It’s an unfortunate gardening fact that some plants which initially seem harmless (even attractive and ornamental) can quickly become aggressive invaders.
Japanese knotweed gets a lot of airtime thanks to its notorious reputation as a fast-growing weed that can crack through patios and even damage property foundations. Its presence in a garden can also affect a property’s value, making it one of the most loathed plants for homeowners.
But it’s not the only invasive plant gardeners should be wary of. “Japanese knotweed gets most of the attention, but it’s far from the only invasive plant gardeners should be aware of,” Luke Newnes, gardening expert at Hillarys, tells Country Living.
“In reality, there are several species that can quietly establish themselves in gardens before spreading much further than intended. The biggest mistake is assuming a plant is harmless simply because it looks attractive. A lot of the UK’s invasive species were originally introduced for ornamental reasons.
“One thing I always tell gardeners is to pay attention to plants that seem unusually vigorous. If something is spreading rapidly, popping up where you didn’t plant it, or starting to crowd out everything around it, it’s worth taking a closer look. Invasive plants often reveal themselves through sheer enthusiasm.”
Take a look at the invasive garden plants to keep an eye open for:
Giant hogweed
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Giant hogweed is another troublesome invasive plant. “Giant hogweed is hard to miss once it’s mature – it can tower over people and produces enormous white flower heads,” Luke explains.
“Aside from being invasive, it’s one of the few plants I genuinely tell people to be careful around because its sap can cause serious skin reactions in sunlight.”
How to spot it: Watch out for low-growing clusters of leaves with jagged, spikey leaves and purple-flecked stalks.
Himalayan balsam
Himalayan balsam is a deceptively pretty weed, brought to the UK in the 1800s as an ornamental flower.
“At first glance, Himalayan balsam looks like exactly the sort of pretty wildflower you’d want in the garden,” Luke explains.
“The problem is how quickly it spreads. Its seed pods burst open when touched, sending seeds surprisingly far and allowing it to colonise new areas very quickly.”
How to spot it: You’ll often find this weed near water, and it’s recognisable by its reddish-pink stems, explosive seed pods, and pinky purple flowers.
Three-cornered leek
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“Another tricky weed to spot is the three-cornered leek. “This one often flies under the radar because it doesn’t look particularly threatening,” says Luke. “It has attractive white flowers, and many people mistake it for a charming spring plant. The trouble is that it spreads aggressively through bulbs and can take over borders before you know it.”
How to spot it: This plant is also called a ‘three-cornered garlic’ due to its strong garlic aroma. It’s instantly recognisable due to its bright green leaves and clusters of drooping white bells.
Floating pennywort
For those with ponds in their garden, floating pennywort can be a tricky weed to overcome once it takes hold.
“Anyone with a pond should keep an eye out for floating pennywort,” says Luke. “What starts as a small patch can quickly blanket the surface of the water. Once it’s established, it grows at an incredible rate and can become a real headache to control.”
How to spot it: Look for runners with small round or kidney-shaped leaves creeping across mud or water.
New Zealand pygmyweed
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Another pond plant that can look perfectly innocent in the early stages but be a real pain to deal with is New Zealand pygmyweed, also known as Australian swamp stonecrop.
“The issue is that it forms dense mats that smother other aquatic plants and wildlife habitats. It’s one of those species that’s much easier to deal with when you catch it early,” Luke explains.
How to spot it: Look out for bright green shoots that are starting to cluster around the edges of a pond.
A final word on invasive plants
Luke describes invasive plants as “a bit like weeds on fast-forward”, telling us, “The longer they’re allowed to settle in, the harder they become to remove; that’s why early identification is so important.”
“If something in your garden seems to be spreading far more aggressively than everything else around it, it’s worth investigating before it turns into a much bigger problem,” the expert gardener advises.
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