Australian Swamp Stonecrop
Australian Swamp Stonecrop - Crassula helmsii - was introduced to Britain in 1911 from Tasmania. It was first sold as an “oxygenating plant” in 1927 by Perry’s Hardy Plant Farm in Enfield. The first recorded natural occurrence was at Greensted Pond in Essex in 1956. In recent years it has spread much more widely and rapidly due to the increased availability of the plant at Garden Centres and Aquatic Nurseries.
The plant will grow around the damp margins of ponds and in water up to 3m deep. It forms very dense stands. It first appears as a small light green tussock on the sediment. These tussocks grow and spread rapidly to form a dense mat of vegetation. The dense mat out-competes all other aquatic vegetation, eliminates native flora and creates a poorer ecosystem for invertebrates and fish. Severe oxygen depletion can occur below dense growths of this plant. The plant assimilates CO2 for 20 hours of the day when submerged due to the possession of crassulacean acid metabolism and grows throughout the year. There is no dormant period.
Control
Mechanical control - Cutting and removal of the vegetation can be done with our amphibious Truxor. Care is taken to prevent further infestations downstream.
Chemical control - Once the dense mat of vegetation has been removed a follow-up spray of approved herbicde can be applied
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.
Biological control - There are no known biological control agents for this plant in this country. It will be eaten by grass carp if the infestation is small and not well developed, but dense infestations cause severe fluctuations in dissolved oxygen content of the water and the fish do not usually survive.
Environmental control - Australian Swamp Stonecrop C. helmsii is tolerant of shade for long periods. It is frost tolerant, desiccation tolerant and cannot easily be controlled by any method of environmental control. Covering with black plastic or carpet can effectively eliminate small patches, but the shade material should remain in place for at least 8 weeks, and preferably for 6 months.