PondTools

Weeds

Cattails, duckweed, watermeal: what works, and grass carp per acre

Cattails come out with a shovel or a cut below the waterline repeated through the season, or with an aquatic glyphosate, imazapyr or imazamox product sprayed on the leaves in late summer. Duckweed and watermeal are skimmed, starved of nutrients, and if that fails treated with diquat (duckweed only) or fluridone, flumioxazin or penoxsulam. Grass carp eat none of these reliably. They are a tool for soft submerged weeds, stocked at your state sheet's rate per acre and at 8 to 12 inches so bass cannot swallow them, and they take two seasons to show.

What the three plants are

Cattails are emergent, rooted in the shallows. Cornell Cooperative Extension says their rhizomes "grow underground near the edge of ponds," so a long shallow margin grows a cattail ring whatever you do about seed. Duckweed and watermeal float free and usually arrive together; Purdue's APM-2-W says watermeal "will feel gritty, like rubbing cornmeal between your fingers." Both are a nutrient problem: "These plants require a lot of nutrients (nitrogen and phosphorus) to grow" and "can produce a new plant every 24 hours." Do not fertilize a pond with either (the PondTools fertilization calculator flags that case).

What the extension sources say

Cattails

Texas A&M's AquaPlant page says "Cattails can be mechanically removed by digging up the rhizomes," but "Cutting off the tops of the plant will not kill them." Cornell gives the cut that works: "If you cut the cattails below the water line two or three times in a season, very few cattails will grow back the following year," and dredging should deepen the edge "to a point where cattails are unlikely to grow back (18 to 24")." AquaPlant adds that "There are no available methods for biological control of cattails at this time." The herbicides it lists are diquat, rated good, and glyphosate, imazamox and imazapyr, rated excellent, though glyphosate has "been effective on cattails above the water line, but ineffective on plants in the water." Penn State gives a rule for any spray: "Treating only one-third to one-half of the pond at a time will help prevent an accidental fish kill due to oxygen depletion," and Pennsylvania requires a permit for "Any application of an herbicide to a pond."

Duckweed and watermeal

Purdue's non-chemical options "start with eliminating obvious sources of nutrients." Skimming "can work, but the plants must be disposed of so that they do not wash back into the pond," and "Introducing grass carp is not a viable option because duckweed and watermeal are not preferred foods." Purdue names diquat and fluridone and separates the plants: repeated diquat "can successfully control duckweed; unfortunately, they seldom give effective control of watermeal," while fluridone "will control watermeal in most sites, but sometimes, for unknown reasons, it has little or no effect," and needs "pond outflow be minimized for at least 30 days." AquaPlant adds carfentrazone, flumioxazin and penoxsulam for duckweed and rates flumioxazin excellent on watermeal. Both Texas pages repeat that "the label is the law."

Grass carp

Texas A&M's 2022 sheet says "Grass carp are most effective at controlling tender, submerged species of aquatic vegetation" and that they "do not offer control for emergent plants they cannot reach, or tough, fibrous, woody or waxy species." Missouri says "Grass carp should never be stocked as a preventive measure."

Published grass carp stocking rates and sizes, per surface acre
SourceRate per acreSize with bass presentLegal
Missouri Dept. of Conservation, 2025"2 to 5 carp" at "20 to 40 percent" cover; "5 to 10 carp" at "40 to 60 percent"; "10 to 20 carp" above 60"8 to 12 inches"Screen the spillway
Texas A&M AgriLife, 2022"7 to 15 grass carp per acre" for moderate infestations; TPWD approves "Up to 10 grass carp per acre" a year"10 to 12 inches upon stocking"Triploid, TPWD permit
Oklahoma Dept. of Wildlife Conservation"five to 15 per acre" lightly infested; "15 to 30 per acre" heavily weeded"larger than eight inches""require no permit"
University of Tennessee (Hill)"5 up to 100 per acre"; "The lower rate is advisable""9 to 10 inches long or they will be eaten"Triploid required
Clemson HGIC"20 to 25 triploid grass carp per acre of vegetation""a minimum of 10-12 inches"Triploid, SC DNR permit

Missouri says "weed decline is usually not apparent in a pond until the end of the second year." Texas says carp "may not be an effective control measure 5 years or more after stocking" and, because they "are difficult to catch and remove once stocked," recommends "a conservative approach."

What pond owners on Pond Boss do

For cattails the forum cuts and sprays. Bill Cody (Moderator, Ambassador) says to cut them off underwater in fall so the roots drown for lack of air through winter. esshup (Moderator, Ambassador) says to spray in the fall and bag the catkins before they blow. The most viewed weed thread, Homemade weed killer. Pond safe??, asks about vinegar, salt and dish soap and was told the salt persists and the mix does not belong near water.

For the floaters, the forum agrees with Purdue about carp. teehjaeh57 (Chairman, Moderator) has not heard of grass carp controlling watermeal or duckweed in any scenario. The only dissent is thin: george1 saw duckweed controlled by carp or tilapia, and was not sure which. Fluridone is what worked. mnfish (Hall of Fame) killed a watermeal and duckweed infestation with it, CJBS2003 (Ambassador, Hall of Fame) saw watermeal whiten and die by two months, and the 2003 thread Duckweed control? settled on diquat to knock the plants back and fluridone to eradicate them.

For carp against submerged weeds the forum stocks far below the sheets. Bob Lusk (Editor, Pond Boss Magazine) says no more than 4 per acre to start, and Bill Cody says about 2 per acre if you want to reduce the weeds rather than eliminate them. Dave Davidson1 (Moderator) put 7 in a one acre pond and the pondweed was gone by the second spring. FireIsHot (Moderator) found bushy pondweed gone in months but coontail never controlled, and Bill Cody's quarter acre went bare and turbid for three years after the carp finished. For size, esshup stocks carp at least half the length of the biggest bass, because owners who bought 6 to 8 inch carp never saw them again.

Where the tiers differ and why it matters

The sources disagree on carp and duckweed. Penn State's grass carp page lists duckweed and watermeal as preferred plants; Purdue says they "are not preferred foods"; Clemson says carp eat duckweed without "satisfactory control." The forum's outcomes side with Purdue. Do not buy carp for a floating plant.

Stocking rates run from Missouri's 2 to 5 per acre at light cover to Tennessee's "5 up to 100," while the forum's regulars start at 2 to 4 and add later. The high rates are written for eradication, and the Texas and Tennessee sheets both say to start low. The forum rates sit inside Missouri's lowest band, and the failures people report ran the other way, ponds stripped bare and muddy with no cheap way to take the fish out. Size is where the sources come closest to agreeing, all of them somewhere between 8 and 12 inches, while esshup's half-the-length-of-your-biggest-bass rule has no published basis.

What to do

  1. Identify the plant. If it is gritty with no roots it is watermeal, and if there is a root under each leaf it is duckweed.
  2. For a small patch of cattails, dig the rhizomes, or cut below the waterline two or three times a season with the last cut in fall. Deepen the margin to 18 to 24 inches if a machine can reach.
  3. For an established stand, spray an aquatic-labeled glyphosate, imazapyr or imazamox with an aquatic surfactant on green leaves in late summer, treating a third to a half of the pond at a time, with the permit in hand.
  4. For floaters, cut the nutrients (Missouri suggests "a 100 foot or wider buffer strip"), skim when the wind has bunched the plants and dispose of them on land, then treat early in spring. Use diquat with repeats for duckweed, and fluridone, flumioxazin or penoxsulam where watermeal is present, with no outflow for 30 days after fluridone.
  5. Use grass carp only for a soft submerged weed (naiad, elodea, pondweed, chara). For a 2 acre pond about half covered, Missouri's "5 to 10 carp" per acre means ten to twenty fish, Texas's "7 to 15" means fourteen to thirty, and the forum's 2 to 4 means four to eight. Buy certified triploids at 10 to 12 inches, get the permit, screen the spillway, stock in early spring, and wait two seasons before adding more.
  6. Do not stock carp as a precaution, or for cattails, floaters or algae. Do not spray the whole pond at once, and keep vinegar and salt away from the water.

The PondTools calculator

Every rate above is per surface acre. The PondTools surface-area calculator computes the area from the outline you trace in the app (a shoelace calculation, so an irregular pond is not rounded up to a rectangle); the standalone version takes surface area in square feet, typical water temperature, the coldest night you plan for and the kind of fish. Divide square feet by 43,560 to get acres and multiply by the sheet rate as in step 5. There is no grass carp calculator yet.

Sources