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Twelve more species to avoid
Despite successfully campaigning for five of the worst offenders to be banned from sale in the UK. many other non-native, invasive plants are still at large. Here is our "Dirty Dozen": 12 alien plants that are causing the most damage to the countryside and our native species.
1. American skunk cabbage
A distant relative of our own lords and ladies or cuckoo pint, this invader grows to 1.5 meters high and has creeping roots and unpleasant smelling flowers. It quickly out-sizes the wet patches where it is planted and, once in the wild, its large leaves and rapid spread mean it can cause extensive damage locally. More >
2. Broad-leaved bamboo
This is an easily obtained ‘ornamental’ plant, but what filled a medium size pot at the nursery can quickly produce a patch of undergrowth 2 metres high and 6 metres across in the garden or the wild. Once established, it is hard to eradicate, being vigorous, hardy and robust.
3. Chilean Giant rhubarb
This monster has 2-metre wide leaves. It is popular in formal gardens where it is grown beside ponds for impact. In the wild it now occurs along the coast and beside rivers mainly in the South-west and south Wales. It can render farmland useless and in south-west Ireland manages to out-compete willow trees. More >
This is a large group of pink and white-flowered, red-berried shrubs favoured by gardeners. But just one species is native to Britain with up to another 70 now joining it in the wild after being bird-sown from parks and gardens. Plantlife is particularly concerned about holly berry, wall and Himalayan cotoneaster. More >
5. Himalayan balsam
Also called Indian balsam, this plant can grow to a dizzying 2.5 metres high from seed in a single season. The stems are reddish and the pink-purple flowers deceptively attractive but this is an aggressive coloniser of river, stream and canal banks. The Wye Valley is amongst areas badly affected. More >
6. Hottentot fig
The large yellow or pink flowers of this succulent make it a popular ornamental plant but sadly it is a wolf in sheep’s clothing. It forms dense, impenetrable mats that carpet warm, sunny coastal cliffs to the exclusion of all other species. A single plant can dominate an area up to 50 meters across. More >
7. Japanese knotweed
A scourge of human-dominated environments as well as more natural areas, this is one of the most pernicious weeds in Britain, reproducing from tiny fragments of rhizome in the soil. It can dominate a variety of habitats, happily growing through walls, tarmac and concrete and is so resilient it counts as controlled waste. More >
8. Pirri-pirri bur
Originating from the Antipodes, this short, creeping plant is readily available from garden centres. Pirri-pirri bur becomes especially invasive when it establishes on cool, damp cliffs and upland habitats – the very places threatened native plants often occur. Its hooked burs mean it is easily spread by sheep and other animals. More >
9. Rhododendron x superponticum
The classic Jekyll and Hyde of our countryside, this tall evergreen shrub with showy pink-mauve flowers was planted in Victorian times in estates and country parks. It has since swept across the UK, being most invasive in western and upland areas where it crowds out other species and suppresses the germination of other seeds. More >
10. Spanish bluebell
Widely planted in gardens and able to cross with our own native bluebell, the Spanish bluebell and its hybrids are now often found in woodland and are dominant in urban and suburban situations.
11. Three cornered garlic
You might have thought it was a white bluebell, until you came to smell it... This spring flower is now found along roadsides, hedgerows and woodland and field edges. There it forms thick colonies that push out favourites like primroses and violets. It is most common in south and west Britain but is spreading north. More >
12. Variegated yellow archangel
This innocent-looking dead-nettle is widely cultivated as a garden plant. Once it gets into the wild, it can smother the ground and impact on sites relied upon by plants of conservation interest like spreading bellflower. It prefers shady places, including woodland edges and hedgerows. The leaves are blotched white, distinguishing it from its well behaved relative, the yellow archangel. More >
Background image: Floating pennywort clogging a river © Trevor Renals.