Alien invaders go wild in the country

DR PAUL HUMPHRIES a senior lecturer in microbiology in the schoool of applied sciences at the University of Huddersfield, looks at the growing problem of “escaped” plants

As the weather is finally warming up the thoughts of many people are turning to their gardens.

For some that means tidying up the border, others may have more exciting plans such as replanting or digging a pond.

The colour of our gardens owes much to the work of plant breeders and plant collectors.

Plant collectors were a hardy breed of explorers who toured some of the world’s most inhospitable places searching for new and interesting plants for the gardens of stately homes and botanical collections such as Kew.

They provided much of the genetic material that plant breeders are still working with today.

Although the introduction of all these plants from all over the world has brightened up our gardens considerably, there are a few down sides, the main one being that they do not always stay where we plant them.

The escape of plants from gardens has become a major problem across the country with so called invasive species spreading at an alarming rate.

The highest profile of these alien invaders is Japanese knotweed.

This plant was first introduced by the Victorians as an ornamental garden plant. Little did they know what they were unleashing on the countryside, with the ability to grow at a rate of a three feet a month, a final height of seven feet, and the strength to grow up through concrete this is a formidable invader.

Its rhizomes extend for many metres around the plant and down to seven feet or more.

It also has the ability to re-grow from a small fragment of rhizome making digging it out very difficult. A fragment of rhizome less than an inch long can generate a mature plant in less than two weeks.

Its rapid growth means that it quickly out-competes other plants producing huge colonies if left unchecked.

And its spread across the UK has been rapid with major infestations appearing along water ways, railway lines and waste and derelict land.

It is currently estimated that the control and treatment of knotweed infestations cost £100million a year.

The plant is such a problem that there is a legal duty of care to manage knotweed if it is growing on your land.

The costs associated with removing and treating knotweed infestation has led to the unprecedented step of the planned release of an insect that only eats knotweed. It is these insects and other natural controls that prevent knotweed being a problem in its native Japan.

This biocontrol measure is only being considered after a considerable period of research by the agricultural research organisation Cabi, that screened hundreds of potential knotweed pests.

They finally chose a tiny sap sucking insect known as a psyllid, because it only eats the dreaded knotweed.

The first controlled trials of this biocontrol are currently being planned on a small number of selected isolated sites which will be closely monitored.

Knotweed may be the most prominent escaped garden plant causing problems but it is not the only one.

Himalayan balsam, rhododendrons, cherry laurel and montbretia are all garden escapees causing problems in the countryside.

Another group of plants causing concern are water plants imported for use in ponds.

The nature of these plants means it is very easy for them to escape garden ponds because only small fragments are needed for a plant to re-grow.

These plants are often chosen because they are fast growing generating a quick impact in a new pond. But that also means they can rapidly become a menace in any pond, river or canal if they get there by accident.

The issue of invasive pond plants is such a serious one that it has prompted the Government to launch the Be Plant Wise scheme to encourage the responsible cultivation and disposal of ornamental aquatic plants.

This scheme identifies a rogue’s gallery of five problem plants, namely floating pennywort, parrot’s feather, New Zealand pigmyweed, water-primrose and the water fern azolla which pose a major risk to waterways if they are released into the wild.

The scheme suggests a range of alternative plants and useful advice on the disposal of unwanted plants, waste water and the movement of plants from pond to pond.

Overall our gardens and ponds have benefited enormously by the collection and cultivation of wild plants form around the world.

But it is always worth remembering that plants that are well behaved when carefully tended in our gardens may get out of control if left to grow wild in the countryside.