Olive trees (Olea europaea) are resilient Mediterranean plants, but their health and productivity depend heavily on specific preparations for winter. These evergreen trees require a cold period to initiate the development of flowers and subsequent fruit. Managing the transition to colder months protects the tree’s sensitive tissues from damaging freezes and excessive moisture. Successful winter care ensures the tree survives the cold and thrives the following spring.
Understanding Dormancy and Cold Tolerance
Olive trees enter a state of semi-dormancy in the late autumn, where growth slows significantly as daylight shortens and temperatures decrease. This cold period, called chilling, is required for the tree to set a good crop. Chilling also triggers a physiological hardening process that increases tolerance to freezing temperatures, but this cold acclimation is limited.
The tree’s various parts possess different levels of hardiness. Young, soft wood and leaves are the most vulnerable, suffering damage around 25°F to 28°F (-4°C to -2°C). Mature trees of hardier varieties, such as ‘Arbequina,’ can tolerate brief dips down to about 15°F (-9°C). The root system is highly sensitive, potentially freezing at temperatures as high as 23°F (-5°C). The greatest threat is the combination of freezing temperatures and high moisture, which can quickly lead to fatal root rot.
Protecting In-Ground Trees from Severe Weather
For in-ground olive trees, winter protection requires minimizing exposure to both extreme cold and excess water. Preparatory work begins in late summer by ceasing all nitrogen-rich fertilization, which prevents the tree from producing tender new growth easily damaged by frost. Water management must also be drastically reduced, as waterlogged soil is a threat to root health when temperatures drop.
Insulating the root zone is the first line of defense against deep soil freezes. Apply a thick layer of organic mulch, such as straw or wood chips, four to six inches deep around the base of the trunk. Keep this mulch pulled back a few inches from the trunk itself to prevent moisture buildup that encourages disease or rot.
Protecting the trunk and canopy is necessary when severe weather is forecast, especially below the mid-20s Fahrenheit. Wrap the main trunk with burlap or a similar insulative material to prevent bark splitting from sudden temperature changes. For the canopy, use horticultural fleece or a heavy frost cloth draped over the entire tree to trap a small pocket of warmer air.
Secure the frost cloth to the ground around the base to retain ambient warmth emanating from the soil. Cold, persistent winds are damaging, so setting up temporary windbreaks or positioning the tree near a south-facing wall helps reduce wind chill. During an extreme, short-duration freeze, hanging a string of incandescent lights within the wrapped canopy can provide gentle heat to prevent tissue damage.
Overwintering Containerized Olive Trees
Olive trees grown in pots require a different winter strategy, as their roots are exposed to cold air and susceptible to freezing damage. Move the tree to a sheltered location before the first hard frost, generally when overnight temperatures consistently drop below 40°F. The goal is to provide a cool, dry environment that allows the tree to remain dormant but keeps it safely above freezing.
An unheated garage, a cool basement, or a temperature-controlled sunroom are ideal overwintering locations, with an optimal temperature range between 41°F and 50°F (5°C to 10°C). If the tree is exposed to too much warmth, it may break dormancy prematurely, leading to vulnerable new growth. Although dormant, the tree still requires light, so positioning it near a bright window or using supplemental grow lights is beneficial.
The watering schedule must be significantly reduced indoors, as the tree’s slowed metabolism means the soil dries out slower. Overwatering is the most common mistake for potted olive trees in winter, leading to root rot. Water the tree sparingly, providing just enough moisture to prevent the root ball from completely desiccating until spring arrives.