Invasive Species Aquatic Plants

Australian Swamp Stonecrop (New Zealand Pygmyweed) - Crassula helmsii

Description and Identification
Crassula helmsii appears as small light green tussocks on the sediment, spreading rapidly to form dense, Crassula helmsii - small leaved flowering plantinterwoven mats that cover other vegetation. It is found round damp pond margins and in water to 3m depth. It is a perennial and grows throughout the year. (All images: Peter Spurrier)

There are three typical growth forms: a terrestrial form with creeping or erect stems and aerial leaves which are mid- to yellowish-green in colour and succulent in appearance; an emergent form usually growing as stands of short densely-packed stems in water of 0.6 m or less in depth; and, a submerged form which grows from a basal rosette with long sparsely-leaved stems which may reach the water surface. The emergent and terrestrial forms are flowering. When it is creeping, roots form at the nodes. Leaves on the submerged form are thin and barely succulent.

The stems of Crassula helmsii have pairs of fleshy unstalked opposite leaves (4-24 mm in length) borne alternately on rigid stems. The leaf bases are joined giving a I mm collar which is a distinctive characteristic. The leaf shape is simple and varies from a long narrow near-parallel form, to very slightly elliptical with sharp or bluntish tip. This leaf tip distinguishes the underwater form of the plant from most Starworts which have obliviously notched leaf tips.

The flowers have short stalks and arise singly at the base of the leaves, they are four petalled, white or occasionally pale pink, with filaments and black anthers and appear between July-September.
In water depths of less than 0.5m, growth becomes dense and more branching, with stems becoming emergent during late spring and summer, either by plant growth or a decrease in water level. In these conditions the plant forms a very dense mat of sprawling stems from which many vertically growing side shoots with succulent leaves are produced, producing a turf which is very effective at excluding growth of other species.

Growth on damp mud at water edges is restricted to dense mats, which are not as erect as those in water.

Currently in Britain, C.helmsii colonises static or slow flowing systems.

In Europe two other species growing in damp muddy and sandy places on acid soils that are flooded in winter could be confused with the terrestrial form of C. helmsii. Northern Water Stonecrop, C. aquatica is annual, with pointed linear leaves and solitary, unstalked, white flowers. Southern Water Stonecrop, C. vaillantii differs from the northern species in its blunt-tipped leaves and forking clusters of stalked flowers (Fitter & Mannuel 1994). Submerged growth C. helmsii can also be mistaken for some Callitriche species especially C. hamulata, however Crassula never has notched leaf tips.

C. helmsii may be sold in garden centres, where it is sometimes labelled as Crassula recurva, Tillaea recurva or Tillaea helmsii,

Problems caused
C. helmsii causes major problems in freshwater aquatic habitats. It forms a mat over other vegetation, out-competing and killing submerged plants, causing a poor deoxygenated environment for invertebrates and fish and choking ponds and ditches. Severe oxygen depletion can occur beneath dense growths. It has spread rapidly since it was first introduced in the 1950s. It has driven rare species to extinction in the UK and is threatening others. (Image: Crassula helmsii and Starwort)

Crassula assimilates CO2 for 20 hours a day when submerged, and grows throughout the year. It readily reproduces vegetatively (it doesn’t produce viable seed in Britain). It is most easily spread by fragments, as small as 5mm. In autumn, it also releases short shoots called turions, which are also effective at spreading it in water bodies.

Crassula has not so far been eliminated from any natural site, once it has dominated it. It grows most strongly in nutrient-rich sites.

Crassula may be sold as an oxygenator and deliberately introduced, or it may be a contaminant attached to other plants.

Other vector routes for the expansion and invasion of the species include:
i) Passive transfer of plant fragments via human activities e.g. transfer on fishing nets, during transfer of fish, disposal of aquaria and pond contents, on boat propellers, and during ecological survey and management work.
ii) Passive transfer by movement of wildlife.
iii) Passive drift along canals and drainage channels.

N.B. Extreme care should be taken when visiting waterbodies infested with C. helmsii to ensure that plant fragments are not transferred to other uncontaminated sites.

C. helsii may have taken over about 8,000 sites in the UK. It is reported to have roughly doubled the number of infested sites about once every two years.
It tolerates acid, alkaline conditions and has been recorded in semi-saline conditions.

In Essex it has been reported in the following locations
• Weeley,
• Howlands Marsh, St. Osyth.
• Possibly Bellhouse landfill at Colchester, TL949228
• Aldham
• West Bergholt
• Old Hall Marshes, large infestation.
• Fingringhoe, large areas at Fingrinhoe Wick nature reserve
• Pond on village green at Layer de la Haye (TL975 200).
• Pond on Tiptree Heath SSSI
• Reported at one point on Chelmer Navigation, approx TQ780092
• Large areas at Asheldham Pits (TL973 017).
• Several sites near Harlow & in the River Stort corridor (in particular in the Harlow Marshes LNR)
• Parndon Wood Nature Reserve, approx. TL 446068.
• Parndon Moat Marsh, Harlow Marshes LNR is a concern as it is on a site with Desmoulin's Whorl Snail.
• In Basildon District.
• In Brentwood District.
• At Belhus Woods Country Park, near Aveley; present in car park pond isolated beds, Hunts Hill lake in great quantity throughout, there were a couple of small areas on the first lake but not seen recently.

It may be widespread due to pondkeepers dumping surplus.
The National Biodiversity Network; showed it was present in 45 of 59 10km squares in Essex.

Control Measures
Control is extremely difficult.

Once Crassula helmsii has been identified at a site, any associated plants or animals should also be identified to enable safeguarding important or vulnerable species. For help with identification, see contact addresses at end of page and to whom confirmed occurrences should be notified.

The prime objective is to eliminate Crassula helmsii from sites which it has invaded. This contrasts with the normal use of herbicides which try to significantly reduce ie. control, but not to eliminate, the target species.
Early and effective treatment saves effort and preserves native species. The natural seed bank of native species should be considered a resource for their re-establishment. (Image: Typical view of Crassula helmsii)

Inadequate or incomplete control favours the rapid re-establishment of C. helmsii and results in further reduction of native species. Allowing the plant to re-grow to previous densities before further treatment is also self-defeating.

Non-chemical means are favoured in preference to chemical treatment in sites with significant nature conservation interest. Manual physical removal can be used as a precursor to other methods in order to reduce the major part of the plant biomass, but great care should be taken to prevent the production and spread of fragments in the process.

The choice of method depends primarily on the growth form and extent of the Crassula helmsii stand but also the extent and importance of remaining native plant populations within the site. At sites with large stands (> 1000 m2 or a quarter of the water body), there will be no practical alternative to chemical control although the combined approach of the initial use of shade material or physical removal may reduce the biomass and thus the total chemical loading to the site.

Mechanical Control

It is advised not to use mechanical control because the fragments that are produced by cutting and tearing can regrow and spread the infestation downstream or re-infest the treated area. Fragments as small as one node (5mm) can regrow. However, it has been suggested that 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.

Mechanical removal of dead plant material that has been treated with herbicides is recommended to reduce oxygen depletion by decomposing plant material.

Chemical control

C. helmsii is susceptible to formulations containing glyphosate. Also, dichlobenil as ‘Midstream GSR’ can be used in February or March when the plant is still completely submerged.

It is recommended that at least 70% of dense infestations is treated at one time to reduce recolonisation from untreated areas. Treatment of the remaining 30% should be carried out after one week. The dead material should be removed two to three weeks after treatment if possible, but can remain in the water if necessary.

Glyphosate can be applied to any emergent material, either on the bank or in the water, as long as it is dry. Only formulations of glyphosate which are specifically recommended for use in aquatic situations should be used. Re-treatment after an application is not normally necessary except to treat parts that were missed the first time. Glyphosate should be applied from April to the end of November, when the majority of the plant is emergent.

Apply glyphosate at the rate specified by the manufacturer (see product label) to terrestrial and dry parts of emergent stands. The herbicide is ineffective if used in water. Glyphosate will however be taken up by the dry emergent parts of the plant and moved through the remainder of the plant even if it is growing in water.

For best results with glyphosate, this herbicide should be applied at the first sign of invasion of a bank or shore line, while the leaves are still of the aquatic type and the plant density is Iow. At the turf or high density stage, herbicide will kill the surface layers of plants but regrowth will occur.

Glyphosate becomes less effective below 12 C and below 8 C the uptake of the herbicide may be insufficient for the level for control, but is worthwhile taking advantage of the dornant period of many native plants.

When using ‘Midstream’, this should be done at least twice per growing season at intervals of between 3 and 5 weeks. Treatment should take account of the effect it will have on ANY green vegetation.

Crassula helmsii needs to be "hit hard" at the first signs of re-growth as only then will the plant be weakened. Allowing any re-growth will not only encourage C. helmsii to re-dominate, but will discourage the re-growth of native species and lead to their reduction or loss. One application of herbicide or one physical removal exercise, is very unlikely to eliminate all the C. helmsii at a site and repeat treatments for at least three applications with a minimum interval of three weeks between each treatment during the growing season (check product label) may be needed to eradicate the plant. Re-growth in small areas is probably best 'spot-treated' with herbicide. An effective control plan is likely to include two general applications of herbicide and three spot treatments undertaken over two seasons.

Environment Agency consent is required for any herbicide use in or near water bodies. Site access should be restricted during the application of herbicides, especially if grazing stock are present or the site is open to the public; an informative notice should be displayed. 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.

Generally, pesticides may only legally be applied by operators (or by persons in their direct control), who have an appropriate certificate of competence issued by the National Proficiency Test Council for Agriculture and Horticulture. This is under the Control of Pesticide Regulations 1986, and in accordance with the appropriate Code of Practice under the Food and Environmental Protection Act 1985. Site owners or controllers do not need a certificate of competence to treat their own land.

Care should be taken to reduce further dispersal as a result of the herbicide application because viable fragments are formed as the stems die back towards the tip. These break-away and in more open water may be rapidly moved around the water body to colonise new areas. The risk of spread can be greatly reduced by the use of a fence of fine wire mesh (5 mm) to enclose the area to be treated. This area should be about twice as large as the stand of Crassula helmsii to allow for the inclusion of areas in which the plant has probably already spread but is not yet easily visible.

Biological control

No known biological control agents exist in the UK. Grass Carp will eat small infestations that are not well developed, but dense infestations cause severe fluctuations in dissolved Oxygen content of the water, and subsequent fish mortality.

Manual removal

Clearance by hand allows selective removal of plants when Crassula helmsii 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.

Environmental control

C.helmsii is frost tolerant, dessication tolerant and cannot be easily controlled by any method of environmental control. It is tolerant of shade for long periods. Covering with opaque material, such as, black polythene, e.g. rick sheeting, old carpeting or geotextiles can effectively eliminate small patches, but the shade material should remain in place for at least 8 weeks and preferably 6 months. Large pieces of material in deeper water should be anchored by weights at the corners. 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.

Experiments are in progress at Old Hall Marshes to see if it can be effectively controlled by immersing it in sea water.

General points re control
It is important that the spread of Crassula helmsii should be controlled and that its growth should be halted as soon as possible after discovery and confirmation of identity. Several treatments may be necessary to control dense stands of this plant.

Fragments should be removed from footwear and other equipment, e.g. spades, excavator buckets before leaving a site.

Any fine wire-mesh fence placed around the stand should not be removed until all regrowth has been eliminated.

Sites should be monitored regularly at intervals of 3-6 months for at least 5 years following any apparent elimination of C. helmsii. Treated and adjacent areas must be carefully examined for developing shoots or small buried rhizomes.

These guidelines will continue to be amended to incorporate new information as it becomes available. Circulated comments on successes or failures are therefore important.

Actions the public can take
Recording the spread of Crassula helmsii and monitoring the attempts at eradication will be an important element in the action of suppressing its continued spread. It is therefore requested that any new occurrences are registered with either the CEH, EN or the Biological Records Centre (see below) and that any observations regarding control methods are reported.

  • Do not plant this species.
  • Do not put it into ponds, rivers, streams or ditches.
  • Do not inadvertently spread fragments from one site to another.
  • Do not use herbicides in water without the approval of the Environment Agency.

Further information
Contacts for identification, technical advice, reports of control and notification of occurrences.
Dr. F.H.Dawson, Centre for Ecology and Hydrology – Dorset, Winfrith Technology Centre, Dorchester, Dorset DT2 8ZD Their advice leaflet can be downloaded from here