3/27/2005

The Sunday Telegraph

By Bunny Guiness

Recently, there has been much grumbling among gardeners about invasive foreign plants colonizing their borders. From Japanese knotweed (Fallopia japonica) and policeman's helmet (aka Himalayan balsam or Impatiens glandulifera), to giant hogweed (Heracleum mantegazzianum), the list is expanding rapidly. Getting rid of them takes tenacity, vigilance and hard work. Repeated treatments are often required.

When these foreign invaders leap garden boundaries and take over our native habitats, a small-scale problem becomes a monstrous one. These garden escapes are the second-largest threat to global biodiversity, just behind habitat destruction.  Japanese knotweed had been growing here from about 1850 to 1940, without causing concern. Then, for some reason, it proliferated rapidly throughout the country. Now it is said to be present in more than half of the 10km grid squares that are used to map plant distribution.

Knotweed competes aggressively with the native vegetation, and the subtle balance of ferns, bluebells and woodland grasses cannot cope. The result is ``marching'' knotweed: the wildlife balance changes, and insects and mammals lose their food sources, and disappear, too.

Successful eradication usually involves translocated, non- selective herbicides (which kill every plant they touch), such as glyphosate (Roundup or TumbleWeed). But it is paramount that partially invaded areas are also treated, so prepare to lose some native vegetation initially.

In Japan, knotweed behaves well in its natural habitat and is attractive in small doses. It has more vigorous plants, pests and predators around it than it does here.

We do not just have to worry about plant invaders -- there are also pests creeping in. The harlequin ladybird is the most recent. Pet terrapins and bullfrogs that outgrow aquariums are frequently released into streams, where they reduce the number of fish, amphibians and small mammals. The New Zealand flatworm and the Australian flatworm, which arrived here more than 30 years ago on the soil of plants, feed exclusively on earthworms -- the gardener's best friend. As a result, the earthworm population in affected areas, such as Scotland, Northern Ireland, the North and the South West, has plummeted. The soil in these areas has suffered from poorer aeration and reduced organic matter. If you have flatworms, never give plants in soil to friends.

In countries such as Japan every trace of soil has to be washed off the roots of imported plants. Nothing is imported in any soil- or peat-based growing medium. No restrictions exist here on growing invasive plants, with the exception of some seaweeds, giant hogweed and Japanese knotweed. These plants are listed under Section 14, Schedule 9 of the Wildlife and Countryside Act 1981, which make it an offence to ``plant or cause to grow in the wild''. But the Department for Environment, Food and Rural Affairs launched a new code of practice, on March 3, to draw attention to this problem (www.defra.gov.uk/wildlife-countryside), available only online, although leaflets will be in garden centres soon. It is also evaluating the risk associated with different plants, in order to identify problem plants before they cause havoc.

The list of offenders is growing, but the key plants are Rhododendron ponticum, swamp stonecrop, parrot's feather, marsh pennywort and least duckweed. The Royal Horticultural Society's leaflet on invasive non-native species is useful.

I used to grow hogweed, but now give it a wide berth because of its prolific seed distribution. I deadheaded all the flowering plants well before they started to set seed. Gloves and long sleeves are a must for this, as skin contact in sunlight causes blisters that can take up to a year to heal. I have spotted Japanese knotweed on several of my sites -- some even pushing its way up through tarmac. For knotweed and Rhododendron ponticum, glyphosate is the most effective treatment. Usually, only about 90 per cent of the plant is killed on the first application and respraying over three to five years is necessary to kill the re-growth from the plant's immense root system. Application in autumn, as the plant dies down, is effective, because the chemicals are drawn to the roots.

For aquatic plants, such as duckweed, regular removal with nets is the solution. Large areas of aquatic weeds are difficult to control, as cutting tends to increase the vegetative spread. Chemical control (glyphosate again) can be used, but is approved only for the professional market. It destroys all aquatic vegetation it touches, but does not harm fish. Do not compost or tip any of these invaders.

Although our gardens would be poorer without many non-native species, it is worth knowing what to avoid. Plant Invaders, by Quentin Cronk and Janice Fuller, is a helpful guide.