Boxwood shrubs are perhaps one of the most planted evergreen shrubs in landscapes around the Midwest. Although they are typically 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 to keep this ubiquitous landscape plant happy and healthy.
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, creating a backdrop of fine, textured, glossy evergreen foliage wherever they reside.
Although I most commonly think of littleleaf boxwood (Buxus microphylla) as the primary species utilized in central Illinois (Zones 5 and 6), its cousin, the common boxwood (Buxus sempervirens), is also hearty across the Prairie State. Cultivars of both species abound, creating a boxwood for almost all occasions.
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 or rabbit “proof.” I can certainly understand why many folks may want a deer-resistant hedge somewhere in their landscape, and these hardworking shrubs can meet the demands.
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 the 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, with fully emerged new leaves.
Winter injury appears as dead or dying foliage, typically tan to light brown in color in late winter or spring. It is often limited to the outer canopy of the shrub or one side that is more exposed and does not spread as spring progresses. Conversely, injury or infection from pathogens or pests will oftentimes spread as the spring season unfolds.
This spring, I have also seen a lot of boxwood psyllid (Psylla buxi) damage, which is a very common pest of most all boxwood species. This tiny insect injures shrubs as it feeds on buds and young leaves in early spring. The damage from its piercing and sucking mouth parts leaves a telltale sign in the form of cupped terminal leaves as new leaves emerge. Upon peeling open some of these cupped leaves, you can find the most obvious sign of this pest in the form of waxy, threadlike secretions, similar to a tiny spiderweb, left behind by feeding nymphs.
Minor damage from boxwood psyllids is considered aesthetic and rarely impacts plant health. One simple control method is to prune out (and destroy) affected branch tips before feeding nymphs mature in late May to early June. By this point of spring, nymphs that have not matured are large, green and pretty easy to find if you look for their trail of cottony, threadlike secretions.
From what I have observed this spring, we are likely to have high numbers of boxwood psyllids across central Illinois, so it’s worth inspecting your shrubs and considering control if infestation levels are high or your shrub seems stressed. Beyond simple pruning, there are some lower-impact pesticides that can be applied in early spring next year as new growth emerges, such as neem oil or insecticidal soap. Timing is important to ensure treatment coincides with early emerging nymphs in late April or early May. Later in the season, the cupping leaves serve as good protection.
Some insecticides are also labeled for application and control later in the year (June or later) but function systemically, which may have a larger negative effect on beneficial insects. Options include foliar spray of acephate insecticides or soil drench application of either acephate or imidicloprid insecticides. I only recommend systemic control measures for severe infestations on already stressed shrubs. Most plants that are doing well otherwise will bounce back from this as summer progresses.
Ryan Pankau is horticulture extension educator with University of Illinois Extension serving Champaign, Ford, Iroquois and Vermilion counties.