While weeds are often considered a pesky nuisance by gardeners, some can play a key role in maintaining ecosystems and helping biodiversity – while others can threaten native species.
However, did you know there are a few invasive species that can actually land gardeners and farmers into a spot of legal trouble and lead to a hefty fine?
James Ewens, gardening expert at Alexander Francis, warns of 10 invasive plants that, if left unchecked, could lead to fines or even legal action. Some are dangerous to humans, others to local ecosystems, but all demand attention.
10 dangerous weeds
1. Japanese Knotweed
Arguably the UK’s most notorious invasive plant, Japanese Knotweed is infamous for its ability to grow through concrete and tarmac, causing severe structural damage. It has been seen to do so much damage that it features on house-buying surveys and can seriously impact the value of your home.
While not illegal to grow, you must not allow it to spread to neighbouring properties—or you could face prosecution or a community protection notice.
2. Giant Hogweed
Giant hogweed looks deceptively pretty but harbours a toxic sap that can cause severe skin burns and permanent scarring. Commonly found near rivers and roadsides, it must be handled with care. It’s not illegal to have it in your garden, but it is a criminal offence to allow it to spread.
If spotted, contact your local council for professional removal.
3. Spear Thistle
With its spiky leaves and purple flowers, spear thistle may look striking—but it's currently illegal to grow it in your garden. It spreads aggressively and can harm native plants and crops. Letting it grow unchecked could result in a fine of up to £2,500.
4. Common Ragwort
Though not banned outright, common ragwort is subject to control under a specific Code of Practice. It’s toxic to most mammals, especially horses and livestock. If it poses a threat to animal welfare or food production, you are legally obligated to remove it.
5. Broad-Leaved Dock
Often seen as a folk remedy for nettle stings, this weed is classed as ‘injurious’ under the Weeds Act 1959. It’s not illegal to have in your garden, but its rapid spread—thanks to up to 60,000 seeds per plant—makes it a major nuisance.
6. Curled Dock
Another highly invasive dock species, curled dock is hardy, fast-spreading, and thrives in a variety of conditions. While not toxic, it’s extremely tough to get rid of and often outcompetes native flora, disrupting local ecosystems.
7. Rhododendron Ponticum
This ornamental plant may look lovely with its vibrant purple flowers, but it’s a serious threat to UK biodiversity. It’s legal to grow, but you must prevent it from spreading, as it smothers native species and is increasingly under scrutiny from environmental authorities.
8. Himalayan Balsam
Often likened to Japanese Knotweed, this fast-growing plant can rapidly colonise riverbanks, crowding out native species. It’s not illegal to have in your garden, but it’s against the law to allow it to spread beyond your property.
9. New Zealand Pigmyweed
Originally introduced as a decorative pond plant, New Zealand Pigmyweed forms dense mats that choke water systems and deplete oxygen levels. It is now banned in the UK, and controlling its spread is extremely difficult once established.
10. Three-Cornered Garlic or Leek
With its delicate white flowers, this plant might seem harmless—but don’t be fooled. Its seeds are spread far and wide by ants, and it can easily escape your garden. While legal to grow, you can be penalised if it spreads into the wild or neighbouring land.
Not all greenery is garden-friendly. Some of these plants, while beautiful or seemingly harmless, can pose risks to your property, local wildlife, and even your legal standing. If you have any concerns seek professional help.