CRASSULA HELMSII – AN ALIEN ON THE MOVE

Over the summer whilst surveying for great crested newts with Suffolk Wildlife Trust’s pond project, I was appalled to find Crassula helmsii, otherwise known as Australian swamp stonecrop or New Zealand pygmyweed in a number of garden and farm ponds (including one of our own).  Dispersal of Crassula helmsii results from a wide range of human activities associated with water – water gardening, fishing, pond restoration (fragments on machines move from infected pond to uninfected) and it is often offered for sale with other water plants.  Worst of all, a fragment as small as a single node on a 10mm stem, can be transported with mud or by wildlife between ponds. 

The plant will grow around the damp margins of ponds and in water up to 3m deep where it forms very dense stands.  It first appears as a small light green tussock on the sediment.  These tussocks grow and spread rapidly to form a dense mat of vegetation.  The dense mat out-competes all other aquatic vegetation, eliminates native flora and creates a poorer ecosystem for invertebrates and fish, and great crested newts, as it suppresses their favoured egg-laying plants.  Severe oxygen depletion can occur below dense growths of this plant.  Its three growth forms (terrestrial, emergent i.e. half in and half out of water, and submerged) grow throughout the year and there is no dormant period.

An English Nature control leaflet says “this plant has not, so far, been eliminated from a natural site once it has dominated despite repeated general and spot treatments with herbicides” - depressing words.  However, I’ve gleaned some experiences online from conservation organisations’ attempts at controlling the plant in ponds. Its lack of dormant period makes control in the autumn (when other plants die down) or late winter (before they have emerged) an option without killing everything else.   Crassula needs to be hit hard at the first signs of re-growth as only then will the plant be weakened. 

Mechanical control is not a practical or sensible option as fragments as small as one node can re-grow so it is highly unlikely you will have any success!  In streams it can make the situation worse - the fragments that are produced by cutting can re-grow and spread the infestation downstream or re-infest the treated area.  Chemical control (which many may dislike) is probably the only option worth considering and even that is now fraught with difficulty as the only really effective chemical (diquat) for submerged parts of the plant has been withdrawn from aquatic use by the EU, but the products are the subject of appeals to reinstate their use on this species. The terrestrial and dry parts of emergent stands can be treated only with glyphosate but if the submerged part of the stand has not been treated, it will simply re-grow!  Another less effective chemical (but approved for use in water) is dichlobenil.  No precise details on chemical control can be given here, as you must always refer to full manufacturers’ instructions.

Take care if or when buying aquatic plants (best avoided) – or receiving well-intentioned swaps from friends’ ponds!  There could be an alien lurking.

Juliet Hawkins