Pigmyweed (Crassula helmsii), also known as Australian swamp stonecrop or New Zealand pygmyweed, is a succulent plant native to Australia and New Zealand. This aquatic or semi-terrestrial species is highly invasive, posing a substantial threat to freshwater ecosystems globally by thriving in diverse aquatic environments.
Identifying Pigmyweed
Pigmyweed features stiff shoots with narrow, parallel-sided leaves arranged in opposite pairs along the stem. These succulent leaves measure between 4 and 24 millimeters in length.
Pigmyweed exhibits various growth forms. It can grow terrestrially with creeping or erect stems and yellowish-green aerial leaves, or as an emergent form with dense, short stems in shallow water. It also thrives submerged, forming long, sparsely leaved stems that reach the water surface from a rooted base. Small white flowers with four petals may appear in summer, produced on stalks extending from the upper leaf axils, always above the water.
This plant is commonly found in still or slow-moving freshwater bodies such as ponds, lakes, ditches, and their muddy margins. It forms dense mats that can completely cover the water surface or carpet the surrounding land.
Why Pigmyweed is a Threat
Pigmyweed poses a severe environmental threat due to its aggressive growth and ability to dominate aquatic ecosystems. It rapidly forms dense, impenetrable mats that outcompete native aquatic plants for light and nutrients. This overgrowth significantly reduces biodiversity by displacing indigenous species, leading to a decline in the natural value of water bodies.
The dense mats alter water chemistry and deplete oxygen levels beneath the surface. This oxygen depletion harms aquatic life, including fish and invertebrates, potentially leading to fish kills. Extensive growth can also impede water flow, causing drainage issues and increasing the risk of flooding. Its presence reduces the aesthetic and recreational value of waterways, making activities like fishing and boating difficult.
Controlling and Managing Pigmyweed
Controlling pigmyweed infestations is challenging due to its resilience and rapid spread. Manual removal requires extreme care to ensure all fragments are collected. Even a tiny piece, as small as half a centimeter containing a single node, can regenerate into a new plant, making complete eradication difficult. Proper disposal of removed plant material, away from any water bodies, is important to prevent re-infestation.
Chemical treatments are another option, often requiring professional application due to environmental considerations and the need for specific aquatic herbicides. These treatments can be costly and may have broader environmental impacts if not applied correctly. Biological control methods are also being investigated, focusing on specific mites, such as Aculus crassulae, which can cause abnormal growth and swelling in the plant’s shoots. This approach aims to reduce the plant’s growth and reproductive success.
Preventing Pigmyweed Spread
Preventing pigmyweed spread is paramount, as its fragments can easily hitchhike to new locations. Strict biosecurity practices minimize its dispersal. This includes thoroughly cleaning equipment such as boats, fishing gear, and gardening tools after use in any aquatic environment.
Inspect footwear and clothing for any plant fragments before leaving an infested area, as these can easily transport the plant. Responsible disposal of plant material is critical; never discard aquatic plants into natural water bodies. Finally, always source aquatic plants from reputable suppliers and avoid introducing wild plants into new areas, as this significantly reduces the risk of accidental introduction.