The UK Plant Disease Diagnostic Laboratory (PDDL) has received many broadleaf and needled evergreen samples with discolored foliage recently. Despite relatively mild weather during much of the winter thus far, extremely low temperatures, low humidity, and strong winds during the third week of December resulted in a range of winter injury symptoms in landscape plants. Furthermore, other environmental and cultural stress factors preceded the severe weather in many cases and have contributed to the extent of damage visible now.
Symptoms
Since the beginning of January, most of the samples with winter injury that have arrived in the PDDL have shown symptoms of winter drying. On broadleaf evergreens (boxwood, cherrylaurel, holly, magnolia, rhododendron, etc.) symptoms typically include marginal leaf scorch, irregular spotting, complete browning of the leaves, and occasionally extensive leaf drop. Conifer (arborvitae, Leyland cypress, Cryptomeria, juniper, etc.) symptoms include pale, bronze or brown needles or needle tips, particularly on the exterior foliage and branch tips. Symptoms are often more noticeable on the wind-exposed side of affected plants.
Causes of Winter Injury
Unlike their deciduous counterparts, “evergreen” plants retain foliage year-round. Even during winter months when active growth is not occurring, water is still lost through the leaves and needles of these plant species via transpiration, although more slowly than during times of active growth. Environmental and cultural factors that affect overall moisture availability in plants increase the likelihood of winter drying symptoms, including:
- Areas where soil is frozen, limiting, or preventing water uptake by roots.
- Low moisture retained in leaves/needles due to drought (i.e., late summer and fall 2022).
- Sunny winter days with wind and low humidity which increase transpiration rates.
- Inadequate root systems from recent transplanting (within 3-4 years), a restricted root zone (due to nearby sidewalk, driveway, or building) or mechanical injury to roots.
- Plants or sections of plants in a protected area (i.e., along a warm brick wall) that have not completely hardened off.
Management of Winter Injury Symptoms
Resisting the urge to promptly remove the damaged plant material may be difficult, but for the next several weeks, patience is key. Often the foliage is damaged, but the stems and buds are still viable and will produce new healthy growth in spring. A “wait and see” approach is often best when dealing with winter-injured plants. When new growth begins to emerge in spring, it will be clear which symptoms are “cosmetic” and which symptoms indicate significant plant damage.
To help reduce the risk of winter drying in the future, good general plant care practices, such as watering during periods of drought and applying mulch, are beneficial. Supply adequate irrigation to broadleaf evergreen and conifer plants, especially late in the growing season so that plants have sufficient moisture during the winter months. Fertilizer is not typically recommended as this can encourage succulent top growth that is easily damaged in winter.If fertilizer must be applied, a fall application is best, since spring fertilization may promote more new growth than roots can support during summer droughts. Locate sensitive plants in sheltered locations. If severe cold and wind is predicted, protect plants that are prone to winter drying and located in exposed sites with temporary fencing to block the wind or a light covering of burlap or fabric (e.g., old bed sheets). Wetting the fabric before windy weather will allow the covering to freeze, increasing wind protection.
Categories: Kentucky, Rural Lifestyle