Tackling flooding menace of pennywort

Great rafts of shiny green pennywort stretch over the water's surface, making an otherwise industrial riverbank look almost picturesque, and even those who are hoping to kill the weed have to admit that it looks pretty.

In the background, the flowers of another invasive weed from abroad – Himalayan balsam – add splashes of radiant purple, and virtually screen a line of decaying warehouses.

But behind all this beauty is a dark story. Other plants, species which are native to Britain, are being snuffed out. And ominously, in

the case of pennywort the weeds pose a serious threat to flood defences.

Which is why the Environment Agency put a boat on to the River Calder at Chantry Weir in Wakefield and began the process of trying to eradicate the pennywort before the weed gets beyond control.

The Calder has a history of flooding, and further upstream the Pugney's Country Park acts as an overflow to protect Wakefield at times of heavy rainfall, while the city's regenerated waterfront is protected by computer controlled flood barriers. But dense, interwoven mats of pennywort on the Calder could put these flood defence schemes in serious jeopardy.

Floating pennywort (Hydrocotyle ranunculoides) is a native of North America and was brought to Britain in the 1980s for use in tropical aquaria and garden ponds. However, people soon began to dispose of it in the wild when they realised how hard it was to control. It can grow at a rate of eight inches a day and becomes so thick its mats make water look like dry land. Its prolific spread has caused havoc on the waterways of the Norfolk and Suffolk Broads, and the weed has completely covered the River Soar in Leicestershire, halting navigation.

In Yorkshire, the Environment Agency's Andrew Virtue said: "These mats break off during the winter, when the plant starts to die back. That's when they become potentially dangerous, blocking drains which empty into the river, thus causing the water to back up and producing flood problems.

"Pennywort has only really arrived in Yorkshire in the past couple of years. At present, it is known to be in South and West Yorkshire. Other invasive species such as Himalayan balsam and Japanese knotweed affect only the riverbanks and now are so well-established that they are virtually impossible to eradicate. We want to tackle pennywort before it can become as big a problem."

And between showers on a dull summer's day, Andrew and fellow agency officers took the first steps on the River Calder by spraying the weedkiller Roundup, mixed with an adjuvant to break through the pennywort's hard waxy covering. There will be another spraying operation in September, and by October the agency should know whether it's been effective.

A total of 10,000 has been set aside for controlling the weed in Yorkshire this year since the problem was brought to the Environment Agency's attention by Chris Firth of the Don Rivers Trust. After spraying on the Don and Rother there have been some positive results, but Andrew said the extent of pennyworth growth on the Calder is more serious.

"I think this is because it's coming down the Calder and Hebble Navigation. Up at Horbury, where the river and navigation interlink, there are very big stands of pennywort because it loves very slow moving water like canals, and it's had a chance to get really established there."

This summer, the Department for Environment, Food and Rural Affairs (Defra) launched a "Be Plant Wise" campaign to raise awareness among gardeners, pond owners and retailers about the damage caused by invasive aquatic weeds.

British Waterways is having to spend 50,000 this year in removing floating pennywort. Hardest hit are the River Don at Sprotbrough, the Figure of Three Locks in Horbury on the Calder and Hebble Navigation, and some locations on the Aire and Calder Navigation at Castleford.

It is not the only plant which is putting a strain on the waterways system. But it's pennywort that poses the greatest long-term worry because of its capacity to cause flood risks.

So serious does the Environment Agency take the problem that is has set up a Pennywort Forum in Yorkshire, including interested parties like British Waterways, Doncaster and Wakefield Councils and local rivers trusts.

The Environment Agency has a top 10 list of "nasties". Number one is Japanese knotweed, which is putting so many of our native plants in the shade on riverbanks, and number two is giant hogweed. Both of these have been around for awhile.

"But straight in at number three this year is pennywort," said Andrew Virtue, "and it wouldn't surprise me if it hits number one quite soon unless we do something about it."

There is a potential solution to the Japanese knotweed problem undergoing trials at several secret locations in England, involving the release of an insect which feeds on the plant. If the experiment is successful, it will be widened to more sites in England next year. Sadly, no such solution is yet in sight for pennywort.

CW 7/8/10