My Boxwood is Sick! What’s Happening?

By Julie Boudreau

Boxwood is a small shrub that is increasingly coveted by gardeners. It gives to the landscape an air of nobility. However, year after year, every spring, we lose a few specimens. Some because of the harsh winter, but others give in to various insects and diseases. So let’s find out what could have caused the boxwoods in the garden to die (or almost die)!

Boxwood (Buxus spp.) is a small evergreen shrub very popular in Europe. It has long been an unattainable dream for gardeners in northern North America. But, in recent decades, varieties that are very hardy in USDA zone 4 (zone 5) and even USDA zone 3 (zone 4) have appeared. Northern gardeners can now grow small, low, green hedges all year round. French-inspired gardens, modern and contemporary spaces as well as all gardens in the Mid-Century style movement can benefit of boxwoods.

It would be wrong to say that boxwood is a very easy plant to grow. Because of its evergreen foliage, it’s a plant that will be sensitive to foliar desiccation, especially during very cold, windy winters with little snow cover. It is not uncommon to see one side of a poorly protected boxwood hedge turn orange.

Boxwood grows equally well in full sun and light shade. However, it has a clear preference for rich, fertile and well-drained soils. Less perfect growing conditions can sometimes be the source of a series of diseases, causing damage to boxwoods.

Decline: The Catch-All of Boxwood Diseases

The first source of trouble for boxwood is dieback. In truth, we know very little about what we call boxwood decline, because it seems to be a panoply of diseases that come into play. In addition, most of these diseases take hold due to poor growing or climatic conditions.

For example, prolonged drought conditions often precede the arrival of a parasitic fungus (Paecilomyces buxi). At the other extreme, heavy rains encourage the development of root rot produced by Phytophthora (Phytophthora cinnamomi and P. parasitica). Add to this a hint of leaf spots (Pseudonectria buxi and Macrophoma spp.) and a handful of nematodes (Meloidogyne spp., Mesocriconema spp. or Pratylenchus spp.) and we obtain boxwood decline.

Boxwood decline is first characterized by a general slowdown in growth. We will also note complete death of some branches, all distributed randomly. Occasionally, there will be premature dropping of old leaves or yellowing of foliage. Of course, the signs vary, depending on the cocktail of parasites involved. But, in general, the damage appears very quickly.

There is little to be done about this phenomenon. Certainly, adjusting soil fertility and more regular watering can help. However, if the infestation is important, it’s not recommended to replant boxwood in the same location. We could then fall back on a small hedge of alpine currant (Ribes alpinum).

The Boxwood Miner: Life Between Two Layers of Leaves

If the damage concerns the leaves more specifically, it’s probably boxwood leaf miner (Monarthropalpus flavus). Its presence is likely to increase due to global warming.

The small larva settles in the leaf to chew a comfortable pouch. From an external perspective, the damage appears as the yellowing of part of the leaf. Blistered leaves are also a sign of the presence of this insect. A yellowed leaf here or there is a clear indicator that you are dealing with a serious infestation. The ecological approach offers few solutions for getting rid of leaf miners. The only solution is to quickly throw away all the attacked leaves, in order to prevent these larvae from becoming pupae, then adults who will lay new eggs and so on.

Boxwood Psyllids: The False Mealybug

Initially, you might think it was an infestation of mealybugs. Large mealybugs. With wings! Boxwood psyllids are actually cousins of aphids and mealybugs (Order Hemiptera). They suck sap from young leaves. This causes the leaves to deform and bulge. Young leaves often curl up on top of each other. This forms a kind of rosette or cabbage cluster, easily detected at a glance. When we discover the insect, under the leaves, it is camouflaged under a cottony mass, hence its confusion with mealybugs.

This is not the worst problem that can arise. All the same, a heavy infestation can harm the development of terminal shoots. Heavily affected shoot pruning will contain the infestation and reduce the number of eggs laid in fall. The American Boxwood Society recommends an application of horticultural oil or insecticidal soap early in the spring. Personally, I have always hesitated to apply this type of product to evergreen plants. This would be worth testing.

New for 2023: The Box Tree Moth

Until recently, this harmful insect was only present in Ontario, around Toronto. But, since June 2023, it has been found in several nurseries in Quebec and the Maritimes, and even on private properties.

The caterpillar of the box-tree moth (Cydalima perspectalis) is a great devourer of leaves. In the case of heavy infestations, a small shrub can become completely bare within a few weeks. The webs woven by those caterpillars can also affect the esthetic of plants. These webs look a bit like the nests that tent caterpillars weave… in a less impressive version.

In spring, monitor the presence of eggs, in groups of 10 to 20, under the leaves. Three days after laying eggs, the young caterpillars are born and begin to feed. This will last approximately two weeks. Then, the caterpillars will enter their metamorphosis phase, to become beautiful butterflies with white and brown wings. This life cycle is so rapid that we can expect to see two generations per year (late June and late August, according to current data).

This butterfly is native to Japan, Korea and China. It entered Europe around 2007 and its proliferation was encouraged by the presence of two native species of European boxwood (B. sempervirens and B. balearica). The damage is so severe that the United States has banned the importation of boxwood from Canada.

Moreover, if you detect this insect outside of Ontario, the Canadian Food Inspection Agency asks you to make a report .

Of course, several other diseases and insects can be of interest to boxwoods, but the above represent most of what we are likely to discover on our less beautiful than natural boxwoods. We can predict that the damage caused to boxwoods in the coming decades will increase. Perhaps there is reason to think about alternatives now.