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Salvinia is a free-floating aquatic fern that forms large, dense mats. With its hairy leaves and floating habit it can be hard to recognise as a fern. The spongy leaves are green to bronze, up to 5cm long, and their shape varies with the age.
Young, small leaves lie flat on the water surface but mature leaves tend to become crowded and fold up. The upper surface of the leaf is water repellent and covered with distinctive white hairs with an egg-beater like tip. Salvinia has no true roots but has a root-like structure underneath each leaf pair.
Salvinia grows on wind-protected ponds, small lakes, artificial waterbodies (e.g. dams and reservoirs) and swampy backwaters. It grows best in nutrient enriched waters but can survive in frequently inundated, damp mud.
Salvinia quickly forms extensive mats, completely smothering waterways. It can double in area within 10 days. The mats exclude native plants, block dams and waterways, impede drainage, disrupt recreational activities and reduce water quality by lowering oxygen levels. The mats also create a drowning risk for people and animals. The only conditions which are not suitable for the species are in cooler seasons, extreme shade or frosty mornings in winter, where the species can suffer from 'winter die-back'.
In New Zealand, salvinia does not produce spores. It spreads vegetatively from plants and plant fragments.