Managing invasive aquatic plants, in plain English.

Why invasive aquatic plants become a problem.

An invasive aquatic plant is one that arrived from somewhere else, has no local herbivores or pathogens holding it in check, and can reproduce fast enough to dominate a waterway within a single growing season. In Florida, that list includes hydrilla (Hydrilla verticillata), water hyacinth (Pontederia crassipes, formerly Eichhornia), water lettuce (Pistia stratiotes), and salvinia (Salvinia molesta). Each one arrived through the aquarium or ornamental trade in the 20th century and has been under active state management ever since.

When those plants are unmanaged in a nutrient-rich system, four things follow. Surface mats block sunlight from reaching submerged native seagrasses and native submerged aquatic vegetation. Dense stands prevent navigation, recreational fishing, and stormwater flow. When the mats die back (naturally in fall, or after cold snaps, or after any treatment), the decomposing biomass pulls oxygen out of the water column just like the 2016 brown-tide collapse did — localized fish kills follow. And large mats trap sediment and become physical barriers to water movement, worsening flood risk in the canals and stormwater systems they occupy.

The Indian River Lagoon proper is a saltwater estuary, so the freshwater invasives above are less of an issue in the open lagoon than they are in the freshwater canals, stormwater ponds, and tributary creeks that drain into it. But those upstream systems do feed the lagoon, and unmanaged plant biomass in them ultimately becomes nutrient loading in the lagoon downstream. This is why the state manages them at all.

Native vs. invasive vegetation.

Not every aquatic plant that grows in a canal or shoreline is invasive. Florida's native submerged aquatic vegetation includes tape grass (Vallisneria americana), Southern naiad, and pondweeds; native emergent species include pickerelweed, arrowhead, spatterdock, and native water lilies. Along the Indian River Lagoon's shoreline, mangroves, cordgrass, and salt marsh are native and are actively being planted under the Restore Our Shores living-shoreline program — the opposite of what gets removed under invasive-plant management.

This distinction matters because untargeted removal — whether mechanical or chemical — can damage the native plants that filter the water and provide fish and manatee habitat. Any competent management program identifies what it is treating before it treats. When treatment is not properly targeted, the ecological cost can exceed the benefit.

The four available tools.

State agencies, water management districts, and municipal stormwater operators have four categories of tool available for managing an invasive aquatic plant population. None of them is a silver bullet. Each has a role, and the honest debate is about when each is appropriate — not whether any of them should exist at all.

Appropriate does not necessarily mean preferred. In most situations, several management options are available, each with different costs, environmental impacts, and long-term consequences. A tool being appropriate for a site means it belongs in the conversation about that site — not that it is the only reasonable choice, or the one this page endorses.

1. Mechanical harvesting.

Mechanical harvesting means physically cutting or removing aquatic plants using a specialized harvester, an excavator with a rake attachment, or in smaller systems a truck-mounted vacuum. The cut biomass is loaded, hauled off-site, and composted or landfilled.

What it does well. Immediate visible removal — a heavily-vegetated canal can be opened for navigation in a single day. No chemical is introduced to the water. Native plants can be avoided if the operator is trained to identify them. Removed biomass is physically taken out of the system, which means the nutrients bound up in the plant tissue also leave the system rather than decomposing in place.

What it does poorly. High cost per acre — equipment, fuel, labor, and haul-away expense make it the most expensive per-acre option, often several times the cost of a targeted herbicide treatment. Limited to areas the equipment can physically reach; excavators need shoreline access, harvesters need enough water depth. Cutting can disturb bottom sediments and briefly re-suspend nutrients and muck. And many aquatic invasives (hydrilla in particular) regrow from fragments — harvesting without a follow-up treatment often accelerates the next bloom by breaking up and dispersing the plant.

When it is appropriate. Dense surface mats blocking navigation channels or boat ramps. Sensitive waterways where herbicide is not appropriate. Sites with strong shoreline access. Situations where the operator wants to physically remove nutrient-bound biomass from the system rather than let it decompose in place. Small, targeted, or high-value sites where the higher cost is justified.

Brevard County example. Brevard County’s Save Our Indian River Lagoon program has funded muck-dredging equipment that also functions as targeted mechanical removal in the tributary creeks that drain into the lagoon — particularly Turkey Creek and Crane Creek, where accumulated muck and plant biomass are being physically removed and hauled off-site rather than decomposing in place and re-releasing nutrients downstream.

2. Targeted herbicide treatment.

Herbicide treatment means applying an EPA-registered aquatic herbicide (in Florida, most commonly diquat, endothall, fluridone, glyphosate for emergent-only species, triclopyr, or 2,4-D depending on target) to selectively kill the invasive plant. Application methods range from foliar spray (for surface-floating plants like water hyacinth) to sub-surface injection (for hydrilla) to targeted bank application (for shoreline invasives).

What it does well. Cost-effective per acre — often 5 to 10 times cheaper than mechanical removal for equivalent coverage. Can reach areas mechanical equipment cannot — the middle of a lake, dense stands, tangled canal networks. Selective herbicides (fluridone for hydrilla, glyphosate for emergent-only, triclopyr for certain shoreline species) can be tuned to affect the target species without wiping out natives. Every aquatic herbicide registered for use in Florida has been through EPA review, has published tolerances, and carries specific label restrictions on drinking-water intakes, irrigation withdrawals, and swimming.

What it does poorly. Requires trained, licensed applicators to hit the target species without collateral kill of natives — and applicator error, in a hurried or under-supervised program, can and does happen. When the killed biomass decomposes in place, the resulting oxygen demand can locally suffocate fish — the same mechanism that killed fish in the 2016 brown-tide collapse. Public concern about chemical inputs to public waters is real and deserves to be addressed transparently — agencies that respond to that concern only with technical dismissals lose trust. Repeated application on the same water body can select for herbicide-resistant plant populations. And certain older or high-drift formulations do carry non-target risk that newer selective chemistries do not.

When it is appropriate. Large infestations where mechanical removal is impractical or prohibitively expensive. Species where selective chemistry exists and is well-characterized (hydrilla via fluridone, for example). Locations far from drinking-water intakes and non-target sensitive habitat. Programs that combine treatment with in-place biomass management, so the killed plants do not decompose in a way that suffocates fish. Programs that are transparent about what was applied, where, and why.

Indian River Lagoon example. The Florida Fish and Wildlife Conservation Commission conducts aquatic herbicide applications in freshwater bodies statewide, and publishes annual work plans identifying which lakes and canals are treated. In the IRL watershed, applications occur primarily in freshwater tributaries and stormwater systems that drain into the lagoon rather than in the saltwater estuary itself; a resident wanting to know what was applied where can request the FWC treatment records for a specific water body.

3. Biological control.

Biological control means introducing (or maintaining) a natural predator, herbivore, or pathogen of the invasive plant. In Florida, the classic example is the sterile triploid grass carp (Ctenopharyngodon idella), which is used in closed or semi-closed water bodies for hydrilla control. Other examples include the water hyacinth weevils (Neochetina eichhorniae and N. bruchi) and the salvinia weevil (Cyrtobagous salviniae), all introduced under USDA and Florida DEP-permitted host-specificity testing.

What it does well. Long-term, low-maintenance suppression — once a biocontrol population is established and the host-plant population is reduced, ongoing costs drop dramatically. No chemical input. No mechanical disturbance. Sterile grass carp cannot reproduce, so the population is controllable by not restocking. Works especially well in closed or semi-closed water bodies where the biocontrol agent cannot spread to non-target sites.

What it does poorly. Slow — often years between introduction and meaningful population suppression, which does not help a canal that needs to be navigable this month. Species-specific — a grass carp only eats certain plants, and if the invasive shifts or a different invasive arrives, the biocontrol may not help. Ecological risk if not properly host-tested — a poorly-chosen biocontrol agent can itself become a problem, which is why every new biocontrol release in Florida goes through years of USDA host-specificity work before permits are issued. Not effective in the middle of an established, well-connected infestation on the scale of hundreds of acres.

When it is appropriate. Established invasive populations where a proven, host-tested biocontrol agent exists. Closed or semi-closed ponds and canals. Long-time-horizon management where an operator has years to let the biocontrol establish. Programs that combine biocontrol with short-term mechanical or chemical suppression to buy time while the biocontrol population builds.

Florida example. Sterile triploid grass carp are stocked under FWC permit in closed and semi-closed freshwater bodies across Florida, including retention ponds and canals in the IRL watershed, for long-term hydrilla suppression. The water hyacinth weevils (Neochetina spp.) were introduced to Florida waters starting in the 1970s under USDA host-specificity testing and now suppress water hyacinth populations statewide with minimal additional intervention — a slow success that took decades to become visible.

4. Nutrient reduction as long-term prevention.

The three tools above all treat symptoms. The reason those invasives are able to form mats fast enough to dominate a Florida canal is that the water in that canal is nutrient-rich — because upstream septic loading, stormwater runoff, and legacy muck are still feeding it. Reduce the nitrogen and phosphorus entering the system, and the plants have less to grow on. See How a lagoon gets fixed for what nutrient reduction actually looks like: septic-to-sewer conversions, muck dredging, stormwater retrofits, and living shorelines.

What it does well. Addresses the root cause. Reduces the recurrence rate of every downstream problem — not just aquatic plants but also harmful algal blooms, low-dissolved-oxygen events, and seagrass die-off. Every dollar spent on nutrient reduction reduces the dollars needed on plant-management treatments in future years.

What it does poorly. Slow — measured in years to decades. Cannot solve today's navigation blockage. Requires sustained public funding and political consensus over multiple election cycles. Only works if all the other pieces of the nutrient budget are also being managed; a single septic conversion in an otherwise-loaded watershed produces measurable but modest improvement.

When it is appropriate. Always, in parallel with whatever short-term treatment is being applied. Nutrient reduction is not a substitute for mechanical or chemical management on a specific canal this year, but it is the reason the mechanical or chemical treatment can eventually stop being needed every year.

Brevard County example. The Save Our Indian River Lagoon Project Plan, funded by the half-cent sales tax Brevard voters approved in 2016, is the largest nutrient-reduction program in the IRL watershed — septic-to-sewer conversions, muck dredging, stormwater retrofits, and living shorelines. Every one of those projects reduces the nitrogen and phosphorus load that would otherwise be feeding the next generation of invasive plant growth in downstream canals and stormwater ponds.

A simple decision flow.

The chart below is a teaching tool, not a substitute for site-specific expertise. It shows the questions a plant-management program is roughly working through when it decides what to do at a given site. Real decisions include water body use, applicator licensing, permit conditions, non-target sensitivity, budget, and political context that no simple chart can fully capture.

Decision matrix: when each tool is appropriate.

The table below is a simplified teaching version of what state and district plant-management staff actually work through when deciding how to treat a specific site. The real decision includes water body use, applicator licensing, permit conditions, and non-target sensitivity that this page cannot fully capture.

The honest position.

There is a legitimate debate about how much aquatic-herbicide application is happening on Brevard's waterways, whether it is properly targeted, whether it is properly disclosed, whether public concern is adequately taken into account, and whether nutrient reduction is being pursued aggressively enough to reduce the need for it. Those are real questions worth asking of state and county programs, and they should be asked in public.

What this page will not do is present any one tool as a villain or any other as a savior. Mechanical removal alone leaves the underlying nutrient problem in place and often accelerates the next infestation by fragmenting the plants. Chemical treatment alone can suffocate fish and generates real public distrust when it is not transparent. Biological control alone cannot solve an urgent navigation blockage. Nutrient reduction alone cannot open a canal that is blocked this month.

A responsible plant-management program uses all four tools, in the right combination for the specific water body, with transparent disclosure of what is being applied where. That is the standard by which local programs should be judged. The Florida Fish and Wildlife Conservation Commission's Invasive Plant Management program and the UF/IFAS Center for Aquatic and Invasive Plants maintain the underlying science for how this is meant to be done.

Frequently asked questions.

Is the state spraying herbicide in the Indian River Lagoon itself? The Indian River Lagoon is a saltwater estuary; the freshwater invasives above are not the primary target there. Most aquatic-herbicide application in Brevard County occurs in the freshwater canals, stormwater ponds, and tributary creeks that drain into the lagoon, and in freshwater lakes managed by the FWC. Where and when applications occur is a matter of public record; the FWC publishes annual work plans and, on request, spray records for specific water bodies.

Is aquatic herbicide safe to drink or swim in? EPA-registered aquatic herbicides carry specific label restrictions for drinking-water intakes, irrigation withdrawals, and swimming. Those restrictions vary by product; some products have zero-hour swimming re-entry, others have longer intervals; some have permanent restrictions near drinking-water intakes. A responsible program discloses which product was used, where, and when, so that a resident can look up the specific restrictions that applied. The answer is not "yes" or "no" in the abstract — it depends on the specific product, dose, and location.

Why not just switch to mechanical removal everywhere? Cost. Mechanical harvesting is often 5 to 10 times more expensive per acre than a targeted herbicide treatment. In a program that has to cover thousands of acres of infested canals and lakes with limited public funding, exclusive mechanical treatment would either dramatically reduce the total acreage managed or dramatically increase the tax cost to residents. That trade-off should be discussed openly — it is a legitimate choice a community can make — but it should be discussed with the actual cost numbers on the table.

Do the plants come back? Yes, unless the underlying nutrient conditions change. Every short-term management tool — mechanical, chemical, biological — is a treatment, not a cure. Nutrient reduction is the cure, and it takes years. This is the same story as the lagoon itself: how a lagoon gets fixed.

What can a resident do? Ask your county and district agencies for their annual plant-management work plan and the treatment records for water bodies near you. Attend the Save Our Indian River Lagoon Citizen Oversight Committee meetings, which review the nutrient-reduction side of the work — see who watches the money. Cut fertilizer use on your own lawn during the summer fertilizer ban (Brevard County has an ordinance restricting fertilizer application from June through September). Support nutrient-reduction infrastructure projects on the ballot — those are what eventually make aggressive plant management unnecessary.

Scientific references.

Aquatic invasive plants in Florida — overview and management framework:

• Florida Fish and Wildlife Conservation Commission, Invasive Plant Management Section. Annual work plans, treatment records, and program overview. myfwc.com/wildlifehabitats/habitat/invasive-plants/
• University of Florida IFAS Center for Aquatic and Invasive Plants. Scientific and educational resources, species identification, treatment guidance. ifas.ufl.edu/media/ifasufledu/plants/
• Florida Department of Environmental Protection, Aquatic Habitat Restoration and Enhancement. floridadep.gov/rcp/aquatic-habitat-conservation-restoration

Mechanical harvesting — cost, effectiveness, and downstream nutrient implications:

• Bartodziej, W., et al. "Mechanical harvesting of aquatic plants: current practice and future needs." Journal of Aquatic Plant Management, various years. Peer-reviewed literature indexed via the Aquatic Plant Management Society.
• Madsen, J.D. "Advantages and disadvantages of aquatic plant management techniques." US Army Engineer Research and Development Center technical publications.

Aquatic herbicides — EPA registration, label restrictions, and selective chemistry:

• US Environmental Protection Agency. Pesticide registration and label databases. epa.gov/pesticide-labels
• Florida Department of Agriculture and Consumer Services, Pesticide Registration. Product-specific Florida-registration data.
• Netherland, M.D., and Getsinger, K.D. Selective chemistry for hydrilla and other aquatic invasives. US Army Engineer Research and Development Center technical publications; peer-reviewed Journal of Aquatic Plant Management.

Biological control — host-specificity testing, sterile grass carp, weevils:

• USDA APHIS. Biological control agent host-specificity testing framework. aphis.usda.gov/aphis/ourfocus/planthealth
• Cuda, J.P., et al. "Biological control of invasive aquatic and wetland plants in the southeastern United States." UF/IFAS extension publications.
• Florida FWC. Sterile triploid grass carp permitting and stocking guidance. myfwc.com/fishing/freshwater/regulations/triploid-grass-carp/

Nutrient reduction as long-term prevention:

• Indian River Lagoon National Estuary Program, Comprehensive Conservation and Management Plan. onelagoon.org
• Brevard County Save Our Indian River Lagoon Project Plan. brevardfl.gov/SaveOurIndianRiverLagoon/ProjectPlan
• Marine Resources Council, Indian River Lagoon Report Card. See the summary of the 2025 report on this site.