This leafy collage is mostly made of giant salvinia – a fast-growing, free-floating fern that grows into mats that
can be up to two feet thick.
Pretty it may be, but salvinia is a noxious aquatic weed. Not native to North America, salvinia was accidentally introduced a hundred years ago and has now colonized ponds and lakes across the continent. Its small, oval leaves form dense mats that sometimes double in size in just a few days.
The mats can extend across entire bodies of water, blocking the sunlight needed by other plants. Salvinia also soaks up valuable oxygen, in effect suffocating the fish and other critters below.
Salvinia is a tough species – native to tropical areas with many competitors, the plant has evolved more than a few techniques to oust its rivals from crowded territory. It tolerates freezing temperatures and other kinds of environmental stress. These qualities make it a danger to freshwater environments.
A tiny weevil, Cyrtobagous salviniae, is under study as a possible natural control for this nasty invader.
Photo by Scott Bauer, USDA.
