Aquatic Herbicide Selection Guide

Understanding Aquatic Herbicides

Aquatic herbicides are pesticides specifically formulated and EPA-registered for use in water environments - ponds, lakes, channels, and other aquatic systems. They differ fundamentally from lawn or garden herbicides, which are toxic in aquatic systems and illegal to apply to water. Many states also require permits before herbicide applications in certain water bodies. Always check your state's regulations before treating.

The herbicide market is highly regulated. The EPA requires all aquatic herbicide products to demonstrate safety to non-target plants, fish, invertebrates, birds, and mammals at recommended application rates. Labels are legally binding - they represent the tested safety parameters. Using a product outside its label directions is illegal and risks environmental damage and personal liability.

Contact vs. Systemic Herbicides

The two major classes of aquatic herbicides differ in how they kill plants and how fast results appear. Understanding this distinction is crucial for selecting the right product for your situation.

Contact Herbicides: Fast-Acting, Shallow Control

Contact herbicides kill plant tissue on direct contact, disrupting cellular respiration or photosynthesis. They work quickly - visible damage appears within 24-72 hours, and plants are largely dead within a week. Examples include diquat, endothall, copper, and hydrogen peroxide.

The critical limitation: contact herbicides do not kill roots. Plants that grow from rhizomes or underground tubers (water lilies, cattails, arrowhead) regenerate quickly - typically within 2-4 weeks. Floating plants like duckweed and filamentous algae, which have no roots, may be controlled more permanently by contact herbicides, though some regrowth from survivors always occurs.

Contact herbicides are best for visible, quick results; for treating infestations you need to address immediately; and for use in ponds where you cannot afford to wait weeks for control. They require clear, clean water to work effectively - suspended sediment and algae reduce their efficacy.

Systemic Herbicides: Slow, Root-Killing, Long-Lasting

Systemic herbicides are absorbed by plant leaves or roots and move (translocate) throughout the plant's vascular system, disrupting growth hormones or energy metabolism. This translocation means the entire plant - stems, roots, rhizomes - dies from the inside. Results are slower: 7-30+ days for visible decline, 30-90 days for complete plant death, depending on the herbicide and water temperature.

Examples include glyphosate, fluridone, triclopyr, 2,4-D, and imazapyr. Systemic herbicides are excellent for preventing regrowth from rooted plants (water lilies, cattails, pondweed) because they kill the entire plant, including belowground structures. They require slower water circulation to work effectively - high flow rates reduce translocation and lower efficacy.

Systemic herbicides are best for long-term control; for rooted aquatic plants; for situations where you can wait for results; and for integrated management where you're combining herbicide use with aeration and bacteria treatments.

Herbicide Selection by Target Weed

Selecting the right herbicide starts with positively identifying your target weed. Not all herbicides work on all plants. The table below shows the most effective herbicides for common aquatic weeds and algae:

Target Weed / Algae	Best Herbicides	Type	Speed	Notes
Duckweed / Watermeal	Diquat (Harvester) Fluridone (Sonar) Flumioxazin (Propeller)	ContactSystemic	1-3 days (contact) 30-60 days (systemic)	Contact for fast knockdown; systemic for prevention. No roots to kill, but survivors regrow quickly.
Water Lilies / Lotus	Glyphosate (Shoreline Defense) 2,4-D (Navigate, granular) Triclopyr	Systemic	7-21 days	Must be applied to emerged pads for glyphosate. 2,4-D granular broadcast to water. Triclopyr for persistent control.
Cattails / Emergent Plants	Glyphosate (Shoreline Defense) Imazapyr 2,4-D	Systemic	7-30 days	Apply to foliage when actively growing (spring/early summer). Imazapyr best for persistent control and regrowth prevention.
Submerged Weeds (Milfoil, Hydrilla, Pondweed)	Fluridone (Sonar) Endothall 2,4-D Triclopyr	SystemicContact	7-60 days	Fluridone requires whole-pond treatment; endothall is contact; 2,4-D selective for broadleaf. Triclopyr good for woody stems.
Filamentous Algae	Copper (Cutrine Plus) Peroxide (GreenClean) Diquat	Contact	1-7 days	Copper requires high alkalinity; peroxide safer for fish. Diquat fast-acting. Aeration critical - dying algae consumes oxygen.
Chara / Muskgrass	Copper Endothall	Contact	3-7 days	Copper dosing critical - alkalinity-dependent. Endothall for faster results. Both benefit from aeration.

Key Active Ingredients Explained

Copper (Chelated Copper)

Copper is one of the oldest algicides, used for over a century to control planktonic algae and filamentous algae in ponds. Chelated copper (copper that is chemically bound to a complexing agent) stays suspended in the water column longer than copper sulfate, allowing controlled release of copper ions that kill algae cells.

Dosing is alkalinity-dependent. Water alkalinity (measured in ppm of CaCO3) determines how much copper can be applied safely. High-alkalinity water (above 100 ppm) can tolerate higher copper doses. Low-alkalinity water (below 50 ppm) is at risk for copper toxicity to fish. Never apply copper if total alkalinity is below 40 ppm. Fish deaths can occur from copper overdose, especially in soft water. Always run aeration when treating - dying algae depletes oxygen as it decomposes.

Products: Cutrine Plus, Captain Copper, and similar chelated copper formulations.

Diquat (Herbicide/Algicide)

Diquat is a fast-acting contact herbicide that kills plants and algae within 24-72 hours by disrupting photosynthesis. It works on many aquatic plants including duckweed, watermeal, and filamentous algae. Results are visible within 48 hours on a sunny day.

Because diquat is a contact herbicide, it does not kill roots. Plants with rhizomes or tubers (water lilies, cattails) regrow after 2-4 weeks. Floating plants and algae may have limited regrowth, depending on whether the application killed all individuals. Diquat is excellent for rapid symptom relief and is often used in combination with longer-acting systemic herbicides.

Products: Harvester, Aquazine.

Fluridone (Sonar RTU, Sonar SRP)

Fluridone is a slow-acting systemic herbicide that blocks plant pigment production, eventually killing the entire plant over 30-90 days. It works on a wide range of submerged weeds including milfoil, hydrilla, and pondweed, as well as duckweed and watermeal.

Critical application rule: fluridone must be applied to the entire pond. Spot treatments are ineffective because the herbicide degrades slowly and concentrations must be maintained throughout the treatment period. Water flow must be minimal - high circulation reduces effectiveness. Fluridone is best for closed systems like small ponds and private lakes where you can control water exchange. Not suitable for ponds with significant inflow or outlets.

Advantage: because results are slow, oxygen depletion is minimal - plant death is gradual and doesn't create an anoxic crisis. This makes fluridone safer for fish than faster-acting herbicides.

Products: Sonar RTU, Sonar SRP.

Glyphosate (Shoreline Defense, Rodeo)

Glyphosate is a systemic herbicide that kills most broadleaf plants by disrupting protein synthesis. It works on water lilies, lotus, spatterdock, alligatorweed, and many emergent plants when applied to foliage. It does not work on submerged plants because it cannot enter leaf tissue that is continuously wet.

Application method matters: glyphosate must be sprayed on emerged (above-water) plant parts. For water lilies, spray the floating leaf pads on a sunny morning - spray dries on the leaf and enters the plant. Results take 7-21 days. Glyphosate has no residual activity in water; it degrades quickly and does not bioaccumulate.

Only aquatic-formulated glyphosate (Rodeo, Shoreline Defense) should be used in water. Consumer lawn products are not labeled for aquatic use and may contain surfactants toxic in water systems.

Products: Shoreline Defense, Rodeo.

2,4-D (Navigate, others)

2,4-D is a selective herbicide - it kills broadleaf plants (like water lilies) but not grasses or narrow-leaved plants. It can be applied as a spray or granular formulation. Granular 2,4-D (Navigate) is broadcast into the water as a dry product, where it dissolves and slowly releases the active ingredient over days.

2,4-D works through multiple pathways in plants, making resistance unlikely. Results take 10-21 days. It's excellent for mixed infestations where you want to target broadleaf plants without affecting desirable narrow-leaved vegetation.

Products: Navigate (granular), various spray formulations.

Flumioxazin (Propeller)

Flumioxazin is a granular herbicide that controls a broad spectrum of aquatic weeds including duckweed, watermeal, and many submerged plants. It works by disrupting photosynthesis. Application is simple: dissolve granules in water and spray onto plants.

Results appear in 7-14 days. Flumioxazin requires pH below 8.5 for effectiveness. It's nonsystemic, so like all contact products, it doesn't kill roots - regrowth from rhizomes and tubers occurs. However, it's very effective on surface-floating plants.

Products: Propeller.

Triclopyr

Triclopyr is a broadleaf herbicide used for water lilies, lotus, and woody aquatic plants. It works as an auxin mimic, disrupting plant growth. Results take 10-21 days. Triclopyr is highly selective - it kills broadleaf and woody plants but not grasses. It's excellent for persistent control because it translocates to roots and rhizomes.

Products: Several triclopyr formulations are EPA-registered for aquatic use.

Endothall

Endothall is a contact herbicide that works on submerged and filamentous algae, as well as many aquatic weeds. It disrupts cellular respiration and kills plants within 3-7 days. Endothall is often combined with diquat in products like Aquastrike for enhanced efficacy.

Because endothall is a contact herbicide, it doesn't kill roots. It's best for plants without significant belowground structures (algae, free-floating plants) or as part of an integrated approach with systemic herbicides.

Products: Various formulations with endothall as an active ingredient.

Critical Safety Rules for Herbicide Application

Safety means protecting fish, other aquatic life, and the people and animals that use the water. Follow these rules without exception:

1. Run Aeration Before, During, and After Treatment

This is non-negotiable. Dying plants consume dissolved oxygen as they decompose. In a 24-48 hour period, decomposing vegetation can deplete all oxygen from a pond, causing fish kills even though the herbicide itself is not toxic to fish. Aeration provides oxygen and water circulation, preventing anoxic conditions.

Aeration can be surface aeration (fountain, spray), subsurface aeration (bubbler), or circulation (pump). Any method that creates water movement and gas exchange helps. Keep aeration running continuously throughout the treatment period and for at least 2-3 days after herbicide application as vegetation continues to decompose.

2. Treat in Sections, Not the Whole Pond at Once

Apply herbicides to only 1/3 to 1/2 of the pond at a time. Wait 10-14 days between applications. This approach accomplishes several goals:

• Fish have untreated areas to retreat to, reducing stress and escape space
• Oxygen depletion is localized rather than pond-wide
• You can observe effectiveness and adjust subsequent treatments
• Removes more biomass progressively, which is healthier for water quality

3. Observe Water Use Restrictions on the Label

Every EPA-registered herbicide has restrictions on secondary water uses. Common restrictions include:

• Swimming: 0-21 days (some products have no restriction)
• Livestock watering: 0-60 days (varies by product)
• Irrigation: 30-120+ days (many products have lengthy restrictions)
• Fish consumption: Always check the label

Always read the label before treating. If your pond supplies drinking water for livestock or is used for irrigation, choose an herbicide with appropriate restrictions or delay treatment until irrigation season is over.

4. Watch for Cyanobacteria Before Treating

If you suspect cyanobacteria (blue-green algae) - visible as blue, green, or reddish-brown scum or foam - be extremely cautious with treatment. Killing cyanobacteria with algicides releases cyanotoxins into the water, creating a human and animal health hazard. If cyanobacteria is present, consult your state environmental agency before treating. Some states recommend no herbicide use during confirmed cyanobacteria blooms.

5. Read and Follow the Label - It's the Law

The herbicide label is a legal document. It specifies approved uses, application rates, timing, and water use restrictions. Applying a product outside its label directions is illegal and can result in fines and environmental liability. Labels change as new safety data emerges - always use the current label, not old ones.

Water Use Restrictions Reference Table

This table summarizes typical restrictions for common aquatic herbicides. Always check your product label for current information:

Herbicide (Active)	Swimming Restriction	Livestock Watering	Irrigation Restriction
Glyphosate (Shoreline Defense)	No restriction	No restriction	No restriction
Diquat (Harvester)	24 hours	3 days	7 days
Fluridone (Sonar)	No restriction	No restriction	60-90 days
Copper (Cutrine Plus)	No restriction	No restriction	No restriction
2,4-D (Navigate)	7 days	14 days	21 days
Imazapyr	No restriction	Varies by formulation	Varies by formulation

Note: Restrictions change as labels are updated. Always verify current restrictions on the product label before application.

Application Methods

Spray Application (Tank Sprayer)

Spray application is the most common method for herbicides. Equipment includes a hand-pump sprayer (small ponds), a backpack sprayer, or a larger tank sprayer on a boat. Spray is applied directly to plant foliage, especially to emerged leaves and stems. Best results occur on sunny mornings when leaves are dry - this allows herbicide to dry on the leaf surface and penetrate effectively.

For submerged weeds, spray herbicides that work on submerged tissue (2,4-D, triclopyr). Glyphosate only works on emerged leaves, so it's not appropriate for submerged plants.

Granular Application (Broadcast)

Granular herbicides are broadcast into the water by hand or from a boat. They dissolve slowly, releasing active ingredient over hours or days. Products like Navigate (2,4-D granular) and Propeller (flumioxazin granular) are applied this way. Granular products are effective for whole-pond or large-area treatments because they distribute more evenly than sprays and are easier to apply from a boat without specialized equipment.

Subsurface Injection

For rooted plants like water lilies or cattails, herbicides can be injected directly into the stem or rhizome using an injection device. This method delivers herbicide directly to the plant with minimal environmental exposure, reducing overall application rates. Injection is labor-intensive but highly effective for persistent plants.

Timing: When to Treat

Herbicide efficacy depends heavily on water temperature and plant growth stage. Most aquatic herbicides work best when plants are actively growing:

• Spring (April-June): Ideal window. Plants are beginning active growth, herbicides translocate quickly. Water temperatures are warming but not yet peak summer heat. This is when most successful treatments occur.
• Summer (July-August): High temperatures can reduce glyphosate efficacy in some situations, but many herbicides (diquat, copper, fluridone) still work well. Be aware that high heat can stress fish.
• Fall (September-October): Effective for most herbicides, though water temperatures are dropping. Plants are still actively growing as they prepare for dormancy. Many professionals treat in fall for winter suppression.
• Winter (November-March): Most herbicides do not work in winter because plants are dormant or dead. Treatment is not recommended. Exception: emergent weeds like cattails can be treated with glyphosate in early fall before senescence.

Treat early and small. Small infestations are always easier and cheaper to control than large ones. If you see duckweed or watermeal starting, treat immediately rather than waiting for coverage to expand. Early intervention prevents the exponential growth that makes late-season control so difficult.

Recommended Products

Here are some of the most effective EPA-registered aquatic herbicides available through Natural Waterscapes:

Integrated Aquatic Weed Management

Herbicides are a tool, not a complete solution. The most successful long-term weed and algae management combines multiple approaches:

1. Herbicide Use (Symptom Management)

Target herbicides control active infestations. Select the right product for your weed and follow safety protocols. Herbicides buy time but don't address why weeds are growing.

2. Nutrient Management (Root Cause)

Aquatic weeds are symptoms of elevated nitrogen and phosphorus. Address these nutrient sources:

• Reduce fertilizer runoff from lawns and gardens
• Inspect and maintain septic systems (leachate is a major nutrient source)
• Manage waterfowl populations - their waste is nutrient-rich
• Reduce fish feed application if applicable

3. Beneficial Bacteria (Active Nutrient Consumption)

Products like Pond Cleanse contain beneficial bacteria that consume excess nitrogen and phosphorus, reducing the food source for weeds. Applied every two weeks during warm months, bacteria treatments reduce nutrient availability and support long-term control.

4. Muck Reduction (Nutrient Release Control)

Bottom muck accumulates from years of decomposing plants and organic matter. This muck is a reservoir of stored phosphorus that releases nutrients into the water column as it decomposes. Muck Remover pellets accelerate muck breakdown in a controlled way, while beneficial bacteria help manage the resulting nutrient spike.

5. Phosphorus Binding (Direct Nutrient Control)

Products like MetaFloc bind dissolved phosphorus in the water, removing it from the system and making it unavailable to plants. This is especially effective in spring when phosphorus is released from bottom sediments as water warms.

6. Aeration (Oxygen & Circulation)

Aeration circulates water, prevents stratification that traps nutrients at the bottom, and supports beneficial bacteria. It also reduces anoxic conditions that fuel algae growth. Aeration is the foundation of long-term water quality management.

Common Questions About Aquatic Herbicides

Will pond dye control duckweed or watermeal?

No. Pond dye reduces light penetration, controlling submerged algae and plants that need light. Duckweed and watermeal float at the surface where they receive full light regardless of dye. Dye is not effective for floating plants.

Can I use the same herbicide every year?

Yes, for most aquatic herbicides there is no documented resistance in aquatic plants. However, repeated use of the same herbicide over many years in the same water body creates selection pressure. Rotating herbicide classes (contact one year, systemic the next) is a good practice for long-term management.

What if my water is very alkaline? Which herbicides work best?

High-alkalinity water (above 150 ppm) can actually reduce efficacy of some herbicides. Copper is pH/alkalinity dependent - high alkalinity means you can use higher copper doses safely. Most systemic herbicides (fluridone, glyphosate, 2,4-D) work well in high alkalinity. Flumioxazin (Propeller) requires pH below 8.5, so it may not work in very alkaline water.

Can I treat a pond where I fish or swim?

Yes, but only with herbicides that have no or minimal restrictions on swimming and fishing. Glyphosate (Shoreline Defense) has no swimming or livestock restrictions. Diquat has a 24-hour swimming restriction. Always check the product label for restrictions specific to your water use.

My pond has no aeration system. Can I still treat it?

You can, but it's risky. Running aeration before, during, and after treatment is the single most important factor in preventing fish kills from oxygen depletion. If you must treat without aeration, treat very conservatively - only 1/4 of the pond at a time, and monitor water quality closely. Consider renting or purchasing a small aerator for treatment periods.