What is Crassula, and why is it a problem?
Crassula helmsii is a non-native aquatic plant, also known as New Zealand pygmyweed or Australian swamp stonecrop. It can reproduce rapidly and creates an unsightly green "carpet" on the water surface. It has no native competition here in the UK, so it can get quickly out of control and disrupt the balance of the ecosystem.
Crassula really is a strange beast. It's able to grow in layers, rooting into the base of a waterbody as well as floating in the middle and on the surface. It can "crawl" out of its pond or lake and start growing on land; and it can even grow at night without sunlight. The weed can completely overtake a waterbody, blocking out light for other vegetation and severely reducing the reflective quality of the water.
It’s important to note that Crassula is a nationwide problem, and not just something which occurs at Brockholes. Most nature reserves or parks with lakes or ponds live in dread of it colonizing, or else have already been infested already .