Hydrilla
Hydrilla verticillata
Washington State Classification
Class A — Eradication Required Statewide
Hydrilla forms dense canopies that often shade out native vegetation.
Quick Reference
Identification
Growth Habit
Hydrilla is a submersed perennial herb, growing rooted to the bottom, in either still or flowing water. Hydrilla may be moneocious plants (male and female flowers on the same plant) or dioecious (male and female flowers on different plants).
Leaves
Leaves are in whorls of 3 to 10 along the stem, each 2 to 20 mm long by 1 to 4 mm wide. Leaf margins generally have sharp spines, giving them a toothed appearance. Spines occur on lower midvein, causing plant to feel rough.
Flowers
Female flowers have 3 translucent petals and 3 whitish sepals and float on the water’s surface. Male flowers have 3 white to red narrow petals and 3 white, red or brown sepals and form in leaf axils to break free when mature to float on water’s surface.
Fruit & Seeds
Hydrilla can produce seeds but it mainly spreads by tubers, turions and stem fragments.
Impact
Hydrilla forms dense canopies that often shade out native vegetation. Extensive single species stands provide poor habitat for fish and other wildlife. Once established, it destroys freshwater recreational opportunities by forming extensive surface mats.
Ecology & Spread
Habitat
Hydrilla grows rooted to the bottom of lakes, ponds, springs, ditches, marshes, wet ricefields, slow streams, and tidal waters, where it is submersed in either still or flowing water. Please click here to see a county level distribution map of hydrilla in Washington.
Spread Mechanisms
Reproduction
Hydrilla reproduces in many ways: via underground rhizomes and above ground stolons; from stem fragments, fragments can be as short as two leaf whorls; rhizome fragments; sprout from root crowns; seeds; by turions—buds in leaf axils or stem tips; tubers.
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Control Methods
Mechanical
- Because this plant spreads readily through fragmentation, mechanical controls such as cutting and harvesting should be used only when the extent of the infestation is such that all available niches have been filled. Using mechanical controls while the plant is still invading will tend to enhance its rate of spread.
Biological Control
- Triploid grass carp will eat Hydrilla. Because it is an exotic species, its introduction is tightly regulated, and only the sterile triploid fish are allowed to be introduced in most states, including Washington.