Also known as Australian or swamp stonecrop, it was first brought to the UK in the early 1900s as an oxygenating pond plant. Since then, it has spread across Britain and Europe mainly by movement of vegetative fragments on boats, machinery used to manage water bodies, clothing and wildfowl.

Under Schedule 9 of the UK Wildlife and Countryside Act 1981, this plant cannot be caused to grow in the wild.

When rooted in deep water, New Zealand pygmyweed is straggly with thin, sparsely distributed leaves. However, in shallower water side shoots branch out, forming dense mats of vegetation that can extend over a large area. The plant does not die off over winter, but may retreat under the surface before re-emerging in Spring.

The narrow leaves are green or yellow-green in colour, and are arranged along the stem in opposite pairs. They are fleshy and succulent-like, and may have a waxy coating. The stem is normally green though may also be tinged pink-purple in colour.

The tiny flowers are white or pale pink in colour. They comprise four triangular shaped petals. Flowers occur on a long stalk, originating at a leaf join. They are always found above the water’s surface. The plant flowers from July to September.

New Zealand pygmyweed is dispersed entirely by vegetative fragments, with even a single small 10mm portion of stem being able to root to form a new plant.

The plant grows in ponds, lakes and reservoirs but will also grow along the damp margins of canals and reservoirs. It can survive in acidic, alkaline and even slightly salty water.

The dense vegetation shades out other plant species, leading to oxygen depletion and a resulting decline in invertebrates, frogs, newts and fish.

The obstructions can impact recreational activities such as boating, and there may be economic impacts from the obstruction of navigation and flood defences.