2nd April 2014

Plantlife has created a dirty dozen of non native plants that are inasive and are creating problem in the countryside after spreading from gardens over the years. These plants can have a very negative effect on native flora.

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.

Creative commons copyright - Des Colhoun

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 on 1.5-metre bristly stalks. It is nevertheless 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.

4. Cotoneasters

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.

5. Himalayan balsam

Our tallest annual plant, this alien – also called Indian balsam – 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 where it can form dense thickets. The Wye Valley is amongst areas badly affected.

Creative commons copyright - S J Wells

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.

7. Japanese knotweed

Very much 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. The stout zig-zagging stems rise to 2 meters and the plant can dominate road verges, railway land, waste ground, river banks, woodland, grassland and coastal places. It happily grows through walls, tarmac and concrete and is so resilient it counts as controlled waste and must be disposed of at registered sites.

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 – often the very types of site where threatened native plants occur. Its hooked burs mean it is easily spread in the wild by sheep and other animals.

9. Rhododendron

The classic Jekyll and Hyde of our countryside, this tall evergreen shrub with showy pink-mauve flowers was planted in Victorian times on estates and in country parks. It has since swept across the UK, being most invasive in western and upland areas where it crowds out almost all other species and suppresses the germination of other seeds. In Snowdonia, rhododendron has stripped some Atlantic oak woods of the lichens, mosses and liverworts for which they were internationally important.

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. Britain is home to 70 per cent of the world population of English bluebells and the flower very much defines our woodland but for how much longer?

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.

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.