Also known as
Temple plant, costata, Hygrophila costata
Where is it originally from?
Mexico to Argentina
What does it look like?
Perennial aquatic herb (<1+ m tall) with finely fibrous roots and ability to also grow aerially from stem nodes. Hollow, inflated, floating stems (1-1.5 m long and 5-10 mm diameter at first, increasing to 20 mm with age) become prostrate and branching and take root at nodes. Dark green, slightly waxy, lance-shaped leaves (50-200 x 25-50 mm) are paired with opposite stalks joined at stem, and have serrated edges. Clover-like flowerheads (Nov-Apr) with many thin white 'petals' (florets), followed by yellow-brown seeds (5 mm diameter). Dormant over winter and dies back to rootstock if chilled, but resprouts in spring.
Are there any similar species?
Alligator weed (Alternanthera philoxeroides) is similar, but has smaller, narrower leaves and a reddish tinge.
Why is it weedy?
Roots and seeds in shallow water and damp ground, matures and grows quickly, forms dense mats and scrambles over other species that live on the water margins. Produces few seeds, but they are long-lived and well dispersed. Tolerates warm to hot temperatures, partial drying of stems and root crowns, most soils and water nutrient levels.
How does it spread?
Seed, stem and root fragments can move downstream with water flow, and new areas are infested by seed and fragments moved by diggers and on boats and fishing equipment, and by the dumping of aquarium contents and ponds. Seeds and semi-dry fragments can be spread in soil movement and on the hooves of livestock.
What damage does it do?
Excludes all other species in marginal and shallow freshwater habitats and forms mats over deep water. Prevents seedlings of native species establishing, causes flooding, and rotting vegetation ruins water quality.
Which habitats is it likely to invade?
Wetlands and swamps, rivers and streams, and wet ground, especially in the North Island.
What can I do to get rid of it?
Report all sites to regional council or your local Department of Conservation office.
1. Dig out small sites (all year round): dispose of plant material at refuse transfer station, or dry out and burn.
2. Spray (spring-summer): glyphosate (20ml/L).
What can I do to stop it coming back?
Stems and rootstock resprout and seed bank can reinfest bared sites, so followup 3 monthly until this weed is eliminated. Don't graze the area, as stock will release fragments.