Gail Ruhl, Senior Plant Disease Diagnostician, Purdue University
Winter injury comes in many forms. Temperature fluctuation and desiccation are responsible for much of the foliar discoloration and dieback observed this spring on boxwoods and other broad-leaved evergreens including holly, mahonia, and rhododendron species.
Winter desiccation (drying out or winterburn) of foliage commonly occurs on broad-leaved evergreens growing in both wind-swept and sheltered locations. Water evaporates from the leaves on windy or on warm sunny days and cannot be replaced when the water in the soil is frozen and unavailable to the plant roots.
Damage to broad-leaved evergreens and other woody ornamentals can also occur from multiple types of winter-related injury. Sudden temperature fluctuations during winter months, excessive or late season fertilization causing delayed hardening off/delayed tissue dormancy, lack of snow cover/cold temperature injury, and late spring frosts contribute to winter injury.
Plants vary in their susceptibility to winter injury, and factors such as plant age, species, exposure, soil type, location, and individual genetic variation will all affect the degree of damage a particular plant will experience. Winter injury can appear as stem or trunk cracks, foliar discoloration (purpling, browning, yellowing , bleaching), and/or twig and branch dieback. Often the effects of winter injury do not show up immediately, thus complicating accurate diagnosis of a specific causal factor. Secondary or opportunistic diseases on Boxwood such as Macrophoma leaf spot, Volutella Blight and Botryosphaeria canker may colonize injured plant tissue. While there is no cure for the physiological disorders caused by winter injury and desiccation, cultural practices that conserve soil moisture, prevent root or stem damage, and promote hardening off prior to winter may help: