Thirty years ago, when I was a student in Michael Dirr's class on woody ornamental trees and shrubs at the U of I, I learned that boxwoods did not grow well in central and northern Illinois. At that time, a few hardy species were available but not yet well established in the landscape. Although there are now more cultivars that can offer winter hardiness, we still see plenty of problems on boxwoods. The drought of 2006 will most likely intensify all of these problems. |
Most boxwood species are hardy in zones 5 to 6, but some of the cultivar selections have been developed to tolerate colder temperatures. Sensitive boxwoods suffer winter injury if planted out of their natural zones. Even those in the correct zone sometimes show winter injury, especially on sites exposed to wind, salt damage, or other stress factors. Winter injury results in 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 such injury now and prune out any dead wood.
Volutella (syn. Psuedonectria) blight is a disease we frequently see on injured boxwoods in Illinois. I have already received a call about this problem this season. Volutella can be confused with winter injury symptoms. The two usually go hand in hand because the fungus often infects wounds resulting from winter injury. Volutella blight (Volutella buxi) is a fungal disease of English and American boxwoods. The fungus infects leaves and/or stem tips. The leaves turn 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, thereby discouraging fungal growth. The pathogen has also been reported to cause a root and crown rot. The fungal pathogen can survive in the soil, but there is otherwise not much known about disease infection and spread. To manage this disease, remove infected stem tissue, take steps to avoid winter injury, and otherwise maintain healthy plants through sound horticultural practices.
There is also a root and crown rot disease of boxwoods called Phytophthora root and crown rot. The foliage develops a dull green appearance, as you might expect when water is limiting. The foliage may become twisted and eventually turns brown. The crown tissue rots, becoming gray or brown, and roots turn dark brown with sloughing cortex tissues. Infection occurs in warm to hot weather, and excess water is required for infection.
Other problems on boxwoods may result due to their fairly shallow roots. Cultivating around boxwoods or overapplication of fertilizer may injure or kill roots, also resulting in top dieback and decline of plants. It is suggested that boxwoods be mulched but not too deeply. Two problems could result from thick mulch. Roots will grow into the mulch and will be susceptible to drought stress when the mulch dries. In addition, 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.