The olive tree (Olea europaea) is a plant historically cultivated of subtropical origin, meaning the species is naturally sensitive to prolonged freezing temperatures. This sensitivity is the primary factor limiting its cultivation outside of its native range. As olive growing expands into new climates, understanding the plant’s ability to survive winter cold becomes paramount for successful establishment. While the olive is generally considered tender compared to many temperate fruit trees, the species exhibits considerable variation in hardiness.
Understanding Olive Tree Cold Hardiness Thresholds
The degree of cold damage an olive tree sustains is directly tied to the temperature reached and the duration of the freeze. Temperatures around 0°C to -3°C (32°F to 27°F) typically cause only slight injury, such as damage to new growth, leaves, and one-year-old shoots. If temperatures drop lower, reaching approximately -6°C to -7°C (21°F to 19°F), the damage becomes moderate, affecting nearly all parts of the plant, including older branches and buds.
Established, cold-acclimated olive varieties can often tolerate brief dips to about -9°C (15°F) without experiencing permanent structural damage. However, prolonged exposure to temperatures in the mid-teens can cause serious damage to the main branches and the trunk’s vascular tissue. The most destructive cold events happen when the tree’s root crown is compromised, which often occurs at a lethal temperature (LT50) of around -6°C (21°F) for the root cambium, leading to the death of the entire plant.
Physiologically, freezing temperatures cause water to move out of the cells and freeze in the extracellular spaces, which also leads to cell dehydration. If the cold is severe enough, the formation of ice crystals and subsequent freeze-thaw cycles can cause the xylem vessels to clog, resulting in shoot dieback. The extent of injury is influenced by the speed of the temperature drop, with sudden cold snaps causing more damage than a gradual descent into freezing.
Genetic Differences in Cultivar Cold Tolerance
Cold hardiness is not uniform across the olive species. Choosing a variety with superior cold tolerance is often the first line of defense for growers in marginal climates. Cultivars like ‘Arbequina’ and ‘Mission’ are widely recognized for their moderate to high tolerance, making them popular choices in areas prone to winter freezes. ‘Leccino’ and ‘Picual’ are also noted for their resilience, having survived significant cold events.
In contrast, varieties such as ‘Coratina,’ ‘Manzanillo,’ and ‘Koroneiki’ are considered less tolerant to cold stress. The genetic mechanisms behind cold tolerance involve the differential expression of genes that help the plant cope with stress. Tolerant genotypes are better at maintaining photosynthetic efficiency during cold periods and activating genes responsible for producing cryoprotective proteins and soluble sugars. These internal changes, such as the conversion of starch to soluble sugars, help lower the freezing point inside the plant’s tissues, enhancing survival capability.
Environmental Factors Influencing Cold Survival
Tree maturity is a significant variable, as young trees are far more susceptible to cold damage than older, established trees. Older trees develop thicker bark and a larger, more insulated root system, which helps protect the vital cambium layer and root crown from freezing.
The moisture content and drainage of the soil also play a role in cold survival. Waterlogged soil is particularly dangerous because it transmits cold more efficiently than dry soil and increases the risk of root rot during winter dormancy. Watering the root zone a few days before an expected freeze is beneficial, as moist soil retains heat better than dry soil, providing a small thermal buffer.
Cold acclimation involves gradual cooling in the fall, which triggers physiological changes that increase frost resistance. Conversely, a sudden, sharp temperature drop after a warm spell can catch the tree in a de-acclimated state, leaving it highly vulnerable to injury. Planting trees in a sheltered location, such as against a south-facing wall, can maximize solar exposure and provide protection from harsh, drying winds that exacerbate cold stress.
Essential Strategies for Winter Protection
Preparing an olive tree for winter begins well before the first frost by managing cultural practices in the fall. Growers should stop applying nitrogen-heavy fertilizers by late summer to halt new, tender growth that is easily damaged by cold. It is also best to avoid major pruning in the late autumn or winter, as this encourages a flush of new, susceptible growth and exposes the fresh cuts to potential frost injury.
Protecting In-Ground Trees
For trees planted in the ground, mulch should be applied around the base to insulate the roots. This layer should be kept a few inches away from the trunk to prevent moisture buildup and disease. Wrapping the trunk with materials like burlap, jute, or horticultural fleece provides a physical barrier against prolonged freezing and wind burn. In anticipation of severe cold events, covering the entire canopy with a frost cloth or fleece can raise the temperature inside the cover by a few degrees.
Protecting Potted Trees
Potted olive trees are especially vulnerable because their roots are exposed to the air temperature in the container. The best strategy is to move them to an unheated, brightly lit shelter where the temperature remains between 5°C and 10°C. If the pot is too large to move, the container must be wrapped with an insulating material like bubble wrap or hessian to protect the root ball from freezing solid. Even when moved indoors, the trees still require minimal watering to prevent the soil from completely drying out.