It sounds like something out of a bad horror movie. But this time it is real, and coming to a riverbank near you.

An alien weed that scatters its seed for more than 20ft is threatening to destroy Wiltshire's riverside wildlife.

Experts at the Wiltshire Wildlife Trust have become so alarmed at the rapid reproduction of the Himalayan Balsam they are assembling an emergency hit squad to fight it back from the banks of the River Mead.

Once it spreads to the water it could choke native species, doing serious damage to Swindon's delicate riverbank ecosystem.

Himalayan Balsam, also known as policeman's helmet, is an annual plant that can grow up to four metres tall, sprouting purplish-pink slipper-shaped flowers in the summer.

A native of the Nepalese mountains, the Balsam escaped from garden flowerbeds and can survive with virtually no light.

But its deadliest quality is its ability to reproduce. When one of its seedpods is touched it explodes, ejecting its seeds up to 20ft.

Plants growing near water can propel seeds into the current, helping them to travel great distances.

"It is blessed with an extraordinary reproductive system that makes it virtually unstoppable once it has set seed," said Swindon Water for Wildlife project officer Jo Sayers.

"Once seedpods have formed, any disturbance will cause them to burst open, hurling seeds with incredible force that can only be compared to projectile vomiting.

"If you are unfortunate enough to brush against one at eye-level, it could actually damage your eyesight."

The seeds are spat out by a coiled spring mechanism within the seedpod, which can be seen dangling from the pod afterwards.

When the Balsam takes hold in a river it can choke native riverbank plants such as comfrey and willow herb.

These are the plants which attract insects, and without them the insect population will be affected, which could in turn affect birds, fish and even otters.

"An absence of insects means the whole ecology of the river has been damaged," said Jo.

The Wildlife Trust is hoping to recruit an army of volunteers on Sunday to clear a stretch of the River Mead at West Mead by pulling the Himalayan Balsam up by their roots.

A blanket of the plants is in flower and close to forming the seedpods.

If the seeds are introduced there is a risk they will get into the River Ray, which it joins near Mannington recreation ground, one of the wildlife treasures of Swindon, and home to otters.

Willing weed-fighters should meet on Sunday at 10am at the underpass under Mead Way, near the turning for Whitehill Way. For more information call Sharon Charity on 01380 725670.