Crassula helmsii aka New Zealand Pigmyweed

Crassula helmsii is an aquatic plant from Australia and New Zealand. It belongs to the stonecrop family, Crassulaceae. Its common names include Australian swamp stonecrop and New Zealand pygmy weed. Swamp stonecrop is another name used for this species. Older records may list it as Crassula recurva. Kew places its native range in a subtropical climate. The RHS records it as non-native to Britain and Ireland.

This pond weed has trailing stems that form dense mats. Small, fleshy leaves grow along the stems. The RHS lists its foliage as deciduous. It grows in wet mud at the water’s edge. It can also float on the water’s surface. The listed garden height is 0.3–1.6 feet [0.1–0.5 metres]. Its listed spread is also 0.3–1.6 feet [0.1–0.5 metres]. These figures do not describe the full depth of its mats. The RHS reports mats reaching 9.8 feet [3 metres] deep. Plants reach their listed mature size within two to five years.

The flowers are small and whitish. The supplied sources give no clear flowering season. Plants produce seeds, but the RHS considers them unlikely to be viable. Broken stem pieces provide a means of spread. This makes stem fragments important when dealing with an existing patch. The evidence does not give details of the fruit. It also provides no species-level account of pollination.

The RHS lists full sun and a sheltered site. Suitable aspects face south, west or east. Its listed soils include chalk, clay, loam and sand. It tolerates acid, neutral and alkaline conditions. Poor drainage suits its wetland habit. It is not a plant for dry soil. The RHS gives it a UK hardiness rating of H5. This covers minimum temperatures from 5 to 14°F [−15 to −10°C].

Its invasive status outweighs any case for garden use. The RHS lists it under Schedule 9 of UK wildlife law. It also reports a ban on sale. Plants must not be introduced or allowed to spread into the wild. The RHS advises owners to control plants already present. These restrictions matter more than its tolerance of varied soils. The supplied evidence gives no supported edible or medicinal use. It gives no clear toxicity assessment or species-specific pest and disease information.