Hydrilla (Hydrilla verticillata) is an aggressive submerged aquatic plant that has become one of the most problematic invasive species in freshwater systems across the United States. It grows rapidly, forming dense, tangled mats that cover the water’s surface, which limits light penetration to native vegetation. This growth clogs irrigation canals, interferes with boating and recreation, and alters aquatic ecosystems. Effective management requires understanding the plant’s biology and strategically applying multiple control methods.
Identifying and Understanding Hydrilla
Proper identification is the first step before beginning any control strategy. Hydrilla is a rooted plant with slender, branched stems that can reach lengths of up to 25 feet in deep water. Its distinguishing characteristic is the small, pointed leaves, which grow in whorls of four to eight leaves around the stem. The leaf margins also possess noticeable serrations, or small teeth.
Hydrilla’s aggressive reproductive capacity complicates control efforts. It spreads primarily through fragmentation; a tiny piece of stem can drift away and establish a new colony elsewhere. It also produces specialized, potato-like underground storage organs called tubers. These tubers remain viable and dormant in the sediment for several years, allowing the plant to rapidly re-establish after initial removal.
Mechanical and Physical Removal
Physical removal methods offer an immediate, chemical-free way to clear infested areas, but they must be executed carefully to prevent further spread. For small, localized infestations, simple hand-pulling or raking may be practical. This method is labor-intensive and risks leaving behind plant fragments, so it is most effective when the entire root crown and associated tubers are removed from the sediment.
Large-scale infestations often require specialized mechanical harvesters, which cut the stems and collect the biomass for disposal. While these machines quickly restore navigation and recreational access, they only remove top growth and cannot reach the tubers in the sediment. Regrowth is almost certain. A more precise technique is Diver Assisted Suction Harvesting (DASH), where divers meticulously pull the plants and tubers, feeding the material into a suction hose that minimizes fragmentation.
Another effective physical method is the use of benthic barriers, which are large, weighted mats placed directly on the waterbody floor. These barriers physically crush the existing Hydrilla and block sunlight from reaching the plants below. The lack of light prevents photosynthesis, effectively smothering the growth. Benthic mats provide long-term control for small areas like docks or swimming zones, provided they remain securely anchored and kept free of accumulating sediment.
Biological Control Options
Biological control uses natural enemies to manage the Hydrilla population, offering a self-sustaining, long-term strategy. The most common agent is the Triploid Grass Carp (Ctenopharyngodon idella), a sterile, plant-eating fish. These fish are intentionally bred to be triploid, meaning they possess three sets of chromosomes and cannot reproduce, eliminating the risk of establishing a wild population.
Grass Carp readily consume Hydrilla, which is one of their preferred food sources. Stocking density is determined by the waterbody size and infestation severity, typically ranging from 7 to 15 fish per vegetated acre for significant control. Complete eradication of all aquatic vegetation, if desired, may require stocking up to 15 fish per acre.
Introducing these fish requires careful planning and often necessitates obtaining a permit from regulatory agencies. This ensures proper stocking rates and prevents ecological damage. Control is not immediate, as it can take several months for the fish to reduce dense mats.
Chemical Treatment with Herbicides
Chemical control involves applying aquatic herbicides, often the fastest method for treating large, widespread infestations. Herbicides are classified based on their mode of action: contact or systemic. Contact herbicides, such as Endothall and Diquat, work quickly by damaging plant cells upon absorption, leading to rapid die-off within days.
Contact treatments are best for fast, localized control but do not translocate throughout the entire plant, making them less effective against underground tubers. Systemic herbicides, such as Fluridone and Penoxsulam, are absorbed and move throughout the plant’s tissues to the roots. These treatments are slower, often requiring 30 to 90 days of continuous exposure, but they are more effective at killing the entire plant, including the tubers.
Choosing the right chemical requires professional consultation, as factors like water flow, alkalinity, and the size of the treatment area influence effectiveness. A significant safety concern with large-scale chemical application is the risk of dissolved oxygen depletion. As the massive amount of Hydrilla biomass dies and decomposes, the process consumes oxygen from the water, which can lead to fish kills. Consequently, dense infestations often require phased, professional application and a mandatory regulatory permit to ensure compliance with water quality laws.
Preventing Future Infestations
Long-term management requires a proactive strategy focused on preventing new introductions and reducing conditions that support growth. The most direct prevention method is the “Clean, Drain, Dry” protocol, which should be followed by all water recreationists. This involves meticulously removing all visible plant material from watercraft, trailers, and fishing gear before leaving a waterbody.
All water must be drained from bilges, live wells, and engine cooling systems before transport to prevent the movement of microscopic fragments. Allowing all equipment to completely dry for several days before entering a new water source is the final step to ensure any remaining organisms are killed. Furthermore, Hydrilla thrives in nutrient-rich water, so reducing external nutrient loading is an important control measure. This involves managing stormwater runoff and minimizing the use of phosphorus- and nitrogen-rich fertilizers near the waterbody.