Invasive weeds: Guidance for the control of invasive weeds in or near fresh water

Australian swamp stonecrop

Fact file

Australian swamp stonecrop (Crassula helmsii) was introduced from Tasmania to Britain in 1911. It was first sold as an oxygenating plant in 1927.

The first occurrence in the wild was reported in Essex in 1956. In recent years, it has spread much more rapidly due to the increased availability of the plant at garden centres and aquatic nurseries. It has been recorded at about 1,500 sites although this is probably an underestimate of the true distribution. It is sometimes referred to as Tillaea recurva, Tillaea helmsii, or New Zealand pigmy weed.

The plant is easily dispersed and, although not always sold by suppliers, it is often found as a contaminant with other water plants. Introductions to new sites are associated with a wide range of human, water-based activities, and awareness and education programmes can dramatically reduce transport of the plant between sites. Local dispersal is aided by the high viability of small fragments, which can be carried on mud to new sites.

The success of Crassula lies mainly in its ability to colonise virtually any suitable static-to very-slow-flowing freshwater habitat across a wide range of water chemistry. It has vigorous, year-round growth, and can grow equally well either on damp ground or in water up to 3m deep.

Source: Biological Records Centre

Where Crassula invades, it quickly out-competes native vegetation, and maintains its dominance by very rapid growth and uptake of almost all the available nutrients.

There are three typical growth forms: (i) a terrestrial form with creeping stems and aerial, succulent leaves; (ii) an emergent form with densely packed stems, found in water less than 0.6m deep; (iii) and a submerged form that grows from a basal rosette with long, sparsely-leaved stems reaching the surface. The three forms change according to prevailing conditions. The rigid stems have pairs of fleshy leaves that vary in shape from long and narrow in deeper water to slightly elliptical, with sharp or bluntish tips in air. The leaf tip is never notched, which distinguishes it from the native water starwort (Callitriche spp.). The leaf bases are joined, forming a distinctive 1mm collar around the stem. In summer, white flowers grow in the axils of the leaves on emergent and terrestrial forms.

Control

This plant is best treated at the early stages of infestation. Delay will make the problem several orders of magnitude worse in each successive year.

Chemical control of submerged material with diquat (which is permitted only until June 2004) and emergent material with glyphosate are the best options.

Cutting is not recommended, but dredging out marginal and emergent material can be effective, as the plant is shallow-rooted. The area around any infestation should be fenced to prevent movement of fragments by livestock. Dredged material should be piled in heaps and covered with thick black polythene sheeting or at least 20cm of soil.

Shading of terrestrial or emergent forms with opaque material such as black polythene for about three months may be effective, but is difficult to install and manage, and vandalism can be a problem.

Non-chemical control

Cutting

Not recommended.

Dredging

Dredging of marginal and emergent material throughout the year can be effective, although it is necessary to ensure that plant fragments cannot be transported elsewhere.

Shading

Covering with black polythene or similar for up to 10 weeks during the growing season.

CONTACT THE ENVIRONMENT AGENCY FOR DISPOSAL ADVICE

Chemical control

Emergent

Submerged

In general

Glyphosate

Application of glyphosate at 6 litres/ha to emergent stands from March to October. Regular treatment is required, and at least two applications may be necessary each year.

Submerged material can be treated with dichlobenil in March, or with diquat throughout the year. Submerged material will also die if connected emergent vegetation has been sprayed with glyphosate.

Regular treatment is necessary. Weed wiping may be appropriate in mixed marginal vegetation. Spot treatment of small patches will prevent complete dominance. Treat early and regularly.

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