Chemical Control of Aquatic Weeds

Imazypyr is being sprayed to kill lily pads.
W. Cory Heaton, ©2015, Clemson Extension

Aquatic weeds in ponds or lakes can be controlled by physical removal, biological control, or herbicides. The method or combination of methods used will depend on factors such as target weeds, non-target plants, and the uses of the water (fishing, swimming, livestock watering, and irrigation).

Physical removal can be accomplished manually or with machinery. It is time-consuming, expensive, and normally used alone if other methods are not feasible. However, a certain amount of physical removal may be necessary in combination with the use of biological control and herbicides

Biological control is an option for certain aquatic weeds. The major advantages are ease of application and no concern over damage to plants irrigated with treated water. Triploid grass carp can be very effective for controlling many submerged vascular aquatic weeds. Grass carp are usually used to control all vegetation in a pond rather than selectively controlling certain vegetation. Replacement stocking of grass carp is necessary when fish are lost. A permit is required to stock grass carp, and only triploid fish can be legally used in SC. Tilapia are stocked in the spring and control most algae species. The concern with tilapia is that they are tropical animals and usually die during cold winters, requiring an annual stocking. Tilapia are legal for use in SC. The South Carolina Department of Natural Resources (SC DNR) now requires a free-of-charge permit to stock tilapia and triploid grass carp for aquatic weed control in SC. You can obtain a permit from SC DNR at 803-734-3891 or from registered dealers in SC. The short permit can be FAXed (803-734-4748) for a rapid turnaround. A permit number from SC DNR is required prior to stocking tilapia and triploid grass carp. Check with your Department of Natural Resources to determine if grass carp and tilapia are legal to stock and if a permit is required in your state.

A free-of-charge permit from SC DNR is required to stock tilapia.
W. Cory Heaton, ©2015, Clemson Extension.

After spraying Imazypyr, there is a significant kill of waterlilies (Nymphaea odorata) on the pond.
W. Cory Heaton, ©2015, Clemson Extension

2,4-D, carfentrazone ethyl, copper, diquat, endothall, fluridone, glyphosate, imazapyr, sodium carbonate peroxyhydrate compounds, and triclopyr are registered for use in ponds by US EPA and can be used safely in ponds if the manufacturer�s label directions are followed. Certain waiting periods may be required before certain water uses are allowed after the herbicide is applied, while in some cases, waiting periods are not required. Various chemicals have different product formulations; only aquatic labeled pesticides and surfactants/adjuvants may be used in aquatic applications by law. Labels change frequently; refer to the current herbicide label for specific application information. Never exceed the rates recommended on the label of the specific product applied. The label is the law.

Amount of Formulation for Application

Effectiveness of Herbicides for Aquatic Weed Control

Effectiveness of Herbicides for Aquatic Weed Control Continued

Waiting Period (Days) Before Using Water After Application of Herbicides for Aquatic Weed Control