Pond & lake care
Pond and lake weed control: a complete guide
Clear your water the right way, without harming your fish or breaking the rules.
A weed-choked pond is more than an eyesore. Overgrown aquatic vegetation crowds out open water, tangles fishing lines, clogs pumps, and can throw off the whole pond ecosystem. The good news is that pond weeds are very manageable with the right product, but aquatic weed control has a few rules that lawn care doesn’t, and getting them wrong can cost you your fish. This guide walks through how to identify what you’re dealing with, choose the right product, and treat safely.
First, identify what kind of weed you have
Aquatic weeds fall into a few groups, and where the weed grows determines which product controls it:
Submerged weeds grow under the surface, rooted in the bottom, like pondweed, hydrilla, coontail, and milfoil. You often don’t see them until they reach the top.
Floating weeds sit on or just under the surface, like duckweed and watermeal (tiny green specks that carpet the water).
Emerged / shoreline weeds are rooted in the water but grow up out of it, like cattails, reeds, and other bank vegetation.
Match the product to the weed
Submerged & floating weeds
Diquat
37.3% Diquat Dibromide, a fast-acting contact herbicide for weeds in the water. It controls submerged weeds like pondweed and hydrilla and floating weeds like duckweed, with visible results in just a few days. It’s applied to the water and is safe for fish when used at the labeled rate.
For weeds growing IN the water
Shoreline & emerged weeds
Aquasel 53.8%
An aquatic-approved glyphosate (53.8%) for the vegetation growing up out of the water, cattails, reeds, and bank weeds. Standard glyphosate weed killers are NOT labeled for use around water; this one is specifically formulated and approved for it. Applied to the plant’s foliage, it works systemically down to the root.
For weeds growing OUT of the water
The #1 mistake that kills fish: treating too much at once
This surprises people: the danger to your fish usually isn’t the herbicide, it’s the dead weeds. When a large mass of vegetation dies and decomposes all at once, the decomposition pulls oxygen out of the water, and fish can suffocate. The fix is simple: treat no more than about one-quarter to one-half of a heavily weeded pond at a time, wait about two weeks for the treated section to break down, then do the next section. On a badly overgrown pond, patience here protects your fish.
Know the rules before you treat
Aquatic herbicide use is more regulated than lawn treatment, for good reason. A few things to sort out first:
Private vs. public water. A private pond with little or no outflow can generally be treated by the owner following the label. Public waters, or ponds that flow into them, often require a licensed applicator or a state agency, check your state’s rules.
Water-use restrictions after treatment. The label sets waiting periods before treated water can be used for things like drinking, irrigation, or livestock. Plan around them, especially if the pond feeds anything.
State registration. Some aquatic products aren’t registered for sale in every state. Confirm the product is legal where you are before buying.
Timing and application tips
Treat when weeds are young and actively growing and water temperatures have warmed (generally 50 to 60°F and up), usually late spring into summer. Actively growing weeds absorb the herbicide far better than mature or stressed ones. For submerged weeds, Diquat is applied to the water per the label rate for your pond’s size and depth; for shoreline weeds, Aquasel is sprayed onto the foliage. Always follow the label rates for your water body, more product is not better and can worsen the oxygen-depletion risk.
Common questions
Will pond herbicide kill my fish?
Not directly, when used at the labeled rate, Diquat is safe for fish (it’s applied well below the level that’s toxic to them). The real risk is indirect: if you kill a large amount of weeds at once, the decomposing plants pull oxygen from the water and can suffocate fish. That’s why you treat a heavily weeded pond in sections, a quarter to a half at a time, with about two weeks between treatments.
Which product do I need for my pond?
It depends on where the weeds grow. For weeds in the water, submerged plants like pondweed and hydrilla, or floating ones like duckweed, use Diquat. For weeds growing up out of the water along the bank, like cattails and reeds, use Aquasel 53.8%, an aquatic-approved glyphosate. Many ponds have both and need both.
Do I need a permit to treat my pond?
For a private pond with minimal or no outflow, usually you can treat it yourself following the label. Public waters, or ponds connected to them, often require a licensed applicator or a state agency, and rules vary by state. Check your state’s aquatic pesticide regulations before treating anything beyond a self-contained private pond.
Can I still use the pond water after treating?
The label specifies waiting periods before treated water can be used for things like drinking, irrigation, or watering livestock. These vary by product and rate, so read and follow the label’s water-use restrictions, and plan your treatment around when you’ll next need the water if the pond feeds anything.
When is the best time to treat pond weeds?
When the weeds are young and actively growing and the water has warmed, generally 50 to 60°F or warmer, which usually means late spring into summer. Treating early, before weeds fully take over, is both more effective and safer for your fish, since there’s less vegetation to decompose at once.
How fast will I see results?
With Diquat, floating and submerged weeds typically show visible decline within a few days to a couple of weeks. Shoreline weeds treated with a systemic like Aquasel take a bit longer, since the product has to move down into the roots, but that also gives longer-lasting control of those perennial bank plants.
Ready to clear your water?
Contact Agrisel to find a reseller carrying Diquat and Aquasel 53.8% for your pond or lake.