Boxwoods (Buxus spp.) in Ohio continue to be “in the news” this season as a result of the carry-over of several challenges experienced last season. Those challenges have caused some to question the continued reliance on a long-reliable plant. However, is rejecting boxwoods based on perception or reality?
While it’s a good idea to make plant selection decisions based on avoiding pests and diseases, we don’t apply this decision-making logic across the board. We could provide a long list of diseases and insect pests that target oaks (Quercus spp.) with some affecting aesthetics while others are tree-killers. The same is true for maples (Acer spp.) and frankly, many other trees and shrubs. Yet, we continue to plant oaks and maples.
We’re certainly not suggesting this is wrong; just pointing out that we accept that plants have problems. Our plant selection decision-making is based on accepting a certain amount of risk for the overall rewards. After all, the only boxwood that is free from pests, diseases, and physiological challenges is the imagined cultivar Buxus plasticus ‘PollyEthyl’.
The Rewards of Boxwoods
Boxwoods were some of the first plants used in U.S. landscapes with planting records dating back to the late 1600s. They are currently some of the most common plants found in Ohio landscapes.
Boxwoods play a significant role in landscape designs both in form and function. There are around 70 boxwood species and over 150 varieties, cultivars, and hybrids although boxwood taxonomy remains an ongoing point of contention. The genetic diversity is made evident by the wide-ranging natural growth habits. Of course, boxwoods are exceptionally responsive to shaping and could be considered the “poster child” for sculpted topiary.
Put simply, what other woody perennial plant can realistically replace boxwoods? They are evergreen which eliminates semi-evergreen privet (Ligustrum spp.) from consideration as a replacement hedge. Of course, non-native privets have also displayed an invasive nature.
Boxwoods are resistant to deer which eliminates arborvitae (Thuja spp.), Taxus (spp.), and many junipers (Juniperus spp.) as well as other shrubs with similar growth characteristics. Boxwoods flourish under a range of growing conditions and are not as finicky as hollies (Ilex spp.), although there are limits. For example, we commonly find boxwoods thriving in southwest Ohio where hollies have faded away.
Finally, the value of boxwood bloom in the spring for providing an early-season bounty of pollen and nectar is often overlooked because the small, greenish-yellow flowers somewhat blend with their background. However, if you stand near a heavily blooming boxwood during a warm spring day, plants buzz (literally!) with pollinator activity.
Equally important, the bloom develops like a slow roll with some blooming earlier than others depending on the type of boxwood. This can provide a continuous food source for pollinators. For example, Spring Grove Cemetery and Arboretum in Cincinnati, OH, has an extensive collection of boxwoods as well as consistent populations of soil “mining bees.” I commonly observe large numbers of these bees taking advantage of the early-season gift of pollen and nectar provided by boxwoods.
The Risks to Boxwoods
Last year, the non-native Box Tree Moth (BTM) (Cydalima perspectalis, family Crambidae) was discovered in southwest Ohio. We reported the discoveries in two BYGL Alerts: “Box Tree (Boxwood) Moth Confirmed in Southwest Ohio” (June 26, 2023); and “Box Tree (Boxwood) Moth: New Detection, What to Look For, and Management” (September 27, 2023). Stay tuned for an upcoming Alert on box tree moth.
The discovery of BTM in Ohio could not have come at a worse time. Boxwoods and a range of other woody ornamental and herbaceous perennials in central and southern Ohio as well as parts of Kentucky and Indiana suffered severe winter injury owing to deep-diving temperatures that occurred during the week of Christmas, 2022. The unusual extent of the damage to boxwoods was considered a once-in-a-lifetime event; it had not been seen in decades.
It was common for damaged boxwood stems to become colonized by the “opportunistic” fungi Pseudonectria buxi and P. foliicola which are behind the disease called Volutella Blight/Canker. P. buxi was originally described in 1815 and has been given 25 different scientific names in the intervening years including Volutella buxi, thus the common name for the disease.
The “blight” in the name of the disease refers to the symptoms that appear on the leaves. An important distinction between symptoms of Volutella and those produced by BTM is that with the fungal disease, whole leaves remain attached to the stems as they turn brown and intact leaves are shed. BTM caterpillars consume the leaves to produce “see-through” boxwoods.
The “canker” in the disease name refers to the most destructive form of the disease. The cankers can develop anywhere on damaged stems, but they typically develop towards the ends of the stems. However, the stem infections, and thus the cankers, continue to progress down the stems unless the infected stem tissue is removed.
Unfortunately, the perception may be that the boxwoods are dying from Boxwood Blight produced by the aggressive pathogenic fungus Calonectria pseudonaviculata (previously called Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola). However, while boxwood blight has been confirmed in Ohio, the occurrence of the disease is rare compared to Volutella blight/canker.
Last season, only 3% of the boxwood samples sent to the OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWE-PPDC) were positive for the fungus behind boxwood blight. By contrast, over 80% of the boxwood samples sent to the CWE-PPDC were positive for the fungi behind Volutella blight/canker.
The boxwood disease data from the CWE-PPDC provides an important take-home message. Never “field diagnose” and act upon symptoms that may be caused by a range of causal agents. Send samples to an accredited clinic for confirmation (see below). Otherwise, your professional reputation may suffer.
The good news is that the Volutella blight/canker can be managed by selective pruning as long as the cuts are made below the canker. In general, boxwoods are very “forgiving” when it comes to pruning the stems. The exposure to sunlight allows new stems and foliage to rapidly develop to fill in the openings.
More good news is that the genetic diversity of boxwoods that is made evident by the wide-ranging natural growth habits also provides a range in resistance and susceptibility to both boxwood blight as well as Volutella blight/canker. Numerous online references provide guidance in using plant selection to avoid these diseases.
The bad news is that there are many examples of boxwoods, often in commercial landscapes, that developed dieback last season; however, nothing was done to halt the progression of the dieback. The affected boxwoods are continuing to suffer stem dieback this season.
Unfortunately, the bad-looking boxwoods stoke the perception that boxwoods are not suitable for Ohio landscapes. In reality, it’s common for woody ornamentals, including oaks and maples, to occasionally need some selective pruning to remove dead branches. We don’t automatically apply basal pruning.
Confirm Before Reacting
The CWE-PPDC is a member of the National Plant Diagnostic Network (NPDN). Members must earn accreditation that ensures they meet certain standards. We have two NPDN clinics in Ohio that diagnose pest and disease problems with woody ornamentals as well as other plants important to our Green Industry: the CWE-PPDC on the OSU College of Food, Agricultural, and Environmental Sciences (CFAES) campus in Wooster, OH, and the Ohio Department of Agriculture (ODA), Plant Health Diagnostic Laboratory (PHDL) located in the ODA headquarters campus in Reynoldsburg, OH.
You can learn more about the NPDN and the important work that they do for all of us by visiting their website [ https://www.npdn.org/ ]. If you are located outside of Ohio, you can find an NPDN clinic in your state by using the NPDN state-based NPDN Lab Directory [ https://www.npdn.org/lab_directory?region=NCPDN ].