Managing the Most Difficult Aquatic Invasive Plant
Hydrilla management requires long-term commitment, multi-tool integrated approaches, and realistic expectations. No single treatment eliminates hydrilla permanently. The combination of multiple propagule types (surface vegetation, turions, and long-lived tubers), rapid regrowth from surviving propagules, and documented herbicide resistance in some populations means that hydrilla management is a sustained program, not a one-time event. Successful programs typically operate over 5–20 years before meaningful long-term suppression is achieved.
Regulatory Requirement: Nearly all aquatic weed control activities — herbicide application, grass carp stocking, and many mechanical operations — require permits from your state's department of natural resources, environmental protection, or agriculture. Requirements vary significantly by state. Always verify permit requirements before beginning any control activity. Penalties for unpermitted aquatic herbicide application can be substantial.
Chemical Control: Aquatic Herbicides
Herbicide treatment is the primary management tool for hydrilla in most situations. Several herbicides are registered for aquatic use against hydrilla, each with different modes of action, application timing requirements, and efficacy profiles:
Fluridone (Sonar, Avast)
Fluridone inhibits carotenoid biosynthesis, bleaching plant tissue white before death. It is one of the most effective and widely used hydrilla herbicides in large water bodies. Fluridone requires maintenance of herbicidal concentrations (typically 5–10 ppb) for 60–90 consecutive days to effectively treat hydrilla — a requirement that limits its use to enclosed or slow-moving water bodies where dilution is minimal. Important: Fluridone-resistant hydrilla has been documented in several southeastern states (Florida, South Carolina, Georgia). Before applying fluridone, assess whether resistance testing is warranted based on treatment history.
Endothall (Aquathol, Hydrothol)
Endothall is a contact herbicide that disrupts cell membranes. It acts quickly (days rather than weeks) and degrades rapidly in water, making it suitable for flowing or open water bodies where herbicide residuals cannot be maintained. Endothall is often applied to spot treatments or as part of a rotation with systemic herbicides. It does not effectively reach tubers and must be applied to actively growing above-ground vegetation.
Diquat (Reward)
Diquat is a fast-acting contact herbicide that kills tissue on contact by generating reactive oxygen species. It is most effective on above-ground vegetation in clear, shallow water and is often used for spot treatments and early-season interceptions. Like endothall, diquat does not reach tubers and does not prevent regrowth from propagules.
Triclopyr (Renovate)
Triclopyr is a synthetic auxin that disrupts growth regulation. It is effective against hydrilla at appropriate concentrations but is less commonly used as a primary hydrilla herbicide than fluridone. It is more commonly applied to emergent broadleaf weeds and Eurasian watermilfoil.
ProcellaCOR (Florpyrauxifen-benzyl)
ProcellaCOR represents a newer generation of aquatic herbicide. It is highly effective against hydrilla (including fluridone-resistant biotypes), Eurasian watermilfoil, and several other submersed and floating weeds at very low application rates (parts per trillion). It has a favorable environmental profile and has become an increasingly important tool in resistant hydrilla programs. State registration is still expanding for this product.
Biological Control
Triploid Grass Carp (Ctenopharyngodon idella)
Triploid (sterile) grass carp eat hydrilla — it is among their preferred plant species. Grass carp stocking is permitted in most states (prohibited in a few) and can provide long-term vegetative control when stocked at appropriate densities (typically 10–25 fish per vegetated acre). Disadvantages: grass carp are non-selective — they eat native plants as well as hydrilla, and can eliminate all vegetation if overstocked. They do not eat hydrilla tubers effectively. Stocking density must be carefully calibrated to prevent overgrazing. Grass carp are effective primarily in enclosed water bodies (ponds, lakes with screens at outlets); they escape from flowing water systems.
Tuber Weevils (Bagous affinis, B. hydrillae)
Two species of stem and tuber-feeding weevils have been released as biological control agents for hydrilla. Bagous affinis (from India) and Bagous hydrillae attack hydrilla tubers and stems. These agents are available for large-scale biological control programs through the USDA APHIS biological control program. They work slowly — tuber bank reduction takes multiple years — but provide a persistent, self-sustaining suppression mechanism that complements herbicide programs.
Mechanical Control
Mechanical harvesting using aquatic harvesters removes above-ground hydrilla biomass, providing immediate relief for navigation and recreation. Harvesting typically costs $300–600 per surface acre and must be repeated 2–5 times per season to maintain clearance. Critical limitation: Harvesting does not reduce the tuber bank and can spread hydrilla fragments to uninfested areas if equipment is not properly screened and decontaminated. Harvested plant material must be completely removed from the water and legally disposed of on land — never left at the shoreline where fragments can re-enter the water. Harvesting is most appropriate as a short-term management tool for recreational access and navigation while longer-term chemical or biological programs are implemented.
Water Level Management
Winter drawdown — reducing lake level by 1.5–3 meters in fall and refilling in spring — can expose tubers to freezing temperatures and desiccation. Effectiveness depends on climate (freezing winter temperatures required), drawdown depth (must expose all tuber-bearing sediment), and timing. Drawdown is a management tool in regulated lakes and reservoirs but is not applicable in most natural lakes or rivers. Even successful drawdown reduces (does not eliminate) the tuber bank, and programs typically require repeated annual drawdowns over several years.
Prevention: The Most Cost-Effective Strategy
Prevention of new infestations is dramatically more cost-effective than management of established populations. Key prevention measures: mandatory Clean-Drain-Dry protocols for all watercraft moved between water bodies; public education at boat launches; early detection monitoring programs using underwater cameras, sonar, and diver surveys; rapid response plans that enable treatment of new infestations within the first 1–2 seasons. A new hydrilla infestation of a few acres can sometimes be eradicated with a single targeted herbicide treatment; a multi-decade established population cannot.
References
- Haller, W.T. & Sutton, D.L. (1975). Community structure and competition between hydrilla and elodea. Hyacinth Control Journal 13:48–50.
- Michel, A., et al. (2004). Fluridone resistance in hydrilla. Journal of Aquatic Plant Management 42:89–94.
- Cuda, J.P., et al. (2002). Hydrilla biological control. Journal of Aquatic Plant Management 40:7–12.