Duckweed

Common Duckweed, Giant Duckweed

Common duckweed (Lemna minor) is a very small light green free-floating, seed bearing plant. Duckweed has 1 to 3 leaves, or fronds, of 1/16 to 1/8 inch in length. A single root (or root-hair) protrudes from each frond. Giant (Spirodela polyrhiza) or big duckweed is still relatively small (1/16 to 1/4 inch) with 1 to 4 leaves, or fronds, light green in color. Three or more roots (or root-hairs) protrude from each frond. Duckweeds tend to grow in dense colonies in quiet water, undisturbed by wave action. Often more than one species of duckweed will be associated together in these colonies and can be an aggressive invader of ponds and are often found mixed in with other duckweeds, mosquito fern, and/or watermeal. If colonies cover the surface of the water, then oxygen depletions and fish kills can occur. These plants should be controlled before they cover the entire surface of the pond.

Dense duckweed colonies provide habitat for micro invertebrates but if duckweed completelycovers the surface of a pond for an extended period it will cause oxygen depletions. These colonies will also eliminate submerged plants by blocking sunlight penetration. Many kinds of ducks consume duckweed and often transport it to other bodies of water.

Control Options

Mechanical/Physical

Duckweeds can be removed by raking or seining it from the pond's surface.

Biological

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.

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

The active ingredients that have been successful in treating duckweeds include diquat (G) and fluridone (E). E = excellent, G = good

Redwing 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.

WhiteCap is a fluridone compound and has been effective on duckweeds. 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.

GullWing 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. GullWing requires the use of a spray adjuvant when applying on post-emergent vegetation.

Information and photos courtesy of:
Department of Wildlife and Fisheries Sciences, Texas A&M University
http://aquaplant.tamu.edu/