- Freeze. Winter injury to landscape plants often happens when sudden, unseasonably cold weather occurs in the fall. Many plants are still growing in the fall and have not hardened off for the coming winter. The hardening process takes place naturally as gradually colder weather invades during the fall. Succulence, maintained by late-season fertilizer applications and high soil moisture, predisposes shrubs and trees to cold injury. The primary factor for winter damage is cold that is severe, sudden, and possibly of longer than normal duration.
Winter damage is often not noticed until the following spring or summer. The affects may also be seen 2 or 3 years later. Affected plants may bloom and leaf out but when the hot summer days arrive, they quickly wilt and die. This is usually due to damage in the cambium layer on major limbs or the trunk. The healthy portion of the plant is able to translocate enough water and nutrients in the spring when requirements are light but when the demand for water increases, it fails to keep up. Peaches, apricots, apples, plums and other fruit trees respond in this manner.
In diagnosing trees that die suddenly in late spring and early summer, cut the bark at the crown line to see if the root system is still alive. If the root system is still living or shows greener wood than the above ground portion, then winter damage should be suspected. Freeze damage results in a darkening of the wood just under the bark of the trunk and main branches. By carving on the tree and locating these dark areas, one can often find differences in color from one side of the trunk or branch to the other. The darkest wood is often found on the south exposure. With some woody ornamentals, the bark cracks or splits. Azaleas commonly show this type symptom.
- Water Stress. Evergreens, such as Arizona cypress, cedar, juniper, ligustrum, may suffer winter injury when inadequate moisture is available or when soil temperatures are considerable below air temperatures. Although these plants do not transpire as much water in the winter as they do in the summer, they still remain active and thus may need additional water in the winter months. When soils are cold and warm dry winds blow, roots cannot take up enough water to balance the loss through the leaves. Yellowing and browning of the foliage and even death may occur. These conditions are most likely to exist in late winter or early spring. Evergreens suffer less damage if protected by wind breaks.
Some plant species have a lower tolerance to cold temperature than others. Before purchasing landscape plants, obtain information regarding winter hardiness. In many areas of the state, azalea, boxwood, ligustrum, oleander, pyracantha, pittosporum, fig, crapemyrtle and hibiscus may be damaged during cold winters. Injury may consist of only a few tips or branches dying back. When this is the case, judicious pruning will improve the looks and balance the top growth but it should be done after the exact dead areas are determined. Lawn grasses may also suffer winter injury. As with other plants, healthy turf will be damaged less than turf stressed or weakened by insects, diseases, drought stress or lack of fertilizer.
- Winter injury can be lessened or entirely prevented. The following measures should be considered and practiced when applicable:
- Select winter hardy plants.
- Assist the hardening process by withholding fertilizer and water in the late summer and fall.
- In dry areas, supply adequate water after plants go dormant.
- Avoid poorly drained soils.
- Pick planting sites that protect susceptible plants from high winds.
- Keep evergreens watered.
- Keep plants healthy with proper watering, fertilizing and pruning.