What’s Wrong With Your Boxwood?

Boxwoods are popular evergreen landscape staples. If you don’t have boxwoods in your yard, I am sure your neighbours do.

A few of my clients have reached out regarding their concerns about the sudden decline of their boxwoods, including my dad.

There are a few issues that boxwoods are susceptible to. I thought I would highlight a few here to better explain.

Boxwood Blight

Boxwood blight is caused by a non-native fungus. It is recognizable by the circular black spots that form on the leaves or black lesions on the stems. Once infected, the plants defoliate very quickly. Moisture creates ideal conditions for this fungus, and I think this summer of higher-than-normal rain has resulted in more damage to our boxwoods.

Some tips to avoid getting it:

• Plant boxwood in full sun and prune neighbouring plants that may cause moisture accumulation.
• Do not shear plants during the growing season. Instead, prune, not shear, plants for shape and size in the dormant season and disinfect your pruners with rubbing alcohol.
• Boxwood blight spores spread easily with irrigation or overhead watering so be mindful and watchful if that is your situation.
• Mulch existing plantings.
• If boxwood blight develops in one area, remove severely damaged plants and be careful as the pathogen can transfer from your clothing as well as your gardening tools.
• Fungicide treatments are available to protect new infections on remaining plants. But once blight is in your landscape, it can remain in the soil for a number of years (remember the issue with Annual Impatients), so treatment would need to be applied on an ongoing basis.

Source: www.bartlett.com/blog/how-to-prevent-boxwood-blight/

Boxwood Psyllids

Boxwood psyllids are tiny white insects that feed on new growth of boxwood. You may notice the leaves curl by early summer.

This issue is not as severe and mainly aesthetic. Prune the tips and affected branches with clean pruners and dispose of the branches in the garbage. Fertilizing is then recommended to keep the plant as healthy as possible.

The adults lay eggs that overwinter, and the young insects start appearing and eating in early April-May. The best time to treat the boxwoods is when you see the insect begin feeding on your plants. The insects are tiny and bright green. A registered insecticide can be applied to reduce the population.

Source: connon.ca/wp-content/uploads/2023/05/RW-Boxwood-Psyllids-vMay23.pdf

Box Tree Moth

In 2018, the Box Tree Moth was identified as present in Ontario. Europe has been battling this insect as well. This insect is very destructive and poses a huge threat to our region’s boxwood shrubs. They have a huge appetite and reproduce rapidly.

This is the kind of damage that you should be watching for.

It is a small-sized moth with a wingspan of 4 cm or 1.5 inches. The adults have white wings with a brown border, but it is the larvae that are the most damaging. These caterpillars are bright green with black heads and can grow more than 4 cm in length. The caterpillars are voracious eaters and can consume the foliage on a boxwood very quickly.

Taking us all back to grade school science: the life cycle of the moth consists of four stages: egg, larva, pupa and adult. Each adult female moth lays up to 100 eggs on the underside of boxwood leaves during the summer. Hatching occurs after a week, which means the caterpillars can begin eating immediately.

As they grow, they will also create protective webs that can cover the entire boxwood shrub. The caterpillars will pupate within the webs, emerging as adult moths in about two weeks. This all occurs during the warmer months (May to September) and can produce multiple generations per year.

Now that you know more about them, it is crucial to pay closer attention to your boxwood even if they still appear healthy. Proper pruning and fertilizing are helpful to keep them healthy. Keep an eye on your neighbour’s boxwood, too; that can be a bit of an early warning. You can try manually removing caterpillars and egg clusters for a localized infestation. An insecticide will be key to controlling a larger infestation. Use an insecticide that is labelled for the control of caterpillars and follow instructions.

Safer’s BTK is a biological insecticide that controls caterpillars safely. Frequent treatments will be required. Visit your local garden center for the product and more information.

Sources:
connon.ca/the-box-tree-moth-in-southern-ontario-a-growing-threat-to-ornamental-landscapes/
horttrades.com/box-tree-moth-a-new-plant-pest-in-etobicoke