Seas of Green

Simon Hirst – River Esk Project Officer

New Zealand pigmyweed (Crassula helmsii) was introduced to Britain from Tasmania in 1911. By 1927 it was being sold as an “oxygenating plant” for garden ponds and aquariums by Perry’s Hardy Plant Farm in Enfield. The first recorded occurrence of pigmyweed in the wild was at Greensted Pond in Essex in 1956. It spread widely and rapidly due to the increasing availability of the plant at garden centres and aquatic nurseries.

This non-native invasive plant is now listed under Schedule 9 of the Wildlife and Countryside Act 1981. It is an offence to plant or otherwise cause this species to grow in the wild. The plant is now also banned from sale in the UK, which is a significant environmental step forward.

Crassula helmsii - copyright NYMNPA.

Crassula helmsii grows around the damp margins of ponds and in water up to three metres deep. It starts as a small light green tussock on sediment. The tussocks grow and spread rapidly to form a dense mat of vegetation. Severe oxygen depletion can occur below dense growths. The dense mat out-competes all other aquatic vegetation, eliminates native flora and creates a poorer ecosystem for invertebrates and fish.  The plant grows throughout the year and has no dormant period. Thankfully the pigmyweed does not produce viable seed in the UK but it can re-grow from small stem fragments.

New Zealand pigmyweed is very hard to eradicate when it has become well established. The plant is tolerant of shade for long periods, tolerant of frost and dessication, and it cannot easily be tackled by any existing method of environmental control.

Recently New Zealand pigmyweed was discovered growing in two ponds in Bilsdale which are both adjacent to the River Seph. The river is one of our key wildlife corridors. Working with our Apprentice team, we came up with a plan to carry out a programme of control which will hopefully result in eradication of this invasive plant.

One of the ponds in Bilsdale covered in Crassula helmsii - copyright NYMNPA.

One of the ponds in Bilsdale covered in Crassula helmsii - copyright NYMNPA.

We’ve covered the ponds with black plastic sheeting in order to prevent sunlight reaching the pigmyweed. This will prevent the plant photosynthesising, and should eventually kill it. The unsightly but purposeful plastic sheeting will need to be left on top of the ponds for six months, and we will be reviewing the situation next Spring!

Depriving the Crassula helmsii of light - copyright NYMNPA.

Depriving the Crassula helmsii of light - copyright NYMNPA.

Temporary black plastic over the top of one of the ponds - copyright NYMNPA.

One thought on “Seas of Green

  1. Pingback: Attritional conservation | The official blog for the North York Moors National Park

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