Invasive Species

Floating pennywort


Invasive Aquatic Plants

Plants from several parts of the world have been imported for ponds and aquariums in Britain. Sooner or later they escape or are deliberately introduced into the wild. Some have few natural predators here and can rapidly expand to form dense mats of growth that outcompete and displace our native plants and animals. Several species are very invasive because even small fragments can regenerate into new plants. The three species described below are of the greatest concern. They are known from only a few sites in the wild in Snowdonia but they are probably quite common in garden ponds.

Australian Swamp Stonecrop Crassula helmsii
This is a superweed on a par with Japanese knotweed. It will grow in anything from damp ground to water three metres deep, and can survive for months without light. The plant has long thin rather fleshy leaves 1 to 3cm long that join up at the base to form a collar around the stem.

Floating pennywort Hydrocotyle ranunculoides
This North American plant only arrived in Britain twenty years ago and yet is already a problem. The leaves are lobed, round and fleshy and around 15cm across and held above the water.The stems, which root at the nodes, form a tangled mass below the water. Like many invasive aquatic weeds the growth rate is phenomenal. Plants can double their size in three days.

Parrot's Feather Myriophyllum aquaticum
The shoots which are composed of whorls of fine feathery leaves around a central stem can be emergent or submerged and are brittle. Broken fragments soon give rise to new shoots.

What you can do to help

1. Prevent the spread - If you have any of these plants do not offer them or any other plants from the same pond to your friends and neighbours. Be careful you are not introducing these plants to your own pond through bought-in plants or gifts. Do not put anything from your pond back into any natural water body.

2. Control - For small garden ponds, these plants can be successfully brought under control by gently weeding and dredging. Rough handling will cause the plants to break into fragments which will soon grow back into new plants. It is vital that none of the fragments escape into natural watercourses. The removed material can be dried and then composted and burnt. Repeated efforts will be needed to remove remaining plants.

For larger ponds and water bodies, effective control is more difficult and may not be possible. Herbicides can be effective but this will need to be done in consultation with the Environment Agency.

3. Reporting - If you find any of these species growing in the wild or in a pond connected to a river catchment, please inform either the Environment Agency (0845 9333111) or Snowdonia National Park Authority (01766 770274). It may be helpful to send a sample to confirm identification.

Further information: