Boxwood plants are often susceptible to winter injury in central and northern Illinois; and this injury has been evident already in 2003. Most boxwood species are hardy in zones 5 to 6, but some cultivar selections have been developed to tolerate colder temperatures fof zone 4. Sensitive boxwoods suffer winter injury if planted out of their natural zones. Even those in the correct zone sometimes show winter injury.
Winter injury causes bronze to reddish brown foliage, especially on parts of the plant exposed to winter winds or winter sun. In addition, temperature extremes cause splits in the bark, and entire branches may die to the crown of the plant. Look for injury now and prune out any dead wood.
Volutella (syn. Psuedonectria) blight can be confused with winter-injury symptoms. In fact, the fungus often infects wounds from winter injury. Volutella blight is a fungal disease that infects leaves at the tips of stems. The leaves are reddish to bronze, and stem tips may die. Volutella moves down the stem, whereas winter injury happens seemingly at once and does not progress down the stem. If affected foliage is placed in a plastic bag with damp paper toweling (moisture chamber) for 24 hours, the salmon pink fruiting bodies of Volutella will clinch the diagnosis. Prune out dead wood and thin the plant to allow better air circulation, which discourages fungal growth.
Boxwoods have fairly shallow roots. Cultivating around them or over applying fertilizer may injure or kill roots, also resulting in top dieback and plant decline. It is suggested that boxwoods be mulched, but not too deeply. Two problems could result from thick mulch. Roots grow into the mulch and become susceptible to drought stress when the mulch dries; voles are known to live in mulch and feed on the trunk of this plant. For these reasons, keep mulch shallow and away from the trunk.