What Does It Look Like?
What is it?
Indian Swampweed (Hygrophila polysperma) is a low-growing creeping herb that may grow up to 20 cm high on dry land. The stems are white in the younger parts, becoming reddish and leafless at the base, rounded in cross section and hairless in the vegetative parts, but quadrangular in cross section and hairy in the flowering shoots. Roots may form from all of the lower nodes (points where leaves, or roots, arise) of the stem. The leaves are stalkless (sessile), oval (elliptic) to egg-shaped (ovate), 5–20 mm long and 3-5 mm wide, with entire leaf margins i.e. without teeth or lobes, and with the midrib (midvein) raised on the lower surface.
The pale mauve to blue flowers are sessile, and form spike-like clusters in the axils (angles) of the upper bracts (modified apical leaves); flowers have only been observed on plants which are not in the water (Virginia Seymour, Far North Coast City Council, pers. comm.). The bracts are leaf-like and densely hairy on both surfaces. The corolla (collective term for petals) is 4.5–5 mm long, tubular for most of its length and externally hairless. The flower tubes are made up of an entire (unlobed) upper lip, and a three lobed lower lip that may be turned down at right angles to the tube, with the central lobe wider than the two side lobes.
The fruit is a 4.5–5.5 mm long capsule with a hairy tip; it contains 24–30 seeds, each of them covered with hairs that expand on wetting (Barker 2008 pers. comm.).
For further information and assistance with identification of Indian Swampweed, contact the herbarium in your state or territory.
Growth form (weed type/habit)
Where it currently grows? Preferred habitat
In Australia, Indian Swampweed is a potential weed of freshwater lakes, dams and ponds. It prefers warmer climates and is usually found in flowing water, but does occur in slower flowing waterways and lakes (Johnson 2003). It can grow in water up to 3 m deep, and is adapted to low-light conditions and tolerant of a wide pH range (Johnson 2003; Robinson 2003).
Are there similar species?
Indian Swampweed can be confused with the native species Hygrophila angustifolia, but the native species does not grow in submerged situations. H. angustifolia also has well-developed hooks under each of the seeds in the capsule, whereas Indian Swampweed does not have these hooks. H. angustifolia also has 4 stamens (male organs that include the pollen bearing anthers) to a flower rather than 2 for Indian Swampweed (Barker 2008 pers. comm.).
The weedy Hygrophila species (H. costata), also known as Glush Weed (Thorp & Wilson 1998 – ), is more prevalent in Australia, and is found in similar habitats to Indian Swampweed. The flowers of Hygrophila are white, 3.5 mm long and have a two lobed upper lip, while the flowers of Indian Swampweed are pale mauve to blue, 4.5-5 mm long, and have an unlobed upper lip (Barker 2008 pers. comm.).
Dyschoriste depressa has only relatively recently been recognised as now occurring in the Brisbane area. It occupies similar habitat to Indian Swampweed and could be easily confused with it since it has the same clusters of flowers and fruits in the axils (angles between the stem and leaf). However, it may be distinguished from Indian Swampweed by its distinct awn (sharp protuberance) at the base of each of the anthers (pollen bearing parts of the stamen) or the capsules that contain only 4 seeds compared with many more seeds in Indian Swampweed (Barker 2008 pers. comm.). Further information and images of Dyschoriste depressa can be found on the Weed Society of Queensland site (2006).