Hydrilla

Overview

Hydrilla is one of the most problematic submerged aquatic weeds in the United States. Native to Asia, it was introduced through the aquarium trade and has since spread throughout the Southeast, Mid-Atlantic, and other regions. It can grow in extremely low light and a wide range of nutrient conditions, giving it a competitive advantage over native aquatic plants. Hydrilla is listed as a Federal Noxious Weed, meaning interstate transport and sale is prohibited. Its combination of explosive growth, multiple reproductive strategies, and environmental tolerance make it one of the most costly aquatic invasives to manage.

Identification Characteristics

Hydrilla is identified by its small, pointed leaves arranged in whorls of 4–8 around the stem — a key feature that distinguishes it from native elodea, which has exactly three leaves per whorl. Each leaf has serrated edges visible to the naked eye, and a single diagnostic tooth on the midrib on the underside of each leaf. This midrib tooth is the most reliable single identification character. Stems are slender and can grow very long, branching frequently near the surface.

Two biotypes exist in North America: a dioecious (separate male and female plants) biotype common in the Southeast, and a monoecious biotype that has spread into the Mid-Atlantic and Northeast. Both biotypes produce turions — small, dark axillary buds that detach and overwinter in sediment — and underground tubers that can persist in sediment for over 10 years, making eradication effectively impossible once established.

When removed from water, hydrilla leaves collapse limply against the stem, unlike coontail whose stiff leaves hold their shape. The plant is dark green to olive green, though it can appear brownish-red under high light conditions.

Growth Habit & Ecology

Hydrilla is an extraordinarily fast-growing plant that can grow up to one inch per day under optimal conditions, forming dense monospecific mats at the water surface that can extend across entire water bodies. It thrives across an exceptional range of environmental conditions — from near-anoxic, turbid water to clear oligotrophic systems, from fresh water to brackish conditions up to 7 ppt salinity, and in temperatures from 50°F to 85°F.

The plant reproduces through four distinct mechanisms: (1) Vegetative fragmentation — even tiny stem fragments with a single node can root and establish new plants, making mechanical control potentially counterproductive; (2) Turions — small axillary buds that detach from the plant, sink to the bottom, and remain dormant through unfavorable conditions before germinating; (3) Tubers — starchy underground storage organs produced in the sediment that can survive for over a decade; (4) Seeds — produced by monoecious biotypes, these provide genetic diversity and further spread potential.

Hydrilla can grow at depths up to 20 feet in clear water, and its exceptional low-light tolerance — it can photosynthesize at just 1% of surface light intensity — allows it to establish in turbid conditions where native submerged plants cannot survive.

Habitat Preferences

Hydrilla occupies virtually every freshwater habitat type in the United States: lakes, ponds, reservoirs, rivers, drainage canals, tidal estuaries, and irrigation ditches. It tolerates extraordinary ranges of environmental conditions that would limit other aquatic plants. Water depth is not a barrier — hydrilla can establish at depths ranging from less than one foot to over twenty feet in clear water. Its exceptional low-light tolerance (photosynthesis at just 1% of surface light intensity) allows it to thrive even in turbid, heavily shaded water where native submerged plants fail.

Hydrilla grows in both hard and soft water, from pH 5.5 to 10, and can survive in slightly brackish water up to 7 parts per thousand salinity — allowing it to colonize tidal areas where most invasive submerged plants cannot persist. It establishes in both standing and slowly flowing water, and in both cold temperate climates (when dormant in winter) and year-round tropical conditions. This combination of tolerances makes hydrilla uniquely capable of colonizing any water body, anywhere, regardless of water quality. Once established, it is extremely difficult to displace even by improving water clarity and reducing nutrients — strategies that help suppress other invasive species.

Spread Mechanisms

Hydrilla spreads through four distinct pathways, each contributing to its range expansion and making prevention extremely challenging. Recreational watercraft represent the primary vector: stem fragments as small as a single node — invisible to a casual inspection — can lodge in propellers, live wells, bilge water, anchor chains, trailer wheel wells, and fishing equipment. When that boat enters a new water body, the fragment detaches and establishes a new colony. A single fragment the size of a thumbnail can seed an infestation.

Turions — small, dark axillary buds — are produced in abundance throughout the growing season, detach from plants, and are distributed by water currents, flooding events, and birds. Waterfowl that feed in infested water bodies carry turions on their feathers and feet to distant water bodies. Underground tubers can survive for over a decade in sediment, meaning that even after successful surface suppression, disturbance or dredging can expose viable tubers that regenerate the infestation. The monoecious biotype also produces seeds, adding a fifth pathway unique to that strain. Hydrilla's illegal sale through the aquarium trade — under mislabeled names — has also contributed to its spread into new states.

Prevention is the only fully effective strategy. The Clean, Drain, Dry protocol eliminates the watercraft pathway if followed rigorously. Reporting new sightings to state wildlife agencies enables early response before colonies expand.

Seasonal Growth Pattern

In frost-free climates — primarily Florida and the Gulf Coast — hydrilla grows continuously year-round, reaching peak above-ground biomass in summer and fall. In temperate regions with winter freezes, the annual cycle begins with new growth from tubers and turions in March–April as water warms above 50°F. Growth accelerates through May and June, with plants elongating rapidly and branching toward the surface.

By July–September, dense monospecific surface mats develop in shallow to mid-depth water, reaching the surface in areas under 10 feet deep. This peak mat period is when hydrilla causes the most severe recreational, navigational, and ecological impacts — surface mats can extend across entire bays and lake sections. Turion production increases in late summer and fall in response to cooling temperatures and shortening days; turions detach and sink to sediment to overwinter.

In October–November, above-ground growth begins to die back in colder regions as water temperature drops below 50°F. By December and January, plants in freeze zones appear largely absent from the water column — giving a misleading impression of control. Meanwhile, tubers and turions in sediment remain fully viable, ready to resprout the following spring. This seasonal dormancy is why annual inspection and monitoring are essential; hydrilla's "disappearance" in winter does not mean it is gone.

Ecological Impact

The ecological and economic consequences of hydrilla infestations are severe and well-documented. Dense monocultures displace native aquatic vegetation, dramatically reducing plant community diversity. A diverse native submerged plant community of 20+ species can be reduced to near-monospecific hydrilla stands within a few growing seasons.

Dense surface mats block sunlight from reaching the water column, eliminating photosynthesis by native submerged plants. Decomposing biomass depletes dissolved oxygen, creating hypoxic zones that stress or kill fish, invertebrates, and other aquatic organisms. The dense mats alter water circulation patterns, creating stagnant zones that favor mosquito breeding. Navigation becomes impaired; boat propellers are fouled; swimming is made dangerous; water intake structures for irrigation and municipal supplies are blocked.

Economically, hydrilla management costs tens of millions of dollars annually in Florida alone. Property values around infested water bodies decline. Sport fishing, which contributes billions to state economies, is impaired. The U.S. Army Corps of Engineers has spent hundreds of millions of dollars managing hydrilla in navigable waterways since the 1970s.

Control Methods

Effective hydrilla management requires a long-term, integrated approach. No single control method is fully effective, and complete eradication from established water bodies has not been achieved. Management programs aim for suppression to acceptable levels rather than elimination.

Herbicide control using EPA-registered aquatic herbicides is the most commonly used approach. Fluridone (slow-acting systemic), endothall (contact), diquat (contact), triclopyr (systemic), and florpyrauxifen-benzyl (a newer systemic product) are all registered for hydrilla control. Treatment requires state and federal permits, professional application, and careful attention to water use restrictions.

Biological control research has identified several promising agents. The hydrilla tuber weevil (Bagous affinis), native to Asia, attacks tubers in the sediment. The hydrilla leaf-mining fly (Hydrellia pakistanae) and a native insect, the Cricotopus lebetis midge, have shown localized suppression.

Mechanical harvesting can provide temporary navigational relief but does not provide lasting control and risks spreading plant fragments. Aquatic rotovating and bottom shading with benthic barriers are options for small, high-value areas.

Triploid grass carp — sterile fish that consume aquatic vegetation — are permitted in many states as a management tool. Stocking rates must be carefully calibrated, as grass carp will consume beneficial native plants as well.

Distribution in the United States

Hydrilla is documented in at least 30 U.S. states and continues to expand its range. The heaviest infestations occur in Florida, which has some of the most severe hydrilla problems in the world, with millions of acres of lake and river bottom affected. Major infestations also exist in Georgia, Texas, North Carolina, South Carolina, Virginia, Maryland, and other Southeastern and Mid-Atlantic states.

The dioecious biotype (introduced separately from the monoecious biotype) is concentrated in Florida and the Southeast. The monoecious biotype, discovered in the 1980s, has proven more cold-tolerant and has spread aggressively northward through Virginia, Maryland, Delaware, New Jersey, Connecticut, and into New York, Pennsylvania, and other northeastern states.

Western U.S. infestations are more scattered but growing, with established populations in California, Washington, and several other western states. The continued expansion of hydrilla is driven primarily by human-mediated transport via watercraft.

Frequently Asked Questions

References & Further Reading