Mechanical/Physical Control Options
Duckweeds can be removed by raking or seining it from the pond’s surface.
Biological Control Options
Grass carp will seldom control aquatic vegetation the first year they are stocked. Young grass carp will consume duckweeds but are usually not effective control as large fish (over 10 pounds). Grass carp stocking rates to control duckweeds are usually in the range of 7 to 15 per surface acre or higher. In Texas, only triploid grass carp are legal and a permit from the Texas Parks and Wildlife Department is required before they can be purchased from a certified dealer (the list is at the end of the packet).
Tilapia will consume watermeal but are a warm water species that cannot survive in temperatures below 55 F. Therefore, tilapia usually cannot be stocked before mid-April or May and will die in November or December. Recommended stocking rates are 15 to 20 pounds of mixed sex adult Mozambique tilapia (Oreochromis mossambicus) per surface area. Tilapias are often not effective for vegetation control if the pond has a robust bass population due to intense predation. In Texas, stocking of Mozambique tilapia does not require a permit from the Texas Parks and Wildlife Department. Any other species of tilapia would require a permit. Check with out County Extension Agent in other states for legality of stocking tilapia.
Ducks will eat duckweeds but seldom control them.
Chemical Control Options
The active ingredients that have been successful in treating duckweeds include diquat (G), fluridone (E), penoxsulam (E), and flumioxazin (E). E = excellent, G = good
Reward is a liquid diquat formulation that has been effective on duckweeds. It is a contact algaecide and herbicide. Contact herbicides act quickly and kill all plants cells that they contact.
Sonar, Avast , and Whitecap are floridone compounds and comes in both liquid and granular formulations, and have been effective on duckweeds. These are broad spectrum, systemic herbicides. Systemic herbicides are absorbed and move within the plant to the site of action. Systemic herbicides tend to act more slowly than contact herbicides.
Habitat contains the active ingredient, imazapyr, which inhibits the plant enzyme AHAS (acetohydroxyaced synthase). Habitat is a systemic herbicide that is effective on post-emergent floating and emergent aquatic vegetation. Imazapyr is effective at low-volume rates and does not contain heavy metals, organochlorides or phosphates, making it safe to humans and livestock. Habitat requires the use of a spray adjuvant when applying on post-emergent vegetation.
Galleon is a liquid penoxsulam formulation. It is a broad spectrum, systemic herbicide. Systemic herbicides are absorbed and move within the plant to the site of action. Systemic herbicides tend to act more slowly than contact herbicides. Galleon may be sprayed directly onto emergent plants or applied directly into the water. Galleon should not be applied in areas where it will be diluted rapidly. Galleon will take 60-120 or longer to completely kill the target plants. Galleon will need a surfactant for foliar and exposed sediment applications.
Clipper is a flumioxazin product and comes in a water dispersible granule which must be mixed in water first and then either sprayed or injected. It is a broad spectrum, contact herbicide. Contact herbicides act quickly. Flumioxazin should be applied to actively growing plants and a surfactant will be needed if the herbicide is applied foliage of floating or emergent plants. Water pH needs to be below 8.5 or flumioxazin will rapidly degrade and lose effectiveness.
One danger with any chemical control method is the chance of an oxygen depletion after the treatment caused by the decomposition of the dead plant material. Oxygen depletions can kill fish in the pond. If the pond is heavily infested with weeds it may be possible (depending on the herbicide chosen) to treat the pond in sections and let each section decompose for about two weeks before treating another section. Aeration, particularly at night, for several days after treatment may help control the oxygen depletion.
One common problem in using aquatic herbicides is determining area and/or volume of the pond or area to be treated. To assist you with these determinations see SRAC #103 Calculating Area and Volume of Ponds and Tanks.
Many aquatically registered herbicides have water use restrictions (See General Water Use Restrictions).
To see the labels for these products click on the name. Always read and follow all label directions. Check label for specific water use restrictions.
Duckweeds can be very invasive in quiet, nutrient rich waters and careful consideration should be taken before introducing them to ponds. They can be propagated by transferring them in the spring to a shallow area of quite water.