- Types Of Zone 6 Olives: What Are The Best Olive Trees For Zone 6
- Can Olive Trees Grow in Zone 6?
- Growing Zone 6 Olives
- Classic Trees, Professionally Grown
- Cold Hardiness of Olive Trees
- All about olive trees
- Site Selection
- Cold is required for proper blooming and fruit-bearing
- Why do I have to know about olive tree chill units?
- Learn more about the olive tree
Types Of Zone 6 Olives: What Are The Best Olive Trees For Zone 6
Want to grow olives, but you reside in USDA zone 6? Can olive trees grow in zone 6? The following article contains information about cold hardy olive trees, olive trees for zone 6.
Can Olive Trees Grow in Zone 6?
Olives need long warm summers of at least 80 F. (27 C.), along with cool night temperatures of 35-50 F. (2-10 C.) in order to set flower buds. This process is referred to as vernalization. While olive trees need to experience vernalization to set fruit, they freeze from extremely cold temperatures.
Some resources claim that a few varieties of olive can withstand temps down to 5 F. (-15 C.). The caveat here is that the tree MAY re-emerge from the root crown, or it may not. Even if it does return, it will take several years to become a producing tree again if it isn’t too severely damaged by the cold.
Olive trees become cold damaged at 22 degrees F. (-5 C.), although temperatures of even 27 degrees F. (3 C.) can damage branch tips when accompanied by frost. That said, there are thousands of olive cultivars and some are more cold resistant than others.
While variations in temperature occur within a USDA zone, certainly those in zone 6 are too cold for even the most cold hardy olive tree. Generally, olive trees are only suited to USDA zones 9-11, so sadly, there are no zone 6 olive tree cultivars.
Now with all that in mind, I have also read claims of trees dying down to the ground with temps below 10 F. (-12 C.) and then re-growing from the crown. Cold hardiness of olive trees is similar to that of citrus and improves over time as the tree ages and increases in size.
Growing Zone 6 Olives
While there are no zone 6 olive cultivars, if you still want to try growing olive trees in zone 6, the most cold hardy include:
There are a couple other cultivars considered as cold hardy olives but, unfortunately, they are used commercially and not obtainable to the average home gardener.
Probably the best option for growing in this zone is to container grow the olive tree so it can be moved indoors and protected upon the onset of cold temperatures. A greenhouse sounds like an even better idea.
Classic Trees, Professionally Grown
Cold Hardiness of Olive Trees
The cold hardiness of olive trees is greatly exaggerated on some websites. Claims that olive trees can withstand 5 degrees Fahrenheit is true only inasmuch as shoots MAY re-emerge from the root crown several months after the entire top and most of the trunk have been killed. In this case, it will take several years to have a producing tree again and even a moderately hard freeze in the intervening years can be devastating.
Serious cold damage to olive tree branches begins at 22 degrees, though lesser damage to fast-growing branch tips can occur at 27 degrees if frost is present. Temperatures of long duration in the mid-teens can cause very serious damage to larger branches and trunks, and the extent of the damage may not be evident for some time. Damage from cold is exacerbated when the temperature swings sharply from very warm to very cold conditions over a short period of time, as occurs during cold fronts in north and central Florida.
Duration of cold is an important factor. If the temperature falls to the low twenties early in the evening and persists until late morning, damage will be much greater than if the same low temperature occurs for only and hour or two.
Within a particular USDA Cold Hardiness Zone, low temperatures can vary somewhat due to variations in topography, moisture content of surrounding soil and the presence of “heat islands” created by cities and other large infrastructure. In general, however, we at Olive Tree Growers cannot concur with claims that olive trees can thrive in USDA zones 7b or lower and 8a can be questionable, depending upon the particular winter and the particular location.
Many people do grow olive trees in locations generally considered too cold for them and take extreme measures to protect their trees during times of low temperatures. At Olive Tree Growers, we offer our trees only in areas where they can be reliably grown outdoors; anyone who elects to plant olive trees in colder climates should have a well-considered winter-protection strategy ready for implementation.
All about olive trees
Olives are widely planted in gardens but are not always grown for their fruit. This productive, long-lived evergreen tree from the Mediterranean is also valued in gardens for its good looks and heat and drought tolerance. Young olive trees have slender trunks and silver green leaves. Reaching 5 to 7 metres high they are used as shade trees or as hedges or espalier and can be grown as potted plants or as topiary specimens.
To produce a worthwhile harvest of olives, space trees about 6 metres apart to form a grove. This makes maintenance and harvesting manageable and also improves yield. Higher crops are obtained when trees are grown together as olives are wind pollinated so benefit from having other olives and a mix of varieties nearby.
To calculate how many trees to grow to meet your needs, estimate that a well-grown mature tree of nine years of age produces around 40 kilogram of fruit. Seven kilograms of olives yields a litre of oil so a single tree produces about six litres. Crops may vary considerably from year to year depending on growing conditions and tree management.
Varieties and harvest
The most widely grown varieties for gardens are ‘Kalamata’, a dual purpose black olive suited to either pickling or pressing, ‘Manzanillo’ a large green fruiting olive, ‘Verdale’ which is used either for pickling or pressing, and ‘Nevadillo Blanco,’ a heavy cropping black olive that’s the widest grown oil-producing olive in Spain.
Autumn to early winter – when the fruit is plump and either dark green or purple-black – is the time to harvest. Ripe olives are bitter and must be processed in brine before they can be preserved for eating. Olives used for oil are pressed or put through a centrifugal extraction process to extract their oil.
Olives come from the Mediterranean region and this is the best climate zone for olives. They are versatile however and grow in cool zones (although severe frosts can damage olives) and also in parts of the subtropics, although in humid areas they are more prone to pest and disease problems. Most olives require around 200 hours of chilling before flowering and fruiting occurs.
Olives grow best with a slightly alkaline (pH 7-8) well-draining soil. Olives can be grown in large containers but pot-grown plants are not as productive as those grown and managed in the ground.
Olives flower in early spring. Trees take three to five years of growth until they produce their first harvest and most only become fully productive after eight or nine years.
Young plants benefit from early pruning and training to form a single trunk and a framework of three to four main branches. Protect young trees from frost.
To maintain trees, prune them in winter after harvest to remove dead branches and to keep the tree structure open. Olives fruit on the wood from the previous year’s growth i.e. one-year-old wood, so heavy pruning can reduce the next year’s crop.
Olives do not need large amounts of fertiliser but benefit from light applications of organic fertiliser in late winter or spring (prior to flowering) and again in autumn after harvesting. They may also need trace elements.
Major pests include olive lace bug and scale, which attack the leaves, and olive moth caterpillars, which attack leaves and flowers and can reduce fruit production. Pests can be controlled organically with soap sprays or, in the case of caterpillars, the use of Dipel or other organic insecticide.
Diseases include leaf spots such as peacock leaf spot, anthracnose and root rots. These can be controlled with applications of organic fungicides, pruning for good air circulation and growing the trees in well-draining soil.
A note about the African olive plant: In New South Wales, the African olive plant is classified as an aggressive woody weed that invades native bushland. Although related to the edible European olive, African olive plant fruit is not edible.
Soil and Drainage
Where are the ideal places in the U.S. to grow olives?
Olive trees need a subtropical climate and do best with mild winters and long, warm, and dry summers. They are sensitive to hard freezing environments. They will grow in climate zones 10 and 11 (see map below). Some varieties are hardy enough for zone 9 or even 8. Temperatures below 22ºF (-5ºC) will kill small wood and branches. Freezing conditions lasting days or a hard freeze, below 15ºF (-10ºC), will kill or severely damage an olive tree. It is best to avoid planting olive trees in situations where there is a high risk of frost during bloom (late April to mid-May) or where freezing conditions are likely before harvest. Summer rainfall can cause fungal and bacterial infestations.
Can I grow my olive trees indoors?You could grow a tree “bonsai style” and keep it indoors and we have heard of people who have trees in large containers who move them indoors or into a hothouse during very cold weather but this is impractical for most growers.
Although olive trees are non-deciduous, they do require a cold period to go into semi-dormancy. If you have a winter month where the average daily temperature is below 54 degrees or less, the tree will get the message to slow growth and change gears for flowering in the spring.
Where are the ideal places in the U.S. to grow olives?Finally, there can be substantial microclimate differences within one orchard. Small valleys, in particular, can be substantially colder than more open areas and may make a difference between having frost damage or not.
SOIL AND DRAINAGE
Olive trees prefer non-stratified, moderately fine textured soils, including sandy loam, loam, silt loam, clay loam, and silty clay loam. Such soils provide aeration for root growth, are quite permeable and have a high water holding capacity. Sandier soils do not have good nutrient or water holding capacity. Heavier clays often do not have adequate aeration for root growth and will not drain well.
Olive trees are shallow rooted and do not require very deep soils to produce well.
Soils having an unstratified structure of four feet are suitable for olives. Stratified soils, either cemented hardpan or varying soil textures within the described profile, impede water movement and may develop saturated layers that damage olive roots and should be ripped.
Olives tolerate soils of varying chemical quality. Trees produce well on moderately acid (pH greater than 5) or moderately basic (pH less than 8.5) soils. Basic (alkaline) or sodic soils should be avoided since their poor structure prevents water penetration and drainage, creating saturated soil conditions that kill olive roots.
Southern exposures in the West and Southern United States will give better yields and protect from freezes.
Olives can be grown on steep inclines or terraced, although access and mechanical harvesting, spraying, and other orchard maintenance chores can be difficult and more costly. There has been debate on whether terraced trees should be placed on the edge of the terrace or against the hillside. Inclines greater than 20 degrees are inappropriate for most tractors and other vehicles.
Inclined properties cannot be assumed to have good drainage. Dig a hole and fill it with water to do your own “perc” test to see if the water drains.
Before choosing a site, consider the properties around it. Factors to look for include (but are not limited to) organic buffer zone and run-off issues if you intend to grow the trees organically, pest sources such as untreated olive trees infested with the olive fly, dead native growth that may carry boring beetle, grass hopper or weevil infestations, and other damage vectors such as wild pigs, gophers, or ground squirrels.
G. Steven Sibbett and Louise Ferguson: Olive Production Manual, University of California.
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You’re the proud owner of a brand-new olive tree, but recent cold weather got you worrying?
Not only is your olive tree much hardier than you think, it actually needs the cold to bear delicious olives!
- Is my olive tree freezing? How olive tree reacts to cold weather
- How to prepare olive tree for winter
- Olive tree care (planting, pruning, harvest, diseases)
- Grow an olive tree in the North!
This mythical tree connects us to our ancestors who grew it the ancient Middle East, Greece, and Egypt. Modern science is just beginning to unravel how important the environment is to produce olives and great-tasting oil.
Cold is required for proper blooming and fruit-bearing
Interestingly, many varieties of olive trees in temperate climates actually need some degree of cold weather. If planted in areas where winter is too mild, an olive tree won’t enter dormancy and flower buds won’t fully mature. That’s why an olive tree planted in a tropical climate will almost never bloom and bear fruit.
Each variety is different in this respect. Usually, the intensity of the cold required and the duration of it depends on the local climate where the variety was developed.
- For example, the Spanish ‘Arbequina’ olive variety needs to be exposed to temperatures around 7°C for at least 300 consecutive hours. During these hours, flower buds mature and form to get ready to bloom. Winters are short in Catalonia, where Arbequina comes from.
- Italian olive varieties ‘Frantoio’ and ‘Leccino’, on the other hand, require over 600 hours of 7°C temperatures. In Italian regions where olive is grown, winter is longer than winter in Spain.
Of course, temperatures never stay at 7°C for exactly 300 or 600 hours. In reality, time during which temperatures deviate from this optimum count “for less”. Practically, the word “chilling unit” is used to make counting and measuring this easier. It’s more accurate than “chill hour“.
- If the needed “chilling” doesn’t occur, buds on the olive tree won’t turn into flowers. Instead, only leaves or branches would appear.
- That’s why chilling is called a “trigger” for blooming on an olive tree.
After experiencing the required amount of “cold”, flower buds are ready on the branch and remain dormant. Dormancy is lifted when temperatures around the tree warm up.
- The warmer the air, the faster triggered flowers bloom.
- The amount of accumulated heat is measured with a unit called “Thermal time“, “Thermal days” or “Growing degree days” (or hours).
- If it gets too hot too fast, flowers might bloom and wilt before pollination, which also isn’t good.
Experts are able to compute this with mathematical models. When given temperature data for a specific place, they’re able to predict when the olive tree will bloom almost to the day!
Since each variety is different, the mathematical formulas have to be adjusted every time. Research is constantly underway to do this for each cultivar and type. It is a fascinating field of study and there is still a lot to discover!
Why do I have to know about olive tree chill units?
When planting an olive tree, apart from the ornamental olive tree’s value, what’s important is yield and harvest. Selecting the right olive tree variety for your zone or location is key.
- For places with short winters, select olive tree cultivars with low chill unit requirements. If a tree doesn’t get enough cold hours, it won’t flower or bloom properly. You might still get fruits, but not as much as expected. Low chill hour cultivars are called early-flowering cultivars.
- For places with long winters, select olive tree varieties with higher chill unit requirements. If ever you choose a tree that doesn’t require a lot of chill units, it might start blooming in the middle of winter. Blooms would die off at the next frost. Harvest would also be affected. High chill hour cultivars are called late-flowering or tardy varieties.
Here is a list of low chill unit olive varieties
- Chemlali (Tunisia) – 125 chill units
- Arbequina (Catalonia, Spain) – 340 chill units
Note on Arbequina – Research has shown that smaller, potted Arbequina olive trees grown in containers were able to produce fruit and flower even without going through frost and chilling. Since chill units accumulate for temperatures up to 70°F (21°C), it doesn’t necessarily disprove the chill unit requirement. Actual experiment data was not provided.
Currently, an experiment to discover olive varieties that need less than 100 chill units is underway in Wauchula, Florida. Olive tree might be an option to replace citrus in case of disease.
Here is a list of high chill unit olive varieties
- Frantoio (Italy) – 610 chill units
- Leccino (Italy) – 670 chill units
How can I know how many chill units my garden is going through?
Many countries and states have set up a tracking system. Check with your local weather bureau, farmer’s coop, or agricultural service center. They’ll provide accurate temperature readings and may even have computed the resulting chilling unit count.
Learn more about the olive tree
These articles might answer your fears about cold waves and hardy olive trees:
- Winterizing olive trees
- Olive tree growing in a pot
- Olive tree in cold temperatures
- How to grow olive trees
Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Frozen olive tree by Loren Kerns under © CC BY 2.0
Flowering olive by Ulrike Leone under license