Common Names: Florida-elodea, hydrilla, Indian stargrass, water-thyme, Wasserquirl
Genus: Hydrilla
Family: Hydrocharitaceae

Hydrilla, or Hydrilla verticillata, is a submersed aquatic plant, native to Asia, which grows in all types of bodies of water, on every continent. It is an extreme nuisance, hampering boating and fishing, but doubtless serving as a great relief for fish. It was first discovered in American waterways in 1960. Hydrilla is descended from marine autotrophs which colonized the land before the mid-Paleozoic period, approximately 1 percent of which found their way back to a marine habitat.

Hydrilla is a highly polymorphic plant, meaning that it grows substantially differently to take advantage of local environmental conditions. Hydrilla features a slender, branched stem, up to 25 feet long. The leaves are strap-like and pointed, growing in whorls of three to eight around the stem. Leaf margins are distinctly saw-toothed. Hydrilla often has one or more sharp teeth along the length of the leaf mid-rib. The plant produces small white flowers on lengthy stalks. It also produces 1/4 inch turions at the leaf axils and potato-like tubers attached to the roots in the mud. Strings of hydrilla which have become detached from the bed of their home body of water occasionally survive while floating.

Hydrilla has a number of characteristics which enable it to compete more effectively than most native plants. It can grow at up to an inch per day, and when it reaches the surface, it branches furiously, creating a dense mat at the water's surface, blocking light to other plants. The plant is generally 90 percent water by volume, allowing it to spread out quickly. It can grow in both oligotrophic (low nutrient) and eutrophic (high nutrient) environments, and in water with up to 7% of the salinity of seawater. It has been found growing in water 15 meters deep. The plant can reproduce via fragmentation, tubers, turions, and seed. In addition, it can utilize bicarbonate ion as a dissolved inorganic carbon source in the absence of dissolved carbon dioxide. All of this adds up to an extremely hardy plant.

This is unfortunate, because hydrilla is an annoyance at best, and dangerous to boot. It clogs pumps and impedes the flow of canals, which can lead to flooding. It clogs jet drive intakes on boats and personal watercraft, and can easily be wrapped around propellers. The nature of an outboard motor is such that you can easily find yourself attempting to drag literally hundreds of pounds of hydrilla behind your boat.

Hydrilla is not easily controlled once a body of water is infested with the plant. Copper, diquat, endothall, and fluridone-based herbicides can be used to control hydrilla but can have an adverse effect on other wildlife. Various animals and insects have been considered and many are in use to control the plant today, but some of the most efficient consumers of the plant are impractical for some of the same reasons as the Hydrilla itself, because they either displace native species, or inhibit recreational use. Even when the plant is cleared mechanically, a process which may cost as much as US$1,000 per acre, tubers remain dormant and viable in the bottom muck.

Hydrilla control is generally accomplished only by prevention. Boat owners are urged to flush their bilge before leaving any waterway, and to clean the bilge and hull of their craft with a chlorine bleach solution to kill remaining parts of plants, and seeds. Once spread to new waterways, Hydrilla rapidly becomes a problem, displacing native species and making many bodies of water effectively unusable for recreation.


  1. Website: Center for Aquatic and Invasive Plants, Hydrilla verticillata (
  2. Langeland, K.A. 1996. Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic Weed". Castanea 61:293-304.
  3. (
  4. USDA, ARS, National Genetic Resources Program. Germplasm Resources Information Network - (GRIN). (Online Database) National Germplasm Resources Laboratory, Beltsville, Maryland. Available: (14 October 2002)