Boxwood troubles may be caused by something you can’t see

GUELPH – Boxwood troubles may be caused by something you can’t see.

For many home gardeners, boxwood shrubs are the quiet workhorses of their garden. 

They frame walkways, anchor foundation plantings, and provide year-round structure with very little fuss. 

So when a boxwood begins to show signs of stress, most gardeners assume the cause is something familiar: winter burn, drought stress or perhaps salt damage. 

But in recent years, a different culprit has been showing up across southern Ontario, one that often goes unnoticed until the shrub is already in serious decline.

Boxwood blight, a fungal disease caused by Calonectria pseudonaviculata, has become increasingly common in our region. 

Although it was once considered a problem mainly in the United States and parts of Europe, cases are rising in Ontario in both residential and commercial landscapes. 

Research from universities shows the disease thrives in humid conditions, spreads easily on tools and clothing, and can move quickly through densely planted hedges.

Unlike the more familiar winter burn, boxwood blight has a distinct pattern of symptoms.

Look for the signs

Gardeners may first notice tan leaf spots with darker edges, followed by rapid leaf drop that leaves branches bare almost overnight. 

Stems often develop thin black streaks, and entire sections of the shrub can collapse even when soil moisture and growing conditions seem normal. 

Because these signs resemble other issues, many home gardeners don’t realize blight is the cause until the damage is extensive. 

Studies and plant trials of blight-resistant varieties have helped clarify how the disease behaves. 

Researchers have found that once the fungus is present, it can persist in fallen leaves and soil for years. 

This makes management challenging, especially in small suburban gardens where air circulation is limited and boxwoods are often planted close together. 

Boxwood is becoming a less sustainable choice in our climate, where warm, humid summers favour fungal spread.

A home gardener’s best defence against boxwood blight is awareness. Keep an eye on your shrubs throughout the gardening season, watching for the warning signs mentioned.

Remove infected material early, improve airflow and disinfect your tools. These can all help slow the disease. 

If you’re bringing home a new boxwood from a garden centre, take a moment to inspect it closely for those same warning signs before planting it in your garden, or look for blight resistant boxwood varieties.

Consider alternatives

Alternatively, you could consider inkberry holly (Ilex glabra), which is one of the most recommended substitutes for boxwood. 

It has a tidy, rounded form, glossy evergreen leaves, and excellent cold tolerance for the Guelph-Wellington area.