Heavy machinery will be arriving on The Green next week to give the pond some much-needed attention. You’ll also see a large hole being dug nearby — but don’t worry, it will be filled in again!

More light, less silt — a healthier pond

The Parish Council asked the Herts and Middlesex Wildlife Trust to draw up a maintenance plan for the pond, which is available to download. Two recommendations are at the heart of the work:

• thinning out the water lilies to let in more light
• removing some silt to help the pond cope better with drought.

There’s a catch, though: an unwelcome visitor has taken hold.

If you’ve walked past while the water has been low, you may have spotted a small, bright-green, fleshy plant on the pond floor. It’s Crassula helmsii, also known as New Zealand pygmyweed or Australian swamp stonecrop. This non-native, invasive plant can form a thick carpet, crowding out the native plants and wildlife that make a pond healthy. It’s not dangerous, but we don’t want it in our pond.

It may have arrived with birds travelling from other infected waterways, but we just don’t know.

Getting rid of it is far from straightforward. Herbicides can harm other plants and wildlife while the crassula grows back even after repeated treatments. Even being deprived of all light and air for three months has failed to finish it off.

Tiny fragments can also break away into the water, so we expect some crassula to come up with the water lilies and silt when they’re lifted. That means we need to treat all material from across the pond with care.

Why dig a hole?

There are strict Environment Agency rules about handling this plant. Taking affected material away would mean using specialist firms, at a cost we simply can’t justify.

Instead, we’ve decided to keep the material on site and bury it. The hole will be between the pond and the pub car park, and it will be filled in once the material is buried. We’ll use the surplus soil to raise the bank on that side of the pond, where the ground has traditionally been low and swampy.

Keeping an eye on things

Once the contractors have cleared the bulk of the growth, we hope the new Pond Volunteer Group can keep on top of any small patches of crassula that return.

This isn’t a permanent answer — there are only so many holes we can dig around the pond. We’ll keep watching developments in possible biological controls as we look for a longer-term solution.

If you’re interested in how they’ve controlled crassula elsewhere there’s a really interesting article at The Norfolk Ponds Project – Crassula helmsii Guidance Booklet.pdf