Boxwood shrubs are perhaps one of the most planted evergreen shrubs in landscapes around the Midwest. Although they are typically fairly hardy in our area, many suffer from winter injury. In addition, there are a number of other ailments for this shrub that gardeners should be aware of, including a new and very serious threat called boxwood blight.
From the standpoint of functionality in the landscape, these little, green and slow-growing shrubs are matched by few other evergreens. They are synonymous with hedges but have a variety of other landscape uses where a backdrop of evergreen foliage is needed.
Although I most commonly think of littleleaf boxwood (Buxus microphylla) as the primary species utilized in Zone 5 and 6, its cousin, the common boxwood (Buxus sempervirens), is also hearty in Illinois. Cultivars of both species abound, creating a boxwood for almost all occasions and extending their range into warmer regions as far as Zone 9 for some.
Both species are not native, with home ranges spanning several continents and littleleaf boxwood's center of distribution being a bit more tropical. I view both species of boxwood as purely ornamental plants, likely having little value to our native fauna. Foliage actually contains compounds toxic to most mammals, making these plants deer- and rabbit-proof. However, I can certainly understand why many folks may want a deer-resistant hedge somewhere in their landscape.
Over the past few weeks, I have observed quite a bit of winter damage on boxwoods around central Illinois. In most cases, it is not serious and can be pruned out once things green up for the year. Yet, I always caution people to hold off pruning out the dead too soon in spring. Although the foliage may be entirely dead, the limb itself may be alive. Once the shrub comes out of dormancy and begins to grow, you may see a flush of new growth on what previously looked like a dead stem. However, by this point in the season, a stem that is still alive should be showing signs of life, such as developing green leaves or at least swollen buds.
This type of injury appears as dead or dying foliage, typically tan to light brown in color. It is often limited to the outer canopy of the shrub or one side that is more exposed. One great indicator of winter injury is the pattern. All dead foliage on the plant in spring is a result of desiccating winter weather and should not spread after the growing season begins. Conversely, injury or infection from pathogens will oftentimes spread as spring progresses.
In the current issue of the University of Illinois Extension Home, Yard and Garden Pest Newsletter, there is a great article on "Boxwood Blight Look-alikes." My extension colleges who authored this article do a great job reviewing a number of pests and disease that occur on boxwood and appear similar to boxwood blight.
Earlier this week, I had a conversation with Diane Plewa of the UI Plant Clinic, who co-authored the article. We both agreed that one takeaway from the article was the fact that there are quite a few ailments on boxwood. Many times, they may not be that serious, often corrected with pruning or other cultural practices, but odds are you will deal with some issue or another over the life of your boxwood plant.
Boxwood blight is a newer pathogen on this continent, first appearing here in 2011 after already causing several decades of damage in Europe. In Illinois, it was first detected in 2016 and has only been identified 11 times since. Plewa's article points out a key difference between boxwood blight and other common insects and diseases: It causes extensive defoliation. The disease is caused by a fungus, beginning as leaf spots in the lower branches that move upward in the canopy. Infected leaves drop from the plant, unlike many other problems that result in dead leaves that remain on the stem.
Although scouting and early detection of this pathogen is very important, the infections to date in Illinois have primarily been associated with production areas, such as nurseries. Therefore, thoroughly inspect any new plant material for symptoms before planting. It is predominantly spread by human activity with less potential for spread within the landscape by wind or insects.
Nonetheless, if you believe your existing plants have boxwood blight, please contact the UI Plant Clinic (web.extension.illinois.edu/plantclinic/ or 217-333-0519) for information on how to submit a sample.
The current issue of the Home, Yard, Garden and Pest Newsletter is always available at hyg.ipm.illinois.edu/. The newsletter website features an archive of all past issues and an option to sign up for email notifications of each new issue.
Ryan Pankau is a horticulture educator with the UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties.