New Zealand Pigmyweed (Crassula helmsii) is also known as New Zealand Pigmyweed. It was widely sold by garden centres and aquatic centres for its excellent oxygenating properties. However once it had escaped into the environment it has proven to be a problematic non-native invasive species and since April 2014 has been banned from sale.

Spread

Natural dispersal occurs by the transportation of vegetative fragments and the production of special short shoots called turions which break off and disperse by water currents. Fragments can also be transported by birds, other wildlife and livestock and can be unintentionally transported by humans on boots, angling nets, boats, heavy plant, vehicles and other recreational equipment.

Environment

New Zealand Pigmyweed is suited to a wide range of slow moving freshwater systems and forms dense mats of vegetation year-round. It out-competes our native plants by depleting the oxygen levels in water, killing submerged vegetation and suppressing the germination of other native species. The resulting deoxygenated water has an adverse effect on invertebrates and fish.

Effects on Humans

New Zealand Pigmyweed is not harmful to humans but dense mats can be mistaken as dry land and therefore presents a potential hazard to people, dogs and livestock when present on public access sites.

Boating

The dense mats of vegetation can reduce the flow of water along waterways, increasing the risk of flooding and can impact the tourism and recreation of a waterway by preventing the passage of boats and making it impossible for anglers to fish.

New Zealand Pigmyweed is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild.

Mechanical control is difficult and usually not recommended as it can result in small fragments being left in the water, which may spread the plant downstream.  Fragments as small as one node (5mm) can regrow. However, the risk of spread can be reduced by the use of a fence of fine wire mesh (5 mm) to enclose the area to be treated.

Shading out with dark-plastic sheeting has been successful with black polythene covering the plant for at least three months during the growing season, although this will have adverse effects on other species covered up. Shade materials may be laborious to install and move on a regular basis; vandalism may also be a problem at sites open to the public.

New Zealand Pigmyweed is susceptible to formulations containing glyphosate.  However these applications must be made by a certified operator and Environment Agency consent is required for any herbicide use in or near water bodies.  If the site is in or near a conservation site, such as, a Site of Special Scientific Interest, then the control strategy must be discussed with the local Natural England office.

Manual removal by hand allows selective removal of plants when New Zealand Pigmyweed is in the early stages of invading a diverse plant community but it is difficult and very laborious to be effective. Removed plant material should not be taken away from the site but should be stacked and composted under a secured light-proof cover, such as a thick sheeting and/or 0.2 m of soil. Machinery, boots and equipment should be thoroughly cleaned before leaving each site.