Extreme weeding at Claremont

In October 2017 two amphibious, tank-like machines arrived at Claremont armed with giant rakes. Thus began an epic battle on the lake, to reach and destroy almost 16 tonnes of an invasive weed known as Crassula helmsii.

Read our interview below with Claremont's Senior Gardener, Tim Rayfield, to learn more about this troublesome weed and our unique project to control it.

So, Tim, what is crassula, and why is it a problem?

Tim: Crassula 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 and insidious 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.

The invasive pondweed close-up
A close-up of crassula on the water's edge at Claremont
The invasive pondweed close-up

How did crassula end up at Claremont?

Tim: 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 Claremont's lake. 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 profusely.

The weed adds a murky green sheen on the lake surface
A wide shot of the crassula on Claremont's lake during winter
The weed adds a murky green sheen on the lake surface

Isn't it a bit drastic to hire heavy machinery just to get rid of a little pond weed?

Tim: The weed harvesters might look extreme, but we've found it to be the best method of controlling the crassula without having any detrimental effects on the lake.

A weed harvester embarking upon the island at Claremont with a rake full of crassula

The weed harvesters are actually pretty impressive; like a hovercraft, they're able to glide effortlessly between land and water, making it easy for them to collect the weed and deposit it onto the island in the middle of the lake. Here it is stacked neatly and will rot down quickly as compost. Positioning the "crassula mountain" on the island reduces the risk of spreading the plant elsewhere, and will also allow any fish and invertebrates scooped up with the crassula to make their way back into the lake. We estimate that the weed harvesters will have removed more than 16 tonnes of crassula over a period of two weeks.

The "crassula mountain" will decompose by around 40% in six months
A large pile of crassula on the island in the lake at Claremont
The "crassula mountain" will decompose by around 40% in six months

Is this a permanent solution?

Tim: There's currently no known way to completely eradicate crassula, but the weed harvesters have made a huge difference already. The lake is looking much better, and you can see the trees reflecting in the water again.

To help keep the crassula under control, the gardening team have been trained so that we can carry out the weed harvesting ourselves in six-month intervals. Visitors can also help stop crassula from spreading at Claremont and to your own ponds by not pulling it out of the lake.

Thank you

It's all down to you

It's thanks to your support that we're able to carry out projects like this. Visits, donations and volunteers all help us keep Claremont special for ever, for everyone.