Common names include
Australian Swamp Stonecrop; New Zealand Pygmy-weed, Crassula.
Australian Swamp Stonecrop, Crassula helmsii, is an aquatic or semi-terrestrial species of succulent plant, first introduced to Britain from Tasmania in 1911. It was subsequently dispersed by the nursery trade, first being sold as an oxygenating plant in 1927. Having been observed naturalised at the comparatively late date of 1956, it has colonised rapidly, mainly due to its widespread availability at garden and aquatic centres. It was known be naturalised at 750 aquatic sites in 1999, and suggestions are that it may now be found in as many as 2000 locations.
The UK experience of this plant seems far worse than in its Australasian native range, or the numerous Europaean localities where it occurs. Only in parts of the south eastern USA has its potential for invasiveness begun to show any similarity to that which occurs here.
It is a satisfyingly classic invasive plant, comfortably ticking all the boxes described in the section "Weeds and Invasive Plants", It has, furthermore, one of the worst capacities found in invasive plants, in that it is able to radically alter and destroy the habitats in which it is introduced, by means of adapting its form to the conditions it itself creates.
As in the instance of Floating Pennywort, the case for including Swamp Stonecrop in Schedule 9, Section 14 of the Wildlife and Countryside Act, is being strongly pursued. These provisions, whilst making it illegal to plant or cause the plant to grow in the wild, would, in the long run, also be likely to end any further commercial sale.
Australian Swamp Stonecrop is an aquatic to semi-terrestrial evergreen perennial. Stems are coloured yellowish-green, and are of round section. In water of greater than 60cm depth, the stems are thin and barely succulent with long internodes. As water depth decreases, the stems become more succulent and branched, with internodal lengths shortening. Succulence and branching with decreasing internodal length continues as the plants become emergent and then terrestrial. The root basal also forms at the nodes. Its leaves are pale green, in opposite, sessile or connate pairs, arranged alternately. They are linear to oval in shape, becoming increasingly fleshy in shallow water or in emergent and terrestrial forms. Sizes range from 4 to 24mm in length and 0.7 to 1.6mm in width. Connate pairs of leaves form a collar surrounding the stem. Flowers are axillary, white or pinkish, 3 to 3.5mm in diameter, and are produced from June to September. Fruits contain 2 to 5 elliptical, smooth seeds about 0.5mm long. (Seeds are not thought to be produced in the UK.)
The ability to move from aquatic to terrestrial life is facilitated by its metabolic twist. This is known as 'Crassulacean Metabolism'. Most plants absorb Carbon Dioxide through the stomata on the leaves during the hours of daylight, so that the light-dependant energy manufacturing process known as photosynthesis can take place. In a number of Xerophytic (drought adapted) genera, including desert species of Crassula, the stomata remain closed during the day, so that water loss is minimised.
Instead, they open by night to admit and exhaust all the gases required or produced by plant's metabolic processes. The carbon dioxide absorbed is converted to Malic acid, and in this form is stored until dawn, when it is released into the plant's tissues again to take part in photosynthesis.
The results of this remarkable adaptation are twofold. In the first instance we find an aquatic plant able, if needed, to resist hot sun on land by using a mechanism evolved in desert plants. In the second, we find that our Swamp Stonecrop is able to carry on the business of CO2 assimilation for 20 hours per day, throughout the year. This contributes to its super-rapid growth and ability to develop the vast biomass, which smothers all competitors.
This remarkable plant shows adaptability to a great range or environmental conditions, has a variety of strategies in ensuring vegetative reproduction, and adds to this a metabolic twist, inherited from its Xerophytic cousins.
It grows in water up to 3 metres deep; it grows in shallow water; it grows on damp ground. The water can be alkaline, acid, or even slightly salty. Temperature also does not bother it. It grows in its current world-range in areas with a mean temperature range of minus 6°C to plus 30°C, which indicates that the actual temperatures experienced will be greater than either extreme.
Although viable seed is not thought to be produced in the UK, a single node and stem fragment of 5mm length is sufficient to root in the bottom sediment of still or slow-flowing water bodies of up to 3 metres depth. This propagule instigates a colony of stems, at first submerged, but later becoming emergent and covering the surface of the water body with an impenetrable mass of succulent, evergreen foliage. As no period of quiescence occurs during winter, and growth continues through most of the year, everything below this vegetable mattress dies from lack of light and oxygen. As the stems grow upwards they form a huge transpiring sponge, which in ponds lowers the water level, eventually destroying the pond itself. On the boggy to damp ground so produced, the plants adopt their terrestrial form and continue to spread.
In a similar manner again to Floating Pennywort, where open water is covered, the plants pose a risk of drowning to grazing livestock, and indeed to children.
Control, to the point of eradication, has not yet been achieved at any site where the plant has become dominant. Partially effective herbicides include Diquat, which no longer has UK approval; Glyphosate; and the granular herbicide Dichlobenil. In all cases, because of the large quantities of vegetation involved, the amount treated at any one time has to take into account the balance between risking re-colonization and doing even more environmental damage, through de-oxygenation, as killed material Dredging or other methods or mechanical removal have been advocated, though all mechanical methods carry an enhanced risk of rapid re-colonization or even of further spreading of the problem. Removal is probably best considered as a precursor to chemical treatment.
Small infestations are susceptible to periods of light exclusion (usually by covering with heavy black plastic sheeting), which should preferably be continued for 6 months.
Grass Carp will attack small invasions, but are no good in established colonies as fluctuating oxygen levels result in the death of the fish.
No biological control methods for Swamp Stonecrop have yet been proposed, and all other methods, which must by necessity involve some disturbance to the colony, should be preceded by surrounding the infestation with a fence of 5mm mesh wire to avoid fragments spreading to open water.
Cutting should not be used as a mechanical method, and though dredging to reduce biomass prior to spraying used to be employed, the risks of dispersal during removal of material, with fragments clinging to footwear or in mud on vehicle tyres may make this approach too risky.
Two herbicides are still approved for use in the UK, though both are non-selective and harmful to desirable species. Such damage is, however, usually reversible from the native seed-bank, provided that inefficient or poorly thought out applications do not lead to rapid re-colonization by the pest.
For submerged infestations, Dichlobenil granules should be employed during the early part of the growing season during February and March.
For emergent or terrestrial colonies, Glyphosate should be applied at a rate of 61trs/ha between March and October. It is necessary for the plant material sprayed to be dry in order for the Glyphosate to be absorbed and translocated. It must be remembered that Glyphosate, unlike the now proscribed Diquat, does not work at all if actually applied in water or to wet material.
Translocation through thick turves of emergent or terrestrial plants is likely to be only partial, and surviving portions can easily break off and float to fresh areas, should the mesh barrier not be used. Whilst being aware that large masses of dead plant material in a water body can lead to further deoxygenation, at least 70% of a large colony should be treated at once to avoid spread back from the untreated area. This remaining 30% should be treated after one week, followed by removal of all dead plant matter about three weeks later.
The only environmental control found to be helpful in small infestations is to exclude light using black polythene sheeting or geotextile. Such cover must be kept on for at least 8-10 weeks, with 6 months being desirable. back to top