas A&M System. Visit his web site: Gulf Coast Gardening.

Return to HortUpdate – March 2011 Index

Citrus is one of the most rewarding plants to grow indoors or out. The fragrance of citrus blossoms is unforgettable. Seeing your tree covered in ripe fruit backed by dark green foliage looks like a snapshot from the Mediterranean. And, fresh, tree-ripened fruit is simply the best tasting.

If you live in the “Citrus Belt” that stretches from California, along the Gulf Coast to Florida, (USDA Zones 8-10) you can grow citrus trees outdoors all year long. But even in these warm climates, occasional cold snaps can occur. Gardeners in the Northern states can grow citrus in pots and bring them inside for the winter. No matter where you live, get ready to protect your beautiful plants.

Four Tips for Bringing Potted Citrus Indoors for Winter

  1. Wash Citrus. Using a hose, spray foliage and branches to remove insects and allow to drip dry. Next, spray with an organic, insect killer like Espoma’s Insect Soap, covering both the top and undersides of the leaves, to make sure you aren’t bringing any pests indoors.
  2. Re-Home. Citrus trees do well in cool temps from about 50 to 70 degrees with as much bright light as possible. Eight hours of sun per day would be ideal. South-facing windows usually have the most favorable light for citrus trees. Repot plant if it’s outgrown its current container with Espoma’s Cactus potting mix to give citrus plants proper drainage.
  3. Feed Plants. Feed your citrus plant every four weeks with a fertilizer specially formulated for citrus, like Espoma’s Citrus! Unlike trees planted in the ground, potted plants quickly use up the food in the soil and it needs to be replenished.
  4. No Wet Feet. Citrus will not thrive in consistently wet soil, so make sure your citrus pots have excellent drainage. Set the pot on a saucer of pebbles to allow excess water can drain off. Water well when the top two inches of soil feels dry, once a week on average.

Four Frost Protection Tips for Outdoor Citrus Trees

Citrus are subtropical plants and will not survive freezing temperatures. To protect plants, they will need to be covered one way or another. If temperatures dip down to 30 degrees, it’s time to take action. This is especially important for young citrus trees.

  1. Water well. Water-deprived trees freeze faster. Moist soil also absorbs more heat from the sun than dry soil does.
  2. Remove mulch. Expose the soil to the sun for winter months to enable plants to absorb more heat.
  3. Know when to cover up. If freezing temperatures happen often in your area, you can build a simple structure out of wood to surround plants and cover it with plastic, burlap or even old blankets. Fasten the cover with tacks or staples so they can’t blow off.
  4. Let citrus breathe. If day time temperatures are warm, remove the covers to allow for ventilation.

Here are some other citrus blogs we think you will enjoy.

Feeding Citrus for the Most Fruit, Growing Food Out of Your Zone from the Citrus Guy, When Life Gives You Lemons – Grow Them Indoors

Espoma Products for Happy Citrus

2 secrets to getting your fruit trees through a frost

Two proven methods for helping frost-tender trees get through a big drop in temperature.

Words: Ben Gaia

It’s 8am on a cold West Coast morning and as the sun rises, the thermometer reads -5°C for the umpteenth time. I will be watering the trees today. Watering in winter? This is the secret to growing citrus trees and other tender subtropicals like strawberry guavas in a land where the mercury plunges at night to alpine sub zeros. Luckily, frost here on my block means a fine, sunny, almost spring-like day by mid-afternoon.

Most frost damage is done by the plants drying out as their available water freezes up. Watering established citrus in mid-winter can be as valuable to their survival as watering them in a drought summer. These citrus trees are in the ground, but without extra water and frost protection they might not look so happy.

Potted or containerised plants whose root area is limited will need even more attention: saucers, extra water, hay mulch around the stem, and frost cloth thrown over the top through winter’s worst month. Citrus survive best in quite a large capacity tub.

Seaweed solution.

Part of the secret to keeping them alive is to add a seaweed solution to the water. I hand water seedlings, the vege garden, in my tunnel house, and the citrus trees with a seaweed solution in the watering can. There are many good products now on the market including large commercial scale supplies for tractor-mounted sprayers. There must be something magical in seaweed: is it the salt, the iodine, some kind of natural anti-freeze? Or just that breath of mild sea air? Seaweed somehow softens the blow of a hard frost on your plants, and supplies a great variety of trace elements, should any part of your orchard stock be lacking in them.

HOW TO OFFER THE BEST PHYSICAL FROST PROTECTION

MEYER LEMON WITH SHELTER TREES

The photo above shows a -5°C frost in August less than 1km from the sea in the South Island. But with help, these Meyer lemons and lemonade trees survived and are now fruiting after a few years of growth. This is in an area known for its high rainfall and they’re not too far from the glaciers and icefields of the Southern Alps either.

Take a close look and you’ll see that my Meyer lemon is planted up against taller shelter trees (Fuchsia excorticata, Pittosporum crassifolium) which almost overhang it and surround it with shelter from the chilly southerly.

The deep green foliage under the trees is almost frostfree, while there is white frost on the open paddock in front. In contrast, the peach tree (at left) is quite happy with this level of chill and is even budding as it thinks it’s spring and time for a peach. Frost is really good for pip and stone fruit. Late frosts are bad for delicate blossoms like almonds, walnuts, grapes and apricots, nipping the buds and reducing the fruit set.

PROPERLY DRESSED

This is black shadecloth wrapped around the lemonade tree as a temporary measure. Good results can be had from hammering four stakes around a tree, then stapling frost cloth or windbreak in a double fold over the top of the trees. This method protects them from descending frosty air at night, and from hail, but just as importantly, it still allows free air movement around the tree rather than trapping the tree in a cage of cold shade, which too much over-zealous protection can do.

Planting location is important too. Think ahead and don’t plant your trees where winter shade is going to last half the day. They like to warm up and get those rays when the sun is about.

About the author
Ben Gaia grows trees in the extreme climate of the West Coast of the South Island and runs a mail order nursery for organic fruit and forestry trees, www.dialatree.co.nz

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This article first appeared in NZ Lifestyle Block Magazine. Discuss This Article

Cold Hardy Citrus Trees: Citrus Trees That Are Cold Tolerant

When I think of citrus trees, I also think of warm temps and sunny days, perhaps combined with a palm tree or two. Citrus are semi-tropical to tropical fruit crops which are fairly low maintenance and easy to grow, but not usually in regions where temperatures dip below 25 degrees F. (-3 C.). Fear not, there are some cold hardy citrus tree varieties and, if all else fails, many citrus trees can be container grown, making them easier to protect or move if the big freeze hits.

Cold Climate Citrus Trees

Citrons, lemons and limes are the least cold hardy of the citrus trees and are killed or damaged when temps are in the high 20s. Sweet oranges and grapefruit are slightly more tolerant and can withstand temperatures in the mid 20’s before succumbing. Citrus trees that are cold tolerant down into the low 20s, such as tangerines and mandarins, are the most optimistic choice for planting cold climate citrus trees.

When growing citrus trees in cold climates, the degree to which damage may occur is related not only to the temperatures, but a number of other factors. The duration of a freeze, how well the plant has hardened prior to a freeze, the age of the tree, and overall health will all affect if and how much a citrus is affected by a drop in temperature.

Varieties of Cold Climate Citrus Trees

A list of some citrus trees that are the most cold tolerant is as follows:

Choosing a trifoliate rootstock will ensure you are getting the most cold hardy variety of citrus and the smaller sweet citrus, such as Satsuma and tangerine, seem to have the most cold tolerance.

Care of Hardy Citrus Trees

Once you have selected your cold hardy citrus tree, there are several keys to insuring its survival. Select a sunny location that is sheltered from the cold northern winds with well draining soil. If you are not container planting the citrus, plant it in bare, non turf ground. Turf around the base of the tree can significantly lower the temperature, as can situating the tree at the bottom of a hill or slope.

Place the root ball of the citrus 2 inches higher than the surrounding soil to promote drainage. Do not mulch around the tree, as this will retain moisture as well as encourage diseases such as root rot.

How to Protect Growing Citrus Trees in Cold Climates

It is crucial that you take protective measures when the threat of a cold snap is imminent. Be sure to cover the entire plant, taking care not to touch the foliage. A double layered covering of a blanket over layered with plastic is ideal. Bring the covering all the way to the base of the tree and hold it down with bricks or other heavy weights. Make sure you remove the cover when temps rise above freezing.

Do not fertilize the citrus after August since this will encourage new growth, which is sensitive to cold temps. Once your citrus tree is established, it will be better able to withstand and recover from freezing temperature.

Most citrus is descended from four ancestral species.

Most cultivated citrus seems to be descended from four core ancestral species: citron, Citrus medica, from Northern India; mandarin, C. reticulata from China; pomelo (C. maxima, a grapefruit-like fruit) from Malaysia; and papeda (a sub-genus of Citrus) native to tropical Asia. Citrus is very promiscuous and will cross-pollinate with any other kind of citrus fruit – today’s lemons, limes, grapefruits, and oranges are natural or artificial hybrids of these species – and in combination with a possible polyphyletic origin, there is still considerable confusion on the classification of these fruit trees.

A backyard orange tree in San Diego.

The oldest known reference to citrus is in the Vajasaneyi Sanihita, a collection of devotional texts written in Sanskrit prior to 800 BC. The first Chinese references date to perhaps 776 BC, although they may actually refer to conditions well before that time. Citrus accompanied travelers along the Silk Road, migrating to the Middle East and, eventually, Europe. Citron, sanctified in India, was dispersed to the Near East, becoming an important part of Jewish culture. For a long time the only citrus in Europe was the citron which was brought to Calabria, Italy by the Jews around AD 70, and is still grown there. But citrus eventually spread all around the world as a consequence of travel, exploration, war, and politics. During the heyday of the Roman Empire, new varieties of fruit trees came flooding into Europe from all over the world, including probably in order, sour oranges, lemons and sweet oranges. But many of these citrus groves were lost when the empire disintegrated, and the next wave of introductions came with the rise of Islam and the Arab empire, with citron, sour orange, lemon and pummelo found in North Africa and Spain by 1,150 AD.

Orange trees are still grown in southern California.

The Crusades introduced feudal Europeans to lemons, lime and sour oranges, and citrus soon became the fashion of the nobility and rich merchants. By the 16th century, sweet oranges were a well-established commercial crop in southern Europe. The mandarin was first brought to England in 1805, and spread from there to Malta, Sicily and Italy. From Europe citrus spread to the New World, with seeds brought to Haiti on Columbus’ second voyage in 1493. By 1518 the orange reached Mexico, and Spanish settlers brought citrus to Florida in about 1565. By the early part of the citrus had spread to Arizona and California, where they became an important commercial crops.

Many types of citrus can grow more than 20 feet tall in the ground, but dwarf types can be kept at 3-5 feet tall in containers. These trees have ovoid to elongate, glossy, dark green leaves. On many types of citrus the leaf has a small flange along the petiole. There are some cultivars with variegated foliage.

Citrus leaves (L) often have a flange on the petiole (LC) and may be dark or light green (RC) or variegated (R).

Many types of citrus have sharp thorns

Citrus often has thorns at the nodes, especially on new grafts and fruiting wood; these can be cut off if desired (if the thorns are on shoots that originate from the rootstock, below the graft union, the entire shoot should be removed). Thorns evolved for protection from herbivores, especially of the young foliage which is most delicate, so many types only have thorns as juveniles but outgrow them as the tree matures. Lemons and limes generally have sharp thorns, although thornless cultivars are available – but they supposedly have less flavor and are not as productive. Some orange cultivars only have small, blunt thorns at the base of the leaves.

Clusters of fragrant white flowers are produced at the ends of the stems. Depending on variety, most citrus trees bloom in spring to set fruit that are ready to harvest in fall and winter. Others may flower and fruit off and on year-round (especially lemons, limes and kumquats), and specific pruning and fertilization techniques can promote flowering in the off season.

Clusters of buds (L), which may be white or pink (LC) open to white petals and yellow stamens (C, RC and R).

Young fruits develop after the flowers.

Citrus fruits are a hesperidium, a specialized type of berry with a leathery rind.

These are followed by a fruit technically called a hesperidium, a specialized type of berry with a leathery rind called a pericarp. Because most citrus doesn’t require cross pollination to be productive (although it may increase fruit size), they will set fruit on a single plant indoors. Potted trees often set much more fruit than the tree can support, so fruit drop is very common. The time from blossom to fruit harvest varies by type and variety. In general, most lemons and limes ripen in six to nine months, while oranges take about a year. Citrus fruits ripen only on the tree, but can remain on the tree in good condition long after they are mature.

Part of the citrus collection at Villa Medici di Castello.

Once the exotic fruit was brought to the attention of the wealthy, it began to be grown in nearly every European palace and garden either for culinary use, medicine or just for decoration. Since citrus is not frost hardy, in cold climates, including central and northern Italy and France, trees had to be grown in containers and spend the winter in the shelter of a purpose-built structure called an orangerie (orange house) in French and limonaia (lemon house) in Italian. These special structures are intended to protect the citrus from freezing and provide reasonable light, keeping them cool so they don’t grow too much during the winter. The orangery at the gardens of Versailles, with more than 1,000 potted plants, might be the world’s most famous showcase of citrus trees, indoors or out. During the Renaissance it was fashionable among rich families to collect rare mutations of citrus trees. Villa Medici di Castello in Florence, Italy has part of the original Medici citrus collection, with a genetic line that goes back hundreds of years. Some of these unusual cultivars, such as “fingered lemons”, can still be seen in Tuscan gardens.

An assortments of citrus fruits, including fingered lemons (second from right).

Citrus growing in a large container in Italy.

Even without an orangerie or limonaia, citrus is easily grown in containers in cold climates. The plants can remain outdoors during the growing season, but must be brought indoors when temperatures approach freezing. In Italy and France citrus often need to be sheltered in the relatively dim light of orange or lemon houses only for a few months, but in harsher climates the plants will need to remain indoors for longer and will need bright light to thrive.

For the best success growing citrus as a houseplant, place the container where the plant will receive as bright light as possible, such as in a south or southwest facing window. Citrus trees do not go dormant like many other plants, so they need sufficient light and some humidity during the winter (although their growth will slow during this time). Supplemental light will be needed for fruit production if the plants do not receive at least 6 hours of direct light daily (the foliage can adapt to the relatively low light levels typical in a home, but plants are very unlikely to flower). Citrus grow best between 55°F and 85°F. They can tolerate warmer or cooler temperatures (down to about freezing or below depending on the variety) for very short periods of time, but avoid abrupt temperature shifts. They require a 5-10 degree difference in day and night temperatures for flowering.

Citrus in containers are best moved outside once the weather is warm.

Once temperatures are consistently above 50°F citrus trees can be moved outdoors for the growing season. Acclimate them over a period of one to two weeks, gradually moving from a sheltered, partly shaded spot into full sun (leaves will sunburn if moved too quickly from low light conditions in a house to full sun; drastic changes of environment can also cause leaves, flowers and young fruit to drop). Choose a sunny location that isn’t too windy and where the containers will not be standing in water or receive frequent, shallow watering (such as near a lawn with a sprinkler system). Warmer microclimates, such as near a building or where there is reflected heat from a patio or walkway, are good choices. Move them back indoors before night temperatures drop into the 40s.

Grow all types of citrus in a slightly acid, well-drained potting medium, such as cactus mix. Amending regular commercial potting medium with up to 1/3 small pea gravel, pumice, turkey grit and/or other inorganic materials will improve the drainage. Avoid using growing media formulated for moisture retention. Clay, plastic or decorative containers are all suitable, as long as there are sufficient drainage holes. Start plants in smaller pots and move to a larger size as they grow so that there isn’t too much potting medium relative to the amount of foliage (or the soil will remain too wet after watering, making root rot more likely). A deep pot is better than a shallow one, as it will balance the tree when it gets larger and more top-heavy. Citrus can be kept in 10-12 inch pots for several years. Larger containers will allow the tree to grow bigger and more productive, but these may be harder to move. Plants should be repotted every year or two.

Grow citrus in well-drained potting medium and keep it moist, but not wet.

Citrus trees require soil that is moist but never soggy. Water regularly as needed to keep the potting medium moist, but not wet, allowing it to dry slightly between waterings. Infrequent, deep watering is preferable to frequent, shallow applications. The amount and frequency needed will depend on the size of the container and plant, they potting medium, temperature, humidity and other factors. Cupped or yellowed leaves can be a symptom of excessive watering. Reduce watering in winter when plants are not actively growing.

Citrus are heavy feeders and need more nitrogen than phosphorus or potassium. Specialized citrus/avocado fertilizers are available, but any all-purpose or acid-loving plant fertilizer that supplies at least a 2-1-1 ratio can be used. Trace minerals including iron, zinc and manganese may need to be added (most multipurpose fertilizers contain these minerals). Granular, slow release formulations are best, but frequent, dilute applications of soluble fertilizer can be made when plants are actively growing (April through September). Yellowing leaves may indicate a need for more fertilizer.

Cottony cushion scale is an important pest of citrus, but in the Midwest brown scale and spider mites are the most common pests.

The most common pests on citrus trees in the Midwest are brown soft scale and two-spotted spider mite, and occasionally aphids, mealybugs, or whiteflies. Washing the foliage periodically can help deter these pests. Insecticidal soap or synthetic insecticides can be used against all of these, although physically wiping the scales off the leaves and branches may also be needed as any sprays will only kill the crawler stage of that insect. Light horticultural oil may also be effective against scales. With all of these pests, multiple applications may be required to achieve control.

Citrus grown from seed is unlikely to produce the same type of fruit as the parent.

Although citrus can be grown from seed, those plants will be different from the parents, so it is best to purchase a known variety. Cultivars can also be propagated from semi-soft stem cuttings taken in the spring or summer – commercial varieties are almost also grafted as well, with a rootstock that is different from the cultivar grafted on top. Any suckers (shoot growth below the graft union – a diagonal scar between 4 and 8 inches from the soil) should be removed immediately to prevent vigorous growth that will detract from fruiting wood. Citrus can be pruning any time to shape and balance the tree. Lemon trees can even be trained as bonsai specimens. Most varieties of commercial oranges and other citrus grown outdoors in warm climates are too large to be grown indoors. Dwarf cultivars – or those on dwarfing rootstocks that keep the plant small – are especially suited for growing in containers.

Sweet citrus – such as oranges and tangerines – need a lot of heat to ripen, whereas the acid citrus – lemons and limes – need much less, so are more easily grown indoors. These types not only ripen their fruit much faster, but also tend to be nearly ever-blooming. Some of the easiest include:

  • Bearss lime (C. x latifolia) – also known as Tahitian lime and Persian lime, is a vigorous, thornless tree that produces bigger fruit than Mexican limes.
  • Variegated calamondin.

    Calamondin (Citrofortunella microcarpa or Citrus madurensis) – is grown primarily as an ornamental, producing small, round, orange, sour fruit. ‘Peters’ is a variegated form.

  • Citron (Citrus medica) – has yellow fruit with a rough and bumpy surface, very thick and fleshy rind, little juice, and sweet flavor.
  • Kaffir lime (C. hystrix) – is grown for the leaves that are used in Thai and other Southeast Asian, but it does produce a small, bumpy fruit.
  • ‘Eureka’ lemon – is a thornless cultivar with full size, striped fruit
  • Kumquat (Fortunella spp.) – has flowers with a pungent sandalwood-like scent followed by small, elongate, tart orange fruits good for marmalade. Meiwa kumquat (F. crassifolia) has round and sweet fruits.
  • Limequat – a hybrid of Mexican lime and kumquat (C. aurantiifolia x F. crassifolia) with a shrub-like habit and yellow, egg-sized fruit with edible skin.
  • Mandarin or satsuma oranges (C. reticulata) — aren’t really oranges at all but tangerines noted for their abundant fragrant flowers and which require far less heat to ripen the sweet fruit than true oranges.
  • ‘Meyer’ lemon – a lemon-orange hybrid brought from China in 1908 that produces very juicy, medium-sized, seedless fruits with a very thin skin and sweeter pulp than other types of lemons. The Improved Meyer Lemon is virus-free (the original was a symptomless carrier of certain viruses, particularly tristeza).
  • Tahitian orange (Citrus limonia ‘Otaheite’) – a dwarf, thornless tree that should be properly regarded as an acidless or sweet form of the Rangpur lime.
  • ‘Trovita’ orange (C. sinensis) – is fast-ripening with medium-small, few-seeded juicy fruit, but tends toward alternate bearing.

– Susan Mahr, University of Wisconsin – Madison


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Ever thought of growing your own citrus tree? Many are surprisingly hardy, and their evergreen aromatic leaves, edible fruits and scented flowers, often produced in these gloomy days of winter, make them a more useful prospect than you’d think. A sunny windowsill suits compact varieties, while larger trees will grow in frost-free greenhouses or in some cases outside in the garden.

Oranges require year-round warmth, so if you are without a heated glasshouse, other citrus trees are a more productive choice. Meyer lemons (Citrus x meyeri) are cold tolerant to -8C, so can be grown indoors or outdoors in a sheltered spot. Their fragrant, slightly sweet fruits are thin-skinned and do not travel well, so your best chance of getting your hands on Meyer lemons is to grow them yourself. The most useful variety of lime for home growers is the kaffir lime (Citrus x hystrix). The leaves, rind and juice can be used for flavouring food and adding an aromatic note to spirits. I can’t get excited about kumquats (Fortunella species) but some people like eating them. They fruit readily and are a robust plant that tolerates beginners’ mistakes. Some kumquats are considered cold tolerant to -10C, though I have not seen one grown outdoors in the UK.

My favourite citrus tree to grow indoors is calamondin (x Citrofortunella mitis) which is always flowering or fruiting and makes fiery-coloured, tasty marmalade. Japanese bitter oranges (Poncirus trifoliata) may be inedible but are cold tolerant to -20C. Scented flowers emerge in late spring and early summer, and long thorns make them an unusual addition to boundary hedges.

If you have a sheltered spot that gets full sun, a few other citrus trees may grow outdoors for you. Yuzu (C. ichangensis x C. reticulata var. austera) are tolerant of temperatures down to -15C. They produce sour fruits valued in Japanese and Korean cooking for their rind and juice. Grapefruits (Citrus x paradisi) will survive down to -8C. Even plants home sown from pips can fruit when grown outdoors, but will take 12 to 15 years to start producing a harvest.

Trees should be planted out after the risk of frost so they have a full growing season to become established before winter. When cold nights are expected, wrap trees in fleece, or bring pot-grown specimens under cover. Mature trees will survive cold temperatures, but if they are flowering or bearing fruit, covering them on a cold night will prevent flower and fruit drop.

Many citrus pips will readily germinate and develop into trees, but the fruits produced by mature plants may not be the same as the fruit you got the pips from, as citrus are often hybrids. Investing in mature trees means that plants will rapidly fruit, or already be fruiting, and they will be hardier as young citrus trees are much more tender.

The keys to success

• Citrus trees require full sun, neutral or slightly acidic soil. Water with rain water if your tap water is alkaline.

• Hardiness varies between varieties, so check the cold tolerance of your variety. Trees grafted on to P. trifoliata rootstock have increased cold tolerance.

• In the growing season (late March-October) they need high-nitrogen fertiliser.

• Citrus benefit from high humidity but falter in waterlogged soil, so stand pot-grown plants on gravel in trays of water.

• Prune only to remove dead branches and shoots growing from below the graft level, or to keep the plant at a desired size.

• The scourge of citrus trees is sap-sucking scale insects and mealy bugs. Watch out for a sticky black coating on leaves and use a cotton bud dipped in methylated spirits to get rid of outbreaks.

• Pests can also be controlled with sprays based on plant oils or soft soap, or biological controls.

Where to buy

The Citrus Centre; Cross Common Nursery; Larch Cottage.

Reader offer

Buy one Meyer lemon or one calamondin for £11.99, or buy both plants (one of each variety) for £16.99, including free p&p. Call 0330 333 6856, quoting ref GUA626. Supplied as 9cm potted plants. Parts of plant may be edible. Full details supplied with your order.

Protecting Citrus Trees from Cold

Citrus trees such as lemons, oranges, satsumas and grapefruit are some of the most popular fruit trees grown at homes. There is a good reason for their popularity. They are reliable in producing quality fruit and they tend to be relatively easy to care for. They are versatile and they can be planted in the ground or containers.

Citrus trees are relatively small in size: they can grow up to 10 or 15 feet tall and around 10 feet wide. This allows them to easily fit in most landscapes, provided they are planted in an appropriate climate zone.

These trees also produce beautiful, fragrant flowers. Citrus trees are known for their dark, glossy evergreen foliage. This way, these trees are attractive throughout the whole year and they make a perfect addition to your garden.

Planting Your Citrus Tree

There are various types of citrus trees and they usually become available at area nurseries in the fall or early winter. Because of this, they can be planted in the fall. However, it’s important to think about the cold months. It’s always difficult to predict when the severe freezes will occur, it’s risky to plant citrus trees in the fall or winter. It’s best to wait until late winter or early spring. Planting your citrus trees in late February or early March is usually the best way to go. This is when the coldest part of the winter is past but the weather is still cool enough.

In case you wish to buy your citrus tree in the fall, it’s possible to do so, but wait with the planting till next spring. Place a container-grown tree somewhere where it’s warm enough and sunny. Your porch or patio are generally good places. Water regularly through the whole winter. In case the temperatures are predicted to go below 30 degrees F, move the tree inside to protect it from cold.
This will keep your citrus tree alive and healthy through winter even if it’s very cold and snowy outside. Your tree will be ready for planting the next year in late winter or early spring.

Protecting Your Citrus Tree from Cold

In case you do decide to plant your citrus tree in fall of winter, it’s possible to do so. However, it’s important to provide an adequate protection in case of very cold temperatures. Same protective measures should be used for citrus trees already growing in your garden.

To protect your citrus trees from winter colds, you need to take several things into account:

Cold damage. Different types of citrus trees vary in their tolerance to cold.

Protection. You should know how to minimize damage and ensure recovery by maintaining a healthy tree with thick, strong leaves.

Care after damage. You should know how to care for trees that have been damaged during winter months.

Cold damage

It’s important to know that different types of citrus trees have different tolerance to cold and freezes. Here’s the list of citrus trees from the most freeze-tolerant to the least freeze-tolerant: satsumas, kumquats, sour oranges, sweet oranges, grapefruits, lemons and limes.

For example, satsumas and kumquats don’t need any protection until the temperatures drop under below 25 degrees F, and they can survive just well without protection unless the temperatures go below 20 degrees F. Some other types of citrus trees are more fragile and they can be badly damaged if the temperatures are in the low 20s F. Generally speaking, temperatures in the teens F will severely damage or even kill citrus trees that are not protected.

It’s also important how cold it gets and also how long it stays so cold. The longer temperatures stay below freezing, the more likely is extensive damage or even death. If temperatures rise above freezing in a few hours the damage is less serious.

Another important factor is the tree’s age. Older, established trees are more tolerant to severe cold. Even if they are badly damaged they will be able to recover. Older trees have more massive trunks, which enables them to live through freezes that would kill younger trees. This is why young trees require more care and this is one of the reasons why they should be planted after winter has passed.

Also, it’s important to observe the weather just before the freeze. If citrus trees are gradually exposed to lower temperatures they will go through a process called “hardening off”. They will form a natural protection and they will have a decrease in the freezing point of their tissue. Trees can become semi-dormant when hardened in the temperatures of the 40s and mid 30s F range. These trees are less likely to be damaged in winter colds. To make hardening possible, don’t encourage late growth of your citrus trees. For this reason, it’s important to avoid pruning and fertilizing after July.

Protection

You can reduce any potential cold damage and ensure recovery of your citrus trees if you maintain a thick canopy of leaves in your tree. Weak leaves, especially those that are in too much shade, those with insect or disease damage or those with severe nutrient deficiency are the ones that are most severely damaged. These leaves will be the slowest to recover from a damage caused by cold.

Also, don’t forget to water your tree regularly when low temperatures are predicted.

Another thing you should do to minimize cold damage is to plant your citrus trees in the most protected areas of your landscape. Good places are southern exposure enclosed by fences, hedges or buildings on the north side. They provide some protection from cold. Whenever possible, plant citrus trees close to your house (but make sure it’s not closer than 8 feet).

You may also protect a tree that is not too large by making a simple frame and placing it over the tree and encasing it in one or two layers of plastic. The frame should hold the plastic off of the leaves and provide some protection.

Another protection can come from strings of small, incandescent outdoor Christmas lights. Wrap them around the trunk of the tree and throughout the canopy under the cover. This will help raise the temperature within the enclosure and provide additional protection against freezes. Always use outdoor extension cords and make safe connections to keep it safe.

Whenever days become sunny and mild, make sure to remove the plastic coverage in order to prevent the buildup of excessive heat.

If a tree is too large to cover, you can protect it by wrapping its trunk and lower parts of the main branches with insulating material. Fabric or foam rubber works great. In case the upper branches are lost in cold you can always grow a new tree from the living trunk.
Remember, even one night of the 15 degree F temperatures can kill an unprotected tree. For this reason, it’s very important to take protective measures for your citrus trees.

Care after Damage

There are some measures you can take to minimize the damage done and to make the tree recover as soon as possible. There are also some things you can do to make the tree as strong as possible before the winter.

One of the most important things is to refrain from pruning or cutting back citrus trees that have been damaged from cold until June or July. It’s usually difficult to estimate where the regrowth will occur until the tree resprouts.

However, even then, the new growth may collapse during the summer and die. In case the only new growth occurs on the ground level or below the graft union (the graft union is noticeable as a knob or swelling about 1 foot above the ground), you should replace the tree. Citrus trees are grafted onto a rootstock, which helps them grow better. This also makes them a little hardier and more tolerant to cold weather. However, this rootstock can’t produce acceptable fruit.

In case the new growth occurs from the above the graft union, this is a good sign. Such a tree can be saved and it can be trained into a new tree.

Photo credit: cobalt123 and cirox via photopin cc

Grow Your Own

Protecting citrus trees from frost helps to guarantee your annual crop. It doesn’t take long and is so worth the effort!

Here is our shortlist of important things to keep in mind:

  • New citrus trees should be planted in the early spring to allow for root development before summer heat.
  • Plants exposed in open areas to winds, especially in low areas of the garden, are most likely to suffer frost damage first, as cold air accumulates in such pockets. For protection, consider planting citrus trees near walls and fences, which trap and radiate heat.
  • Before an expected frost, water trees well, but don’t get the leaves or trunk wet as they are most vulnerable. Keep the ground as clear as possible of weeds or mulch to allow for more heat to be retained from daylight sun.
  • Protect young tree trunks with cardboard, wrapped tightly around the trunk just before nightfall, from the lowermost branches to the soil. Also consider covering trees with breathable, water permeable frost blankets for the night, and remove during the day.
  • When frost hits, ice crystals form inside the plant cells, disrupting the flow of fluids, causing cells to break down.

Overall, when temperatures fall to 29°F for 30 minutes or longer, some frost damage to tender citrus plants will occur. Certain citrus – citron, lemon, lime, and Satsuma mandarins among them — are more sensitive than others.

  • As applicable, remove frost-damaged fruit with cracked skin immediately to prevent fungus and mold spreading throughout the tree. (Yellowing leaves in winter are common, and may be a sign of over- or under-watering.)
  • Wait to prune damaged branches until spring, to allow for further analysis and recovery in warmer weather. Remember to clean pruning tools to avoid the spread of disease.

Want more? Here’s our guide to California winter citrus.

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