Several boxwood samples were submitted to the Plant Clinic last summer. That was several more than we usually see. In addition, I had some calls this winter about recurring boxwood problems. It is likely that many others are experiencing problems but have not called. Samples that were sent to us last summer had various problems, with disease being secondary to site or environmental stress. Here is a summary of boxwood problems to watch for in your landscape.
Initially, it would help to look in your records to find the species and cultivar you planted so you can determine the expected hardiness. The USDA has developed a map of plant hardiness zones for the United States. You can usually find this map in plant material texts. Southern Illinois is zone 6, but central and northern Illinois are definitely zone 5. Most boxwood species are hardy in zones 5 to 6, but some cultivar selections have been developed to tolerate colder temperatures found in zone 4. Sensitive boxwoods suffer winter injury if planted out of their natural zones.
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 such injury now and prune out any dead wood.
Volutella 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 become 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 clinch the diagnosis. Prune out dead wood and thin the plant to allow better air circulation, which will discourage fungal growth.
If similar problems occur on your boxwood in wet locations in midsummer, it is likely that Phytophthora root rot is to blame. Dig up the plant and examine the root system to find many brown, rotted roots. You cannot eradicate the fungus from the soil, but you can help nearby plants by improving the drainage pattern. Low areas, poorly drained soils, or compacted areas should be avoided as planting sites for boxwood.
Boxwoods are surface rooters, so roots are fairly shallow. Cultivating around boxwoods or over-application of fertilizer may injure or kill roots, also resulting in top dieback and decline of plants in the summer. It is suggested that boxwoods be mulched but not too deeply. Two problems could result from thick mulch. Roots can 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.
Southern states have major problems with root nematodes on boxwood. This is where our cold winters are actually an advantage. We rarely see nematode problems on boxwood in Illinois. Still, soil assays for nematodes can be done at the Plant Clinic if this is a concern.