Chelsea Flower Show warns of alien plant invasion

Visitors to Britain's most famous gardening event, the Chelsea Flower Show, will be confronted with an unusual display this year: a garden they will be warned not to plant.

Visitors to Britain's most famous gardening event, the Chelsea Flower Show, will be confronted with an unusual display this year: a garden they will be warned not to plant.

The event's 157,000 visitors will be told about the increasing dangers posed by invasive species - the often alien flowers, ornamental plants and pond weeds that are devastating the countryside.

The unique display, which will take pride of place at the show's Great Pavilion when it opens on 25 May, will alarm many gardeners because it features 10 flowers, aquatic plants and "architectural" species being promoted by Britain's booming garden centres.

Most of these plants have never been so publicly branded as dangerous - such as gunnera, a South American plant known as "giant rhubarb" which is infesting marshy areas of Ireland, and montbretia, which is spreading out of control along hedgerows and streams.

The display, designed by ecologists and botanists from English Nature and the Environment Agency, will feature several of the plants now choking ponds and streams, such as parrot's feather and floating pennywort.

These plants have already begun overpowering native species in south-west England, said Trevor Renals, an Environment Agency ecologist in Cornwall, and are threatening to spread elsewhere in England and Wales.

Mr Renals said the chief problem was the desire for "instant gardens" being promoted by makeover programmes and garden centres, featuring quick-growing plants that are often invasive species which run out of control.

"There seems to be a climate where people want instant gardens," he said. "But people should learn to tolerate bare patches for a year or two rather than going for a quick fix with these invasive species, and then having to spend their time pulling them up when they spread too far."

The agency, English Nature and councillors formed the Cornwall Knotweed Forum, an umbrella group originally set up to combat knotweed - an invasive ornamental plant which came to the UK in the mid-19th century from Japan. Knotweed is now one of the most hated plants in Britain because of its tenacity and speed of growth.

The forum installed a small knotweed display at last year's Chelsea Flower Show, which won a gold medal from the Royal Horticultural Society. The group has now branched out to campaign for tighter controls on all invasive plants.

This year's exhibit is based on a Cornish field. Drawing on a theme of celebration of the RHS's bicentennial year, the display is split in two.

One half features native Cornish plants typical of 1804, but across a hedge is a modern Cornish field which has been over-run by invasive species, brought there by the illegal dumping of garden waste - one of the most effective ways of spreading garden plants through the countryside.

GROWING MENACE: SOME OF THE 'QUICK-FIX' SPECIES THAT THREATEN TO SPREAD OUT OF CONTROL

Dead nettle: Lamium galeobdolon

Origins: North Africa and Eurasia

Appearance: Short, upright stems of silver-streaked foliage with clusters of small, hooded yellow flowers from late spring to midsummer.

The problem: Also known as yellow archangel and Hermann's pride, it will spread rapidly, especially in woods, crowding out other flowers.

Winter heliotrope: Petasites fragrans

Origins: Mediterranean North Africa

Appearance: Thick, heart-shaped leaves, with pinkish-white flowers between December and March. The problem: Also known as sweet-scented coltsfoot, it will grow thickly in woodland and road edges, smothering native species.

Montbretia: Crocosmia x crocosmiiflora

Origins: South Africa

Appearance: A bulbous plant which dies down in the autumn. Bright green and spear-shaped leaves appear in spring together with long spikes of tubular orange flowers.

The problem: Competes fiercely with indigenous plants in hedgerows and along streams.

Three-cornered leek or garlic: Allium triquetrum

Origins: Western Mediterranean

Appearance: A bulbous plant with drooping white flowers from late winter to June. Its stems are sharply triangular and give off a strong smell of garlic when crushed.

The problem: Looks like a white bluebell, but its quick growth means it will over-run entire areas.

Gunnera: Gunnera tinctoria

Origins: South America

Appearance: An enormous plant with leaves that can grow to up to 4ft wide. Produces large spikes of reddish-green flowers during the summer.

The problem: Known as Chilean or giant rhubarb, gunnera can swamp damp or boggy areas.

Annabel Fallon