Q: I unwisely planted an acanthus seedling in the shady but damp back corner of a large flower bed. Ten years later, it seems set on world domination! I’ve tried to dig out roots to force it back into the original planting space but it just returns with a vengeance. What are my options if I want to remove it completely? LMcD, Dublin
A: A large, fast-growing, semi-evergreen foliage plant, acanthus is famed for its giant, striking leaves, stylised versions of which can be found as carvings decorating the stone temples of ancient Greece and Rome, as well as in the wallpaper designs of the artist William Morris. Commonly known as “bear’s breeches”, the three species commonly found growing in Irish gardens are Acanthus mollis, which has the most rounded leaves, and Acanthus spinosus and Acanthus hungaricus (also known as Acanthus balcanicus), both of which have more deeply divided leaves. All produce tall spiny spikes of decorative, hooded, pollinator-friendly flowers in summer, in muted shades of pink and white.
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This perennial plant’s longevity as a source of inspiration in the world of design is a fitting metaphor for its longevity in the garden. Ultra-hardy, tolerant of a wide range of soils and growing conditions, the only thing acanthus doesn’t like is a very wet, cold soil. As you’ve discovered, once those thick, fleshy roots get a foothold in the ground, they don’t let go easily. Growing in the wrong place, the plant will romp to such an extent that it can suffocate smaller plants beneath the dense tall canopy of its leaves. Some gardeners consider it an invasive for this reason.
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The environmental risks aside, conventional weedkillers won’t get rid of your acanthus plant unless repeatedly applied over a long period of time, because those same deep, fleshy tap roots act like a reservoir of energy that allows it to resprout from small fragments. You might imagine that the most effective environmentally-friendly method of control is to dig out every last inch of root that you can find, but this can actually have the opposite effect, resulting in lots of small root fragments that then individually reshoot. Instead, your best bet is to keep cutting it back hard to the ground every time it attempts to regrow. You’ll need to show no mercy in this regard; even a baby shoot emerging from the ground has the potential to quickly become a large and imposing plant capable of reaching a height and spread of 1-2 metres. Overtime, this process of attrition will gradually weaken the root system and, so long as you’re persistent enough, eventually kill the plant. Best of luck!