CRASSULA HELMSII or ‘NEW ZEALAND STONECROP.’
DESCRIPTION. A small succulent flowering perennial that grows rapidly to form an extensive lush-green ëcarpetí that floats on freshwater or may be submerged. Growth can extend from margins of sheltered waterbodies to completely cover the water surface with tangles of stems and shoots. Plants may range from 10-130cm in length. In deeper water, plants are more elongate and have narrower leaves. Flowers are <4mm and white to pale pink.
BIOLOGY/ECOLOGY. Dispersal is by plant fragments or buoyant shoots carried by birds and mammals, downstream movements and from flooding. Fragments may also be moved with mud. Seeds are not†known to be produced in western Europe (where its range is west of Poland to Italy). It can propagate from fragments as small as 5mm, and has a high growth rate. It grows for most of the year without any serious winter die-back.
DISCUSSION. Crassula’s form is frequently a dense sward-like, green mass which smothers out most other plants and is the cause of much concern; indeed it can resemble solid land and people have been known to try to walk on to it! In the past, it was widely available from aquatic suppliers. Control by physical removal is often recommended but also results in numerous fragments which can germinate and should only be considered if combined with a secondary removal technique. Studies and experience suggest that this plant will remain a problem and great caution is required to prevent its further spread.
Its metabolism allows it to be tolerant of a wide range of environmental variables including dryness. It has been seen growing on bare rock, well above the normal ‘high tide line’ on the shore of Derwent Water in the Lake District. It was ‘on its own’ with no competing species demonstrating the resilience of this species. An aquatic survey conducted there†in 2000, showed significant areas of the gravels at the inflow of the lake to be covered by Crassula ‘lawns.’ The survey also revealed a vast submerged ‘rolling prairie’ of Crassula out in the lake. One sheltered bay was totally dominated by a blanket of Crassula except for the occasional tall emergent native water plants.
Images showing blanket coverage and a close-up of Crassula.
FUTURE.† In the longer term, how Crassula will fit into the wider countryside, away from the ponds, lakes and other water bodies currently associated with it, remains to be seen. It is a pretty insignificant species in small quantities and can be easily overlooked. Those involved in livestock husbandry ought to be made aware, so as to avoid/prevent stock feeding in Crassula infested areas, especially if they are moved between grazing sites. Birds are often cited as one of the vectors but humans (with or without tools and machinery) have also been vectors in the spread of Crassula from one site to another. Some authorities may say that it is already too late. There are however, still plenty of water bodies without Crassula in them and I believe we should do our best to†prevent it spreading to them. If you should find it, report it to the Environment Agency or, your county biodiversity centre.
Compiled from the following sources:
http://www.europe-aliens.org/pdf/Crassula_helmsii.pdf
http://www.sciencedirect.com/science/article/pii/0006320787900711
and from Phil Taylor, retired Ecologist Lake District National Park.