Salvinia

Salvinia molesta
Aquatic
Alternate
Fern-like
No colour
Green

A free-floating freshwater plant forming dense mats of vegetation on the water surface. Its folded ‘leaves’ are borne in pairs and emerge above the water surface.it has submerged 'roots' that are highly divided and feathery in appearance. Its 'leaves' have a covering of egg-beater shaped hairs (about 1 mm long) on their upper surface. Sterile spore sacs are often present among the submerged 'roots'.

Common names 
Also known as: Salvinia, giant salvinia, aquarium water moss, kariba weed, African payal, water fern,
Family 
Salviniaceae
Deciduous 
No
Native/Exotic 
Exotic
Origin 
Native to South America.
Notifiable 
No
State declaration 
Class 2
Council declaration 
Class C – Containment and reduction
Known distribution 

This species is mainly distributed along the eastern coast of Australia, from central New South Wales through to northern Queensland. Scattered infestations are also often found in the Northern Territory and Western Australia, and less often in Victoria and South Australia. Also recently recorded on Norfolk Island.

Habitat 

A serious problem in rivers, streams, lakes, dams, swamps, irrigation channels, drainage lines and other bodies of water and is also a pest of rice crops around the world. Mainly a weed of tropical, sub-tropical and warmer temperate regions, though it will grow on waterbodies in semi-arid areas.

Habit 

A free-floating freshwater (i.e. aquatic) fern with branching horizontal stems and submerged feathery ‘roots'. Its stems break apart readily and it can quickly form dense mats of foliage on the water surface. Younger plants generally have smaller, flat, 'leaves' that are more spread apart (i.e. the primary growth form). As the plants get older, larger 'leaves' are produced that are slightly folded and borne closer together along the stems (i.e. the secondary growth form). Eventually the 'leaves' become very folded in nature and are compactly arranged along the stems (i.e. the tertiary growth form).

Impact and control methods 
Stem and leaves 

Individual plants (i.e. ramets) can consist of as little as a piece of stem with two floating 'leaves' (i.e. fronds) and a third 'leaf', which is modified into feathery 'roots' and remains submerged. Plants produce slender, branching runners and form mats of vegetation very quickly. These slender stems (1-2 mm thick) are much-branched and grow up to 30 cm long (usually only 6-25 cm long) before separating to form new plants. The length of stem between the joints (i.e. nodes), that is the internode length, varies depending on the density of the weed. When plant density is low the 'internode' length is relatively long, but as plant density increases, the 'internode' length may become very short. The 'leaves' (i.e. fronds) are oval or folded, borne on short stalks (i.e. petioles), and are green or yellowish-green in colour. These 'leaves' (20-60 mm long and 10-15 mm wide) have a covering of water-repellent waxy hairs (i.e. papillae) on their upper surface. These hairs (1-3 mm long) are arranged in distinct rows and are tipped with distinctive egg-beater shaped structures that aid buoyancy. The undersides of the leaves are covered in densely matted brown hairs. The 'roots' (i.e. submerged fronds) are brown in colour and highly divided into many filaments (2-50 cm long).

Flowers and fruits 

Flowers are not produced by this species, instead sterile spore sacs (i.e. sporocarps) form along the filaments of the submerged root-like fronds. These small rounded (i.e. globular) spore sacs (1-3 mm across) are covered in hairs (i.e. they are pubescent) and borne in elongated clusters that dangle below the water surface.

Reproduction and dispersal 

This plant reproduces vegetatively, with the floating branches readily breaking apart and forming new plants.Dispersal of these plants occurs during floods and also during water movement caused by water currents or wind. It is most commonly spread to new areas by the dumping of aquatic garden waste, but may also be spread by animals, vehicles and boats.

Similar species 

This a very distinctive floating plant and is rarely confused with other species. However it may appear similar to red azolla (Azolla filiculoides) and ferny azolla (Azolla pinnata) when young. The azollas (Azolla filiculoides and Azolla pinnata) can be distinguished from salvinia (Salvinia molesta) by having tiny overlapping scale-like leaves, not having egg-beater shaped hairs on the upper surface of their leaves, and by the fact that individual plants rarely grow larger than 2 cm in size.