7 Post Pond - Why is it so green?
In May 2025, a local visitor raised concerns about the continued presence of New Zealand pigmyweed, or Crassula as it’s also known, at 7 Post Pond and asked what we would be doing to control it. This is a really good question and we thought it would be helpful for us to explain the challenges and options that come with trying to manage this invasive species.
7 Post Pond is a small pond (0.10 hectares) located along the edge of Parkside, approximately 250 metres south of Tibbet’s Corner. It has long suffered from issues related to litter and the presence of several invasive non-native species, including terrapins, carp, and invasive pond plants such as Parrot’s Feather and the Crassula.
First reported at this site in 2000, Crassula has become the dominant invasive species in the pond. It grows in a range of conditions - submerged, emergent, and even terrestrial - and propagates entirely through stem fragmentation, making it particularly difficult to manage.
Crassula in 7 Post Pond
Introduced to the UK in the early 1900s as a pond oxygenator, the plant has since spread widely through improper pond waste disposal and transfer via other aquatic species. It is now listed under Schedule 9 of the Wildlife and Countryside Act 1981, meaning it is illegal to plant it.
However, controlling Crassula is challenging. Chemical treatments have limited effectiveness, especially on submerged plants, and can harm native species. Mechanical removal, such as dredging, has been carried out several times but only offers a temporary solution. Hand-pulling is possible but labour-intensive and must be done carefully to avoid spreading fragments. Biological controls are being researched but are not yet a practical option.
Previous attempts to remove the Crassula from 7 Post Pond
Our team has met with experts from several eminent organisations such as the London Invasive Species Initiative, the Natural History Museum and the Environment Agency to discuss the future management of 7 Post Pond. One of the main conclusions from this meeting was that any attempt to eradicate the Crassula would be problematic both ecologically and financially. Full eradication is considered unlikely and, considering the surprising good health of the pond (both in water quality and as an amphibian habitat) management of the plant does need to be planned to coincide with other work, such as dredging, so as not to harm the pond’s delicate ecosystem.
Managing invasive species remains a key part of the Commons’ Land Management Plan. However, effective control depends on detailed ecological surveys to fully understand each pond’s unique conditions - like the careful study done before the recent Queensmere restoration. These surveys are expensive, but remain a long-term goal for all of our ponds.