Japanese barberry thickets create the perfect conditions for ticks carrying Lyme disease, columnist notes
Beautiful but dangerous: Japanese barberry thickets create the perfect conditions for ticks carrying Lyme disease.
Last week, I shared how invasive plants aren’t just weeds — they’re ecological bullies that smother biodiversity and, in some cases, threaten human health.
This week we dig deeper: why do we keep planting them? The answer lies in history, horticultural marketing and our attraction to beauty and convenience.
The Canadian Coalition for Invasive Plant Regulation (CCIPR), co-founded by Cathy Kavassalis and Claudette Sims, warns that Japanese Barberry (Berberis thunbergii)—with its glossy leaves and bright berries—is one of the most startling examples of a “pretty killer.”
Barberries were banned in Canada in the early 1900s because of their role in spreading wheat stem rust. Decades later, “rust-resistant” Japanese Barberry (Berberis thunbergii) and hybrid barberries (Berberis vulgaris × B. thunbergii) were introduced in 2001, marketed as tidy, low-maintenance shrubs perfect for hedges. Gardeners loved it.
But behind its tidy exterior lies a hidden danger. Barberry thickets trap humidity close to the ground, creating perfect conditions for black-legged ticks — the carriers of Lyme disease.
Research in the northeastern U.S. (where conditions are comparable to parts of Ontario) has shown that barberry-infested sites can host up to 10 times more Lyme-infected ticks than barberry-free zones. Birds spread the seeds far and wide, and even broken stems can root to form new plants.
Ontario gardener Laura Black learned this the hard way. Growing up in Puslinch she never suspected the shrubs dotting her property. Years later, she was diagnosed with chronic Lyme disease — a story she later shared with CCIPR.
“I lost my dog, battled illness for years, and nearly gave up gardening,” Laura told the coalition. “Please — never buy this plant. If you have it, get it out.”
Despite risks to biodiversity and public health, Japanese barberry is still sold in Canadian nurseries — even in “sterile” forms that experts warn may revert to seeding.
Safer alternatives include native shrubs such as black chokeberry, hardy currants, and winterberry or inkberry holly — just a few of the many beautiful, bird-friendly options available.
If barberry shows how a shrub can threaten both biodiversity and human health, Japanese Knotweed illustrates a different danger: the sheer physical force of an invasive plant overwhelming landscapes and even infrastructure.
Japanese Knotweed can push through pavement and weaken foundations, costing communities millions in repairs. Photo: Japanese Knotweed LTD
One of the most resilient organisms on Earth, Japanese Knotweed evolved to survive hot lava flows. Its roots can grow more than three metres deep and spread more than nine metres wide. Even after decades underground, rhizomes can suddenly re-sprout with explosive growth.
Introduced to Europe in the mid-1800s, Japanese Knotweed was celebrated for its ornamental appeal and won a gold medal in horticulture. By 1868, it had reached North America.
Photo: Landscapes by Terra
The honeymoon was short. By the 1920s, it was infamous as a weed. In Britain, where it earned the nickname “Hancock’s Curse,” property values plummeted in infested neighbourhoods. By 1999, the International Union for Conservation of Nature listed it among the world’s worst invasive plants.
In Canada today, Japanese Knotweed is recognized as one of the most destructive invasives — a true property owner’s nightmare. It cracks pavement, walls and foundations, overwhelms streambanks and wetlands, and spreads relentlessly if cut, mowed or tarped — every rhizome fragment able to sprout a new plant.
The hard truth?
Most conventional control methods make it worse. To address large populations, science shows the only consistently effective approach is the careful application of glyphosate during late summer, when the plant is drawing nutrients into its roots.
Normally, I stand firmly against glyphosate. But here, biology leaves us little choice. Applied sparingly, at the right time, and with extreme caution, it’s currently the only proven way to weaken large infestations.
That said, small patches can sometimes be managed by hand digging — something Cathy Kavassalis herself has successfully done.
In Ontario, invasive honeysuckles are an increasing problem. Japanese, Amur, Morrow’s, Tatarian and their hybrid Bell’s honeysuckle are serious invaders in our forests and are top priorities to control and avoid planting.
Photo: Leslie J. Mehrhoff, University of Connecticut
In addition, other non-native honeysuckles introduced through nurseries, such as European fly honeysuckle, are becoming growing concerns.
Introduced from Asia and Russia in the 1800s, they were planted for their fragrant flowers and quick growth. By the late 19th century, they had already escaped gardens and spread across natural areas.
What makes them harmful?
They form dense thickets that block forest regeneration, while their roots release allelopathic chemicals that suppress the growth of nearby plants.
Their structure interferes with nesting birds, and while their berries look appealing, they offer poor nutrition compared to native shrubs, weakening the food web.
How to tell them apart: break the stem. Invasive honeysuckles have a hollow pith; native honeysuckles are solid. One feeds birds, the other fools them. Which future are we planting?
Coral honeysuckle: a non-invasive native vine with tubular red blooms are the perfect shape for hummingbird beaks (Photo: Plant Native)
Thanks to Cathy Kavassalis and Claudette Sims of the Canadian Coalition for Invasive Plant Regulation for inspiring this column.
Monika Rekola is a certified landscape designer and horticulturist, passionate about gardening, sustainable living and the great outdoors. Contact her at [email protected].Monika Rekola is a certified landscape designer and horticulturist, passionate about gardening, sustainable living and the great outdoors. Contact her at [email protected].
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