Best time to see: Apr to mid Nov

Key facts

Forms dense mats around the edges of ponds and lakes and a dense, tangled mass underwater, destroying all other life

Previously sold as an oxygenating plant, it was banned from UK sale in 2014

Extras

Weak stems and tiny leaves make big tangled mats up to about 20 cm high

Flowers are tiny white stars, very hard to spot!

Leaves are tiny, narrow, pointed