Fighting Them On The Beaches – UK Waterways Attacked By Foreign Invaders
Crassula helmsii is the next big threat for invasive weed management.
Introduction
Britain’s inland waterways are being threatened by a tenacious antipodean weed. First imported from Tasmania in the early 1900s as an ornamental plant, Crassula helmsii (common names include Australian Swamp Stonecrop and New Zealand Pygmyweed) was widely available as an aquatic ‘oxygenator’ from the 1920s and due to its extensive sale throughout UK garden centres and subsequent irresponsible disposal, it has become prevalent in lakes and slow moving rivers across the UK.
Taxonomy
Crassula helmsii is a submerged / semi terrestrial perennial with linear – lanceolate leaves. Small (3mm), solitary, white to pinkish flowers are apparent in leaf axils from June – August in the UK, with four petals and four shorter sepals. It is not thought to produce viable seed in the UK. The plant is a member of the Crassulaceae family, one of a select group able to assimilate CO2 within tissue as an organic acid; known as the C4 pathway. This allows them to reduce water loss during the day through stomatal closure, giving a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures and nitrogen or CO2 limitation. This trait, combined with prolific vegetative reproductive ability is a major factor contributing to the invasive success of Crassula helmsii in the UK.
Range
C. helmsii was reportedly first discovered in the wild in the 1950s in an Essex pond and spread throughout the majority of England, with the South East most affected. It is also found in parts of Wales, Northern Ireland and as far north as Alderney in Scotland.
Control
Control measures have been met with varying amounts of success; the only chemicals permitted by the Environment Agency for use in or adjacent to water systems are glyphosate and 2,4-D amine (Depitox). Numerous novel treatment methods and bio-control systems have been attempted ranging from light exclusion and predation by fish to submersion in hot biodegradable foam. Chemical treatment is only effective on emergent material and only then when the plant is reasonably dry, a situation difficult to encounter when dealing with aquatic plants. Treatment such as this is widely found to have a relatively low success rate (around 50%). The most successful, but labour intensive method of eradication appears to be smothering / light exclusion, which achieves up to a 100% mortality rate, dependant on temporal application of the treatment. Mechanical removal of live material is profoundly dissuaded, due to significant viability of tiny plant fragments to initiate entire re-infestations. However physical removal of treated and decaying Crassula is essential to maintain oxygen levels and limit further damage to aquatic life.
Legislation
In June 2005, C. helmsii was added to Part II of Schedule 9 in the 1981 Wildlife and Countryside Act in Scotland and pressure is mounting to apply this legislation UK wide. Conservation organisation Plantlife are lobbying strongly for numerous such invasives to be banned from commercial sale and make alternative less damaging suggestions for aquatic oxygenating and ornamental plants.
Index
C3 pathway – The usual metabolic pathway for carbon fixation in plant photosynthesis.
C4 pathway – An alternative method of carbon fixation in photosynthesis involving the storage of CO2 as an intermediate compound before its use in the process.
Lanceolate – Leaf shape; Lance-shaped, long and wider in the middle
Linear - Leaf shape; Long and very narrow
Sepals – From the Latin separatus "separate" and petalum "petal", collectively called the calyx. Sepals are generally green and lie under more conspicuous petals.
Stomata - Tiny pore, mostly on the underside of the leaf, used for gas exchange and through which water is lost.
Bibliography / further reading
Blamey, Fitter and Fitter (2003). Wild Flowers of Britain and Ireland. A. C. Black. London.
Bridge T. (2005) Controlling New Zealand pygmyweed Crassula helmsii using hot foam, herbicide and burying at old Moor RSPB Reserve, South Yorkshire, England. Conservation Evidence. (2) 33-34.
Schedule 9, Wildlife and Countryside Act 1981, Joint Nature Conservation Committee - www.jncc.gov.uk
www.plantlife.org.uk
Dr Emma Morley, July 2007
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