Hydrilla, a submerged perennial, is the most troublesome aquatic plant in the United States. It outcompetes native vegetation, acts as a breeding ground for mosquitoes and destroys fish and wildlife. It is among the aquatic species on the federal noxious weed list.
OverviewOther names for this plant include:
- Common names: waterthyme, Florida elodea
- Scientific names: Hydrilla asiatica; H. japonica; H. lithuanica; H. ovalifolica
- It invades lakes, ponds, reservoirs, rivers and ditches. Generally found rooted at the bottom of 20’ or more of fresh, slow-moving or still water.
- Acts as a breeding ground for mosquitoes, and destroys fish and wildlife. Water intake and delivery systems can also be severely impacted.
Classification in Wisconsin: Prohibited
Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for hydrilla was based upon this literature review developed by the department.
Leaves: In whorls of 3-10 around the stem at nodes; 0.75” long and 0.25” wide; small spines give leaf margins a toothed appearance; midrib is often spiny below and reddish when new; lack stems; rough to the touch.
Flowers: Tiny; female flowers are white, located in leaf axils, have 6 petals on long, thread-like stalks; male flowers are green and look like inverted bells.
Fruits & seeds: Loment-like, narrowly cylindrical, 5-15 mm long, smooth or with few to several short to long irregular spines, do not open to release seeds; seeds 1-5, ellipsoid, 2-3 mm long, smooth, brown.
Roots: Reproduces by rhizomatous tubers.
Similar species: Similar characteristics to other aquatic plants, including Brazilian waterweed and the native Canadian waterweed. It also resembles Elodea canadensis, but Hydrilla can be distinguished from Elodea by the presence of tubers, leaves in whorls around the stem, serrations or small spines along the leaf edges, and the reddish midrib of a fresh leaf.
Currently, there have been no reports of hydrilla in Wisconsin. Have you seen it? Send us a report.
Mechanical: Specialized machines are used for mechanically removing Hydrilla, but because of the high cost involved (often over $1,000 per acre) and because of logistical constraints, the practice isn’t widespread. Up to six harvests may be required annually due to the rapid growth rate of Hydrilla. Mechanical removal is used for management only in areas that are in close proximity to domestic water supply intakes, in rapidly flowing water, or when immediate removal is necessary.
Drawdowns can be effective for control if done in the fall and prior to regrowth in the spring. But, even in drained lakes and ponds, subterranean turions may remain dormant and viable in organic substrates.
Chemical: The aquatic herbicides, copper sulfate, fluridone and endothall have been used effectively for control.
Biological: There are studies being conducted for whether sterile grass carp, certain snails and certain insects would be successful biological control agents, but no excellent biological control agents are known to exist at this time.
View hydrilla pictures in our photo gallery!
ResourcesSources for content:
- Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. University of Wisconsin Press. 2005. Pg. 145
- Lake Tides (UW Extension Lakes Program publication) article: Hydrilla? Don’t you mean Godzilla?
- Batcher, Michael S. The Nature Conservancy Element Stewardship Abstract for Hydrilla verticillata