How to Kill Hydrilla: Effective Control Methods

Killing hydrilla requires a combination of chemical herbicides for immediate knockdown and biological or environmental strategies for longer-term suppression. No single method eradicates it permanently, because hydrilla produces underground tubers that survive in sediment for years and sprout new growth long after the visible plant is gone. The most effective programs pair herbicide treatments with triploid grass carp stocking, but even then, managers treat hydrilla control as an ongoing commitment rather than a one-time fix.

Why Hydrilla Keeps Coming Back

Hydrilla’s persistence comes down to how aggressively it reproduces. The plant spreads through fragments, subterranean tubers, and small bulb-like structures called turions that form along the stems. A single stem fragment left in the water can root and produce a new colony, which makes any control method that breaks the plant apart counterproductive if those pieces aren’t removed.¹ The underground tubers are the real problem. They sit buried in sediment, waiting out herbicide treatments, drawdowns, and even years of dormancy before sprouting when conditions improve.

Two biotypes of hydrilla exist in the United States, and they behave differently. The dioecious biotype, found mainly in the Southeast, grows tall quickly, sending stems to the water surface within two to three weeks of planting.² The monoecious biotype, more common in northern states, grows as a denser mat near the bottom and produces tubers and turions at a faster rate. Its tubers also germinate at lower temperatures, which lets it survive shorter growing seasons.³ The practical takeaway is that neither biotype is easy to manage, but they may call for slightly different timing and tactics depending on your region.

The Underground Tuber Bank

Tubers are hydrilla’s insurance policy. Freshly collected tubers hold moisture content between 50 and 60 percent and stay viable at rates above 90 percent. Lab work shows that drying them out kills them, with roughly a 2 percent increase in mortality for every 1 percent drop in moisture. That sounds promising until you look at what happens in real lakes. In a northern Texas reservoir, the buried tuber bank showed no decline in number or viability even after 12 continuous months of water drawdown, because the mud never dried out enough to damage the tubers underneath.⁴

Even aggressive manipulation doesn’t wipe them out. In the same study, an experimental pond with a starting tuber density of roughly 676 tubers per square meter in shallow zones was cycled through six flood-and-drawdown events. Tuber numbers dropped below 30 per square meter, but they were never fully eradicated.⁴ This is a critical point for anyone hoping that draining a pond will solve a hydrilla problem. Drawdowns can reduce the bank significantly, but the remaining tubers are enough to restart an infestation once water returns.

Where tubers have been successfully depleted, it took years of combined pressure. In the North New River Canal in Florida, herbicide treatments ending in 1989 eliminated the parent plants and stopped new tuber and turion production. Grass carp that escaped into the canal around the same time then fed on the sprouting propagules. Within three to four years after the parent plants were removed, turions and tubers were essentially gone from core samples.⁵ The lesson is that suppressing the living plant and simultaneously preventing tubers from resprouting is the only reliable path to depleting the bank.

Chemical Herbicides That Work Against Hydrilla

Several herbicides achieve strong control of hydrilla, but each works differently and has trade-offs in terms of contact time, cost, and risk of resistance. The herbicides with the most research support fall into a few categories.

Endothall: A contact herbicide that kills plant tissue on contact. At higher concentrations, it achieved greater than 90 percent biomass reduction in laboratory trials at both 24-hour and 72-hour exposure times.⁶ Endothall works quickly but doesn’t kill tubers in the sediment, so regrowth is likely without follow-up treatments.
Diquat: Another contact herbicide. Lab studies found that control of both monoecious and dioecious biotypes required at least two days of exposure at lower concentrations, though increasing the dose shortened the needed contact time to as little as 6 to 12 hours depending on the plant’s growth stage.⁷ Diquat also achieved greater than 90 percent reduction in the comparative herbicide trial mentioned above.⁶
Flumioxazin: A newer contact option that showed greater than 90 percent reduction at both 24-hour and 72-hour exposures in the same comparative trial.⁶
Florpyrauxifen-benzyl: An auxin-mimic herbicide registered for hydrilla control in 2018. Its use has grown rapidly. One of its advantages is that it maintains effectiveness even with intermittent exposure, which matters in flowing water where herbicide concentrations fluctuate. In trials testing intermittent dosing, the greatest biomass reduction (89 percent) came from florpyrauxifen-benzyl treatments with a six-day gap between exposures.⁸
Fluridone: A systemic herbicide that was for years the only USEPA-approved option providing systemic control of hydrilla. Unlike contact herbicides, fluridone works slowly over weeks, inhibiting an enzyme the plant needs to protect itself from light damage. However, widespread resistance has emerged, making it unreliable in many water bodies (more on this below).⁹

Combining herbicides with different modes of action is now considered the smartest chemical approach. Pairing endothall with florpyrauxifen-benzyl, for instance, hits hydrilla through two separate pathways, which provides effective control while also slowing the development of resistance.¹⁰

The Fluridone Resistance Problem

Fluridone’s story is a cautionary tale about relying on a single herbicide. For years it was the go-to systemic treatment, applied across hundreds of Florida water bodies. Then managers started noticing it wasn’t working as well. A survey of 200 Florida water bodies found that hydrilla populations with two- to six-fold higher fluridone resistance had appeared in 20 of them.⁹ Genetic analysis revealed that the resistance came from mutations in the very enzyme fluridone targets. Three independent mutations were found at the same spot in the gene, each arising separately through somatic mutation rather than sexual reproduction.¹¹

This finding changed how managers think about herbicide programs. Because hydrilla can evolve resistance through simple somatic mutations, and because the plant spreads vegetatively through fragments, a resistant clone can quickly dominate a lake once it appears. The shift toward multi-herbicide programs using endothall and florpyrauxifen-benzyl together is a direct response to the fluridone resistance crisis. If you’re managing a water body where fluridone has been used heavily in the past, it’s worth assuming some level of resistance may already exist.

Grass Carp for Long-Term Suppression

Triploid grass carp are sterile fish that eat enormous quantities of aquatic vegetation, and hydrilla is one of their preferred foods. Stocking them is one of the most effective long-term tools available. In a study of five Piedmont reservoirs, an integrated approach using herbicides plus 20 triploid grass carp per surface acre of infestation eliminated hydrilla from the water column within one calendar year in four out of five cases.¹² That’s substantially faster than using grass carp alone, which typically involves a lag of several years before the fish population builds up enough feeding pressure.

After the initial knockdown, a maintenance density of at least one triploid grass carp per eight surface acres of the total reservoir prevented regrowth. Only one of the five reservoirs in the study experienced a brief, minor reinfestation.¹² The strategy worked best when hydrilla was caught early, at just 1 to 3 percent of the reservoir’s surface area.

Grass carp don’t last forever, though. Long-term population monitoring in the Santee Cooper reservoirs showed an annual mortality rate of about 32 percent, meaning the stocked population declines steadily and requires periodic restocking.¹³ Other reservoirs in the same study showed erratic survival that made mortality estimates unreliable, so the restocking interval depends heavily on local conditions. Grass carp also eat native aquatic plants, not just hydrilla, which creates ecological trade-offs that managers must weigh. Some states require permits or prohibit grass carp stocking altogether.

Researchers have also explored insect biocontrol agents. A leaf-mining fly, Hydrellia pakistanae, was released in the U.S. in the 1980s and 1990s to attack hydrilla leaves. More recently, a related species showed better performance against the South African biotype of hydrilla, with higher survival, greater egg production, and faster development.¹⁴ Insect biocontrol on its own hasn’t achieved the dramatic results that grass carp deliver, but it remains an area of active research, especially for situations where stocking fish is impractical.

Why Cutting and Pulling Usually Backfires

The instinct to rip hydrilla out by hand or mow it with a harvester makes intuitive sense, but hydrilla’s biology turns physical removal into a dispersal mechanism. Any method that chops the plant into fragments risks spreading it, because each fragment can root and produce a new colony.¹ Even careful hand-pulling leaves behind roots and tubers in the sediment, which resprout. A study that tested manual removal combined with planting native species in a spring-fed river found that cleared plots were recolonized by hydrilla from nearby stands, largely because of the high number of propagules the plant produces from stem fragments.¹⁵

That said, physical removal has a narrow role. In small, contained areas such as around a dock or boat ramp, hand-pulling or suction harvesting can keep hydrilla in check if you’re committed to doing it repeatedly and carefully removing all fragments from the water. It’s not a solution for a lake-scale infestation.

Water Drawdowns Have Real Limits

Lowering the water level to expose hydrilla and dry it out sounds like it should work, and in laboratory conditions, desiccation does kill tubers. But as the Texas reservoir study showed, buried tubers in field conditions retain enough moisture from the surrounding mud to survive a full year of continuous exposure.⁴ Even six repeated flood-and-drawdown cycles only reduced the tuber bank to low levels without eliminating it entirely.

Drawdowns can be useful as one piece of a larger strategy, especially in water bodies where you have the infrastructure to control water levels precisely. Exposing sediment during hot, dry periods offers the best chance of actually drying tubers enough to kill them. But counting on a drawdown alone to solve a hydrilla problem is a recipe for disappointment. The tubers are patient, and they don’t need many survivors to repopulate a lake.

Combining Methods and Managing Costs

The most successful hydrilla management programs treat it as an integrated pest management challenge, layering multiple approaches. Estimated costs vary widely by method. Chemical herbicides run roughly $100 to $500 per acre, making them the most affordable option for large areas. Grass carp stocking costs between $15 and $150 per acre. Hand pulling is far more labor-intensive at $400 to $1,000 per acre, while suction harvesting can reach $1,000 to $25,000 per acre. Benthic barriers, which are fabric mats placed on the bottom to block light and growth, cost $10,000 to $20,000 per acre and only make sense for small, high-priority zones.¹⁶

For most water bodies, the cost-effective approach is herbicide application to knock down the existing biomass, followed by grass carp stocking to provide ongoing grazing pressure that prevents regrowth from the tuber bank. The initial herbicide treatment handles what the fish can’t eat fast enough, while the fish handle the long tail of sprouting tubers over subsequent years. Maintenance restocking of grass carp every few years is part of the cost, given their natural mortality.

Early detection matters enormously for cost control. Treating a lake when hydrilla covers just a few percent of the surface is dramatically cheaper and more effective than trying to knock back a full-blown infestation. Once hydrilla carpets a water body, it can reduce dissolved oxygen levels to the point where other species struggle to survive, and the management bill escalates quickly.¹⁶

Protecting Native Plants During Herbicide Treatment

One of the less obvious challenges of killing hydrilla is that herbicides can also harm native aquatic plants you want to keep. A long-term analysis of Florida water bodies found that native plant richness was actually higher in systems where hydrilla was managed more frequently with herbicides, likely because keeping hydrilla in check gave native species room to grow. However, more extensive herbicide application, meaning treating larger areas at once, was associated with reduced native plant diversity.¹⁷

The practical implication is that targeted, frequent treatments of hydrilla patches are better for native plant communities than infrequent, lake-wide herbicide dumps. Treating hydrilla early and often in smaller areas gives native plants a competitive advantage without subjecting the entire littoral zone to herbicide exposure. Managers who can vary the timing, location, and extent of treatments stand the best chance of controlling hydrilla while keeping the rest of the plant community intact. Selectivity in where and how you apply herbicides matters as much as which herbicide you choose.

The Monoecious Biotype in Northern Waters

If you’re dealing with hydrilla north of its traditional southeastern range, you’re almost certainly facing the monoecious biotype, which has spread into states like Connecticut, New York, North Carolina, and Indiana. This biotype behaves differently from the dioecious plants in Florida. It grows more compactly near the bottom rather than racing to the surface, producing dense tangles of horizontal stems and root crowns.² Its tubers germinate at lower temperatures, and it produces propagules more abundantly under short-day conditions compared to dioecious plants.³

These differences matter for management. The monoecious biotype’s dense bottom growth can be harder to detect from the surface, and its heavy tuber production means the sediment bank builds up fast. Herbicide trials show that both biotypes respond to the same chemicals, but the monoecious biotype’s growth habit, with more biomass concentrated low in the water column, can affect how well contact herbicides reach it. Managers working in northern lakes should expect an aggressive tuber-banking strategy from the plant and plan their herbicide and grass carp programs accordingly, with particular attention to follow-up treatments aimed at preventing the tuber bank from rebuilding after the initial knockdown.

1
UF/IFAS Entomology and Nematology Department. How to Kill Hydrilla: Effective Control Methods
2
Weed Science. Differential Responses to Photoperiods in Monoecious and Dioecious Hydrilla verticillata
3
Weed Science. Comparative Studies of Monoecious and Dioecious Hydrilla (Hydrilla verticillata) Biotypes
4
Weed Science. Effects of drawdowns and dessication on tubers of hydrilla, an exotic aquatic weed
5
Aquatic Botany. Depletion of turions and tubers of Hydrilla verticillata in the North New River Canal, Florida
6
Invasive Plant Science and Management. Comparative herbicide response of three US Hydrilla verticillata biotypes
7
Weed Science. Responses of Monoecious and Dioecious Hydrilla (Hydrilla verticillata) to Various Concentrations and Exposures of Diquat
8
Invasive Plant Science and Management. Monoecious hydrilla (Hydrilla verticillata) response to intermittent herbicide exposures
9
PubMed. Somatic mutation-mediated evolution of herbicide resistance in the nonindigenous invasive plant hydrilla (Hydrilla verticillata)
10
Weed Science. Endothall and florpyrauxifen-benzyl behavior in hydrilla (Hydrilla verticillata) when applied in combination
11
Weed Science. Mutations in Phytoene Desaturase Gene in Fluridone-Resistant Hydrilla (Hydrilla verticillata) Biotypes in Florida
12
North American Journal of Fisheries Management. Hydrilla Management in Piedmont Reservoirs Using Herbicides and Triploid Grass Carp: A Case Study
13
North American Journal of Fisheries Management. Long‐Term Population Response of Triploid Grass Carp Stocked in Piedmont and Coastal Plain Reservoirs to Control Hydrilla
14
Biological Control. A comparison of host range and performance of congeneric leaf-mining flies, Hydrellia pakistanae (Diptera: Ephydridae) and Hydrellia sp., two candidate biological control agents for the South African biotype of Hydrilla verticillata (Hydrocharitaceae)
15
Knowledge and Management of Aquatic Ecosystems. Evaluating the suppression of Hydrilla verticillata by manual removal and planting natives for small-scale restoration efforts in a spring-fed river
16
UF/IFAS Extension. Hydrilla Integrated Management
17
PubMed. Frequency and extent of aquatic plant management with herbicides determines long-term effects on native plants