Giant salvinia is a free floating aquatic fern. An individual plantlet
consists of a horizontal stem that produces two floating leaves (fronds)
up to 25 cm long and a highly dissected submerged frond up to 25 cm. The
floating leaves are green, sessile to short petiolate, broadly ovate in
shape with entire margins. The midrib extends from the base to the apex
of the leaf. The upper surface of the floating fronds is covered with
parallel rows of hairs that have a characteristic "cagelike"
structure at the apex. When plants are young, these leaves are small and
float on the water surface. As plants age, the floating leaves become
crowded and fold against one another resulting in a more vertical leaf
position. The brown, feathery submerged leaf resembles and functions as
a root. This frond bears the sporocarps or spore forming structures. The
globose sporocarps are densely hairy, short stalked and 2-3 mm in diameter.
Spores are rarely formed and if present are deformed and infertile.
Salvinia molesta is native to southeastern Brazil. Introduction
of the mat forming fern is thought to have arisen from the water gardening
and/or aquarium trade where plants are either sold directly or occur as
contaminants in water garden stock
Giant salvinia grows best in stagnant or slow flowing water. Quiet waters
of lakes, ponds, bays, oxbows, ditches, swamps, and marshes may be susceptible
to invasion. In Ceylon, the species has proven to be problematic in rice
fields. The rhizomatous plant can rapidly form dense floating mats of
vegetation. Disturbance usually results in fragmentation and any fragment
having an axillary bud can give rise to a new plant. The optimum temperature
range for growth is 25-28 C and under these conditions plants can double
within 1 week.
Giant salvinia can impact irrigation systems, navigable waters, fisheries,
electric power production, and rice farming. Giant mats reduce light penetration
and result in oxygen depletion. As light becomes limiting, it affects
the growth and survival of phytoplankton and vascular plants. Oxygen depletion
may be so severely reduced beneath a mat that it influences fish survival.
Extensive mats may exacerbate a situation because they prevent water circulation