Slender Knotweed
Persicaria decipiens, Slender Knotweed, is not a weed. It is an Australian native aquatic plant with foliage reminiscent of Vietnamese mint and slender pink flower spikes, an attractive addition to constructed wetland habitats.
It is a trailing plant whose stems grow horizontally, becoming more vertical with length (typically <1m tall). The slender pale pink flowers appear from November to June, with a peak in February, although the timing of flowering will depend on the water regime and season. The plants are summer active and can form dense mats along creek or wetland beds or margins. In winter, their leaves contract in colder, deeper water such that they have dominant red stems and stunted leaves. They may die off and act as annuals or persist over the winter months depending on the water regime and climate.
Likely pollinators are insects, including native bees, flies, wasps and small butterflies. The inundated plants also provide excellent shelter for macroinvertebrates, tadpoles and small fish, making them ideal waterbird habitat as well.
Persicaria decipiens, Slender Knotweed, are excellent plants for creek line regeneration and for stabilising constructed wetland banks. The seeds are readily dispersed and will naturally regenerate in areas that are connected to existing populations. It is important to choose your local species of Persicaria, if possible, all of which will perform similar water treatment, amenity and habitat functions. Larger species can impede water flow, especially at the break of season if dense stands have grown over summer.
Habitat needs: banks of rivers, streams and wetlands; wetland basins during dry phases, still or slow-flowing fresh water. Can withstand wetting and drying regimes and may persist over winter in water to 30-70cm depth, although plants tend to die-off in winter and recolonise or resprout in spring-summer.
Identification notes: narrow elliptic to lanceolate (spear-shaped) leaves are 5-12 cm long and 0.5-1.3 cm across and commonly have a distinctive dark notch on the leaf. Flowers are slender and situated at the end of the main and side branches. The whole flower is between 2.5-3.5 mm long with the tube greenish and shorter than the lobes, which are pale to deep pink and sometimes white. Fruits are c. 3 mm long, triangular, dark brown and shiny.
Conservation status: Unknown conservation status on mainland Australia but listed as Vulnerable in Tasmania. Common in some catchments. It is found across Africa and the Mediterranean, through southwestern Asia, Malesia and all states of Australia, as well as Norfolk Island, New Zealand and New Caledonia. It has become naturalised in Madagascar. Note other Periscaria species may occur in overlapping or alternate regions.
Distribution of Persicaria decipiens (Slender Knotweed) in Australia. Source: Atlas of Living Australia.