Hydrilla image

Invasive Species of the Month: May 2012

Hydrilla (Hydrilla verticillata)

Also Known As

Florida elodea, water thyme, Indian star-vine

General Description

Hydrilla is a submerged, perennial aquatic herb that is native to Asia and Africa. It can form dense mats near the water surface, clogging waterways and restricting native vegetation, irrigation, and recreation. Hydrilla can be distinguished from Brazilian egeria (Egeria densa) and Canadian waterweed (Elodea canadensis) by its toothed midrib and the arrangement of its leaves. It reproduces by stems, stem fragments, turions, and seeds.

Growth Habit

Hydrilla usually inhabits water that is 1.5–10 feet deep, though it can grow at depths of up to 20 feet. It is noticeable by its leafy stems which branch profusely near the water surface.


Lanceolate leaves are narrow with serrated margins, 0.2–0.8 inches long, and occur in whorls of 3–10 along the stem. Small, raised teeth form along the underside of the reddish midrib.


Tiny, white, 6–petal flowers are located in the leaf axis, and grow on long, thread-like stalks. Flowering occurs from June to October, when flowers extend to the water surface. Tiny, greenish, bell-shaped flowers are attached closely to the leaf axils until they break free and rise to the surface, often in large numbers.

Seeds and Fruit

Smooth, narrowly cylindrical fruits contain, but do not release, small brown seeds.


Hydrilla can be found in lakes, ponds, rivers, reservoirs, and ditches. It can tolerate high levels of suspended sediment, drawdown periods, low light, and temperatures from 50° to 95°F. Hydrilla can invade most shallow or still water systems.

Additional Resources

Species Identification and Characteristics

Colorado Department of Agriculture Fact Sheet (PDF) Images and Overview
Montana Weed Control Association Fact Sheet
Texas Invasives Fact Sheet
USDA Forest Service Fact Sheet (PDF)

Past "Invasive Species of the Month" Profiles