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What is Crassula, and what is the problem?

Crassula helmsii, known as swamp stonecrop or New Zealand pigmyweed, is an aquatic or semiterrestrial species of succulent plant in the family Crassulaceae. Originally found in Australia and New Zealand, it has been introduced around the world and is causing problems in wetland environments.

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 .

Crassula

How did Crassula get to Brockholes?

Crassula is sometimes sold as an aquarium plant as it is an extremely good oxygenator, so it's possible that at some point a visitor decided they didn't want their goldfish any more and emptied their tank, along with the Crassula, into an area of open water which has made its way into the lakes at Brockholes, as well as many other ponds and lakes around the country. It may also have come in on the feet of waterfowl visiting from other contaminated ponds.

Once the Crassula arrived, it found itself in the perfect breeding ground as the lake is very still and shallow, and its depth means the sun heats the water to just the right temperature for it to grow thick and fa

Crassula is a nationwide problem, and not just something which occurs at Brockholes.

What can we do about it?

There is no known means of effective control or eradication, though scientists are researching the use of parasitic mites in field trials at select sites in the UK. Whilst we wait to hear the outcome of these scientific trials, a short term solution is being trialed at Brockholes, just to clear some of the huge volume of Crassula and allow more light back into Meadow Lake. Throughout the next couple of weeks, you may see some action with machinery and a paddle boat near the Visitor Village as we try something different The aim is to restore open water areas so that native wildlife can occupy these areas again, and so that foraging Daubenton’s bats can once again use this section of the lake for feeding over. The outcome will only be temporary, until the Crassula grows back, but the respite should be valuable for our wildlif

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