Hydrilla verticillata (L. f.) Royle

U.S. Nativity: Exotic
Habit: Aquatic

Synonym(s): Florida elodea, water thyme, waterthyme

Hydrilla verticillata is a submersed, rooted aquatic plant that can grow in water up to depths of 20 ft. (6.1 m). Plants can survive in depths up to 40 ft. (12 m) in non-turbid water.
Leaves are whorled in bunches of 3-8, but most often with whorls of 5. The midribs of the leaves are reddish in color with the undersides having small, raised teeth. Leaves are 0.2-0.8 in. (5-20 mm) long, less than 0.1 in. (2 mm) wide and have serrated margins.
Only the female flowers of this dioecious plant have been found in the United States, which means no viable seed are produced.
Turions (stem tubers) are bud-like structures which can drop off the plant and successfully survive freezing or drought. Tubers from the rhizomes are another way these plants reproduce and increase their invasive potential.
Ecological Threat
Hydrilla verticillata forms dense mats at the surface of the water. The dense mats can restrict native vegetation, irrigation practices, recreation, hydroelectric production, and water flow. It can invade most slow-moving or still water systems. This plant is believed to be native to Asia or Africa, although it is widely spread across the globe. It was first introduced into North America as an aquarium plant in the 1950s.

Identification, Biology, Control and Management Resources

Biological Control of Invasive Plants in the Eastern United States - USDA Forest Service
Southeast Exotic Pest Plant Council Invasive Plant Manual - SE-EPPC
Element Stewardship Abstract - The Nature Conservancy
Invasive Plant Atlas of New England - University of Connecticut
Identification and Biology of Non-Native Plants in Florida's Natural Areas - University of Florida
Federal Noxious Weed Disseminules of the U.S. - USDA-APHIS
Weed of the Week - USDA Forest Service
Global Invasive Species Database - Invasive Species Specialist Group
Plant Invaders of Mid-Atlantic Natural Areas - National Park Service and U.S. Fish and Wildlife Service