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

Floating pennywort

Fact file

Floating pennywort (Hydrocotyle ranunculoides) is a native of North America. It was first brought to Britain in the 1980s as a plant for tropical aquaria and garden ponds, and was first noted in the wild in Essex in 1991.

H. ranunculoides grows in the shallow margins of slow-flowing eutrophic water bodies (particularly ditches, slow flowing dykes and lakes), and forms dense interwoven mats of vegetation. These quickly cover the water surface interfering with both the ecology and amenity uses of the water body. These mats grow up to 15m from the bank in a single season, with stem growth rates of up to 20cm per day.

H. ranunculoides roots freely from nodes at approximately 40-60mm intervals. The roots are profuse and hair-like. The leaves are emergent, rising on stalks from horizontally growing stems. Both the stem and the petioles are fleshy. The leaf form ranges from circular to kidneyshaped; they are deeply lobed, and up to 180mm across. Leaves are held above the water surface whilst the interwoven mat of roots and stems sink up to 50cm into the water.

Reproduction in Britain is thought to be principally vegetative, and the plant is capable of forming extensive mats from the smallest shoot fragment. Introduction by seed, however, may also have occurred. Floating pennywort can double its wet weight in as little as three days. The plant exhibits seasonally variable growth in Britain. Maximum growth occurs in the late summer when it typically forms the extensive floating mats of vegetation, whilst it over-winters in the margins and on banks as a much flatter and smaller plant.

Source: Biological Records Centre

The plant is relatively restricted in its distribution, largely in southern England and south Wales. Its appearance at the 90 sites so far reported is likely to have been as a result of escapes from aquaria and garden ponds. H. ranunculoides has already proved to be difficult to control because of its rapid growth rates, its ability to re-grow from a single node, and its resistance to chemical control.

Control

Chemical control can be achieved with herbicides containing 2,4-D amine and diquat. Glyphosate is less effective.

Cutting and removal is a very good method of management, but it will not control or reduce the vigour of the plant. The cut or dredged material should be left on site at the top of the bank, well away from water.

Non-chemical control

Cutting

Regular cutting from May to October will prevent complete dominance of this species. Cut material should be removed from the water immediately. Cutting should be followed by hand pulling or by spot treatment with chemicals to reduce the risk of regrowth.

Pulling or dredging

Hand pulling works very well in small infestations and as a follow-up after major mechanical removal. Eradication is possible using this technique.

Grazing

Cattle grazing has been seen to damage the emergent stems, but it has no long-term effect on the dominance of the plant. There are no known biological control agents.

 

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Chemical control

Glyphosate

2,4-D amine

In general

Applying glyphosate at 6 litres product/ha in 400 litres of water is the most effective treatment with this chemical. Repeat treatments will be necessary throughout the growing season as soon as regrowth occurs. The use of wetting agents to increase effectiveness is currently being investigated.

Applying 2,4-D amine at 4.5 kg active ingredient in 200 litres/ha of water per hectare provides temporary control. Re-treatment after exactly three weeks is required for more effective control.

The plant does not rot down very quickly after chemical treatment, and treated vegetation in flood-risk areas should be removed after two to three weeks if possible. Follow-up spot treatment after mechanical removal is recommended. Regular treatment is necessary.

 

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