Hydrilla

• Hydrilla is a declared weed in Tasmania under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of hydrilla are prohibited in Tasmania.
• The legal responsibilities of landholders and other stakeholders in dealing with hydrilla are laid out in the Hydrilla Statutory Weed Management Plan.

What does hydrilla look like?

• Hydrilla is a submerged perennial (long-lived) aquatic plant which can grow as a free-floating mat or attached to the bottom of the water body. The stems are vertical and much branched and can grow to 8 metres depending on water depth. Stolons (or runners) grow on or just below the mud surface and produce tubers, while turions (readily detachable buds) grow on the ends of stems. The leaves are produced in whorls of 4 to 6 along the length of the branches, are brownish-green in colour and have minutely toothed margins.
• Plants are either male or female. Flowers grow in the leaf axils near the ends of the branches. Female flowers rise to the water surface on long floral tubes reaching 10 cm in length, while the male flowers break off from the stems and rise to the surface where they open explosively and release pollen.
• In temperate regions, hydrilla overwinters as dormant shoots, tubers, turions and seed. These commence growth in spring and rapidly grow into a thick mat of interwoven stems which then die off in late autumn and winter.
• Most spread is by vegetative means via stolons, tubers, turions and fragmentation. Local spread of a plant occurs when stolons and tubers germinate along the periphery of the clump. Long distance spread occurs mainly when fragments of stem are dislodged and carried down-stream, where they regenerate and form now plants.

Impacts of hydrilla

• Hydrilla is a native on mainland Australia and is not usually weedy. However the plant can become a serious weed where a water body is artificially enriched with nutrients. Under these conditions, hydrilla can block inland waterways, irrigation systems and hydro-electric installations, as well as limit recreational access to waterways.