Winter Protection of Olive Trees

WINTER PROTECTION OF OLIVE TREES
By Cary Cloud
Olive trees are warm-temperate plants that can survive a light freeze without protection. Even a hard freeze, down to 23 degrees F, is not a problem, as long as the lowest temperatures last for only a few hours. At 22 degrees F, olive trees can suffer minor damage to branch tips, especially if there is new growth. This superficial tip burn does no permanent harm to the tree. Olive trees planted in the earth survive overnight temperatures in the high teens but may sustain some damage to small branches, especially if low temperatures are accompanied by wind or frost.
If overnight temperature falls to 15 degrees, even for an hour or two, olive trees can sustain serious damage, and lower than 15 degrees is life threatening. Claims that olive trees can survive 5 degrees F are simply not accurate. While there are isolated accounts of olive trees surviving single digit temperatures, these were old trees of low vigor growing in sheltered locations and "survival" does not address the condition of the surviving tree. Usually, olive trees that have experienced 15 degrees F make little or no fruit the next year and trees that have suffered 13 degrees lose all their leaves and many large branches; those exposed to wind at the same time may be killed. Trees that survive 13 degrees often have dead areas of the main trunk on the windward side.
While the susceptibility of olive trees to low temperatures generally precludes commercial olive farming in areas colder the USDA Hardiness Zone 9a, the home gardener or hobby grower can successfully grow olives in colder areas with careful siting of the trees and a strategy for winter protection. Olive Tree Growers, Inc. has a customer in Atlanta whose large trees survived 10 Degrees with careful covering and made fruit the following spring. We have even heard many accounts of olives being grown in Chicago but the winter protection strategy is quite extreme: the trees are uprooted and buried in a trench for the duration of winter, then resurrected in spring. That is really wanting an olive tree, but then they really are that special.
In areas where winter lows fall below ten degrees, the home gardener can easily grow olive trees with a well-considered and implemented protection strategy. Remember that low temperature, duration of cold, and frost are all relevant factors. The lower the low temperatures, the longer the duration of cold, the stronger the wind, the more protection will be needed.
The siting of the tree is also important. Olive trees in cold winter areas should be planted in the lee of the coldest winds. Heavily wooded areas to the north are good. Small-stature cultivars such as Arbequina can be planted on the south side of a house for wind protection. Paved areas near the tree help retain some heat. Large bodies of water also provide some protection, especially if they are deep.
Covering of trees should never be done with plastic sheeting. This does little or nothing to stop the transfer of cold and, if left on until the sun is high and warm, can cook the tree.
Implementation of the following guidelines will help with the overwintering of olives in cold areas.
IN-GROUND TREES (For temperatures 10-20 degrees overnight):
*Site tree in most protected area available (see above).
*Have a cold-protection strategy and materials ready. The best material for wrapping trees is "frost fabric", unwoven cotton sheeting, used by professional nurserymen and often sold by garden centers. Old sheets and bedspreads also work, but sure you have enough to cover them thoroughly.
*Wrap the trunk from the lowest branches to the ground. Allow the covering to reach the ground and spread around the trunk for 2 to 3 feet.
*Cover the tree from top to bottom with a big sheet of fabric. Use a double layer if temperature is expected to fall below 17 degrees or if very heavy frost is expected. NEVER USE PLASTIC SHEETING.
*Leave covering in place as long as dangerously low temperatures continue but remove it as soon as possible. For areas with extended periods of sub-20 degree weather, olive trees will need to be grown in pots.
CONTAINER-GROWN TREES
*Leave container-grown olive trees outside in a sunny location until temperatures fall below freezing.
*Remember that trees in pots will be more vulnerable to cold than trees in the ground.
* When sub-freezing temperatures are expected, bring potted olive trees indoors to a cool, well-lighted place. A room with a south-facing windows and a temperature of 40-50 degrees is ideal.
*Move it in and out. When temperatures are above 40 degrees, move your tree outside to a sunny location during the day. Move it back inside if freezing is expected overnight.
*Water. Remember that your tree will need some water during the winter, too. Keep the soil slightly moist through and through. Check the top of the soil and drain holes to be sure soil is not just damp on the top.

  http://www.olivetreegrowers.com/blog.php?view=detail&id=114