Aquatic Weed Control in Irrigation Water Supplies

Jack M. Whetstone, Department of Forestry and Natural Resources
(revised by L.B. McCarty)

To download the 2015 Pest Control Guidelines for Aquatic Weed Control in Irrigation Water Supplies, please click here.  (80 KB, pdf)

Aquatic weeds in ponds or lakes used as sources for irrigation water 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 what the water is used to irrigate. 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 control 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 thereby 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 prior to stocking tilapia and triploid grass carp for aquatic weed control in SC.  A permit can be obtained 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 turn around. 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.

Diquat, endothall, glyphosate, fluridone, triclopyr, copper, sodium carbonate peroxyhydrate, 2,4-D, carfentrazone,  imazapyr, penoxsulam, and imazamox compounds can be used safely in ponds used as irrigation sources if the manufacturer’s label directions are followed.  Certain waiting periods may be required before using water for irrigation 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 label of the specific product applied.  The label is the law.

Table 1.  Amount of Formulation for Application. (18 KB, pdf)

Table 2.  Effectiveness of Herbicides for Aquatic Weed Control. (61 KB, pdf)

Table 3.  Waiting Period (days) Before Using Water After Application of Herbicides for Aquatic Weed Control. (21 KB, pdf)

Lake Hartwell Fishermen