Cape Province Pygmyweed, or Crassula helmsii, is an aquatic and semi-terrestrial succulent perennial herb. It is also recognized by common names like Australian swamp stonecrop and New Zealand pygmyweed.
What is Cape Province Pygmyweed?
Crassula helmsii is a succulent perennial herb native to Australia and New Zealand, known for its adaptable growth forms. Its stems are round, ranging from 10 to 130 cm, and can grow floating, creeping, or erect, rooting at the nodes. Leaves are small, sessile, and succulent, measuring approximately 4 to 20 mm long and 0.7 to 1.6 mm wide, with acute tips.
The plant’s appearance changes with its environment. Terrestrial forms have creeping or erect stems with yellowish-green aerial leaves. Emergent forms show short, densely packed stems in water up to 0.6 meters deep, while submerged forms grow from a basal rosette with long, sparsely leaved stems reaching the surface. Flowers are small and typically inconspicuous, white or pinkish, with four petals, borne singly in leaf axils.
Where Does It Grow?
Crassula helmsii is an invasive species in many parts of the world, particularly in Europe, the United Kingdom, Australia, New Zealand, and the southeastern United States. It thrives in a wide range of aquatic and semi-aquatic environments.
Preferred habitats include ponds, lakes, rivers, canals, ditches, and wetlands. It adapts to different water depths, growing submerged in waters up to 3 meters deep or as an emergent plant on damp ground. The plant tolerates various substrate types and colonizes both acid and alkaline freshwater habitats.
Why Is It a Problem?
Crassula helmsii forms dense mats that significantly alter aquatic ecosystems. These mats can cover the entire water surface, outcompeting native aquatic vegetation by blocking sunlight and monopolizing resources. This reduces local biodiversity and leads to the loss of native plant species.
Its dense growth also impacts water quality and flow. It depletes oxygen levels, harming fish and other aquatic organisms. The thick mats impede water flow in rivers and canals, increasing flood risks. Recreational activities like boating, fishing, and swimming are also hindered.
The plant’s rapid growth and efficient spread contribute to its problematic nature. It primarily reproduces vegetatively through fragmentation; small pieces can break off and root to form new plants. These fragments are easily transported by water currents, wildlife, or human activities, including contaminated equipment. While less common in its introduced range, the plant can also produce apical turions, which are specialized buds that float and establish new growth.
Managing Cape Province Pygmyweed
Managing Crassula helmsii is complex due to its ability to regrow from small fragments. Manual removal, like hand pulling, is generally ineffective as any remaining fragments quickly re-establish the plant. Mechanical removal can reduce plant biomass, but it carries a risk of spreading fragments, potentially worsening the infestation. If mechanical removal is attempted, strict biosecurity protocols for cleaning equipment and containing plant material are necessary.
Chemical control involves the careful application of herbicides. Glyphosate is used for emergent or terrestrial growth, and dichlobenil for submerged plants; both are effective treatments. These treatments require specific aquatic formulations and careful timing to maximize effectiveness and minimize harm to non-target species. Combining herbicide treatment with methods like covering treated areas with black plastic or UV sheeting for several months can improve success by blocking light.
Biological control research has identified potential agents, such as the mite Aculus crassulae, which can reduce plant growth and reproductive success. However, biological control is still being investigated and has had limited success in widespread eradication efforts. Prevention is a key strategy, emphasizing early detection and rapid response to new infestations. Proper disposal of plant material, such as composting or burning away from water bodies, helps prevent further spread. Eradication is challenging once Crassula helmsii is established, highlighting the importance of preventing its introduction and spread.