We have received many phone calls over the past few weeks concerning aquatic weeds and algae. This time of year is when aquatic weeds are the most active, due to warm water and long daylight hours. My recommendations are almost one of the following: herbicide treatment, stocking the pond with triploid grass carp or physical control methods, or a combination of all three. This article is a brief introduction to the three control methods that I rely on.

Herbicide treatments are very common in aquatic weed management. Some herbicides need to be directly applied to the plants, while others can be applied directly to the water or injected into the water using a boat. The method of application generally depends on what type of weed. Aquatic weeds are classified into algae, submersed plants, floating leaf, emergent and finally, free floating plants. Algae is generally very fine and looks like “goop” or scum on the surface of the water and can generally be controlled with herbicides containing the active ingredient diquat. Submersed plants have all or most of the plant below the surface of the water; examples include hydrilla, coontail, southern naiad, and pondweed (there are over 80 species that are called pondweed!). Floating leaf weeds include lily pads, yellow floating heart, water pennywort and others. Emergent weeds include cattail, parrot’s feather, water primrose and many others. Finally, the free-floating weeds include, but are not limited to duckweed and watermeal.

As you can see, there are many different types of aquatic weeds and each class requires different types of chemistry to control. Identification is key to selecting the correct herbicide. Several things to note here: do not treat the entire pond at once when multiple applications may be needed. Aquatic herbicides are also very expensive, so make sure you are following the herbicide label and do not be afraid to hire a professional to administer the application if you are not comfortable doing so.

Triploid grass carp can be used to control almost all of the submersed weed species and one type of algae. Grass carp offer poor to no control of floating plants and emergent plants, such as duckweed, watermeal, cattail, or parrotfeather. The general recommendation is 10-15 triploid grass carp per surface acre of water. Triploid grass carp are a very economical way to control various aquatic weeds. Please take note that a permit is required from the North Carolina Wildlife Commission when stocking more than 150 triploid grass carp at one time, or introducing them into a body of water that is larger than 10 acres.

Finally, physical control methods for aquatic weeds include many methods including dredging sediment, hand removal, the use of pond dye, and draining the body of water. Oftentimes these methods are combined with herbicide applications and the use of triploid grass carp. Hand removal may clean up the pond for a few days but almost always fails to control the aquatic weeds long term. Pond dyes help keep sunlight from penetrating to the floor of the pond. Dredging to a minimum depth of 4 feet will also cut down on the amount of light getting to the bottom of the pond. This is more of a prevention measure than a control measure. In extreme cases, the pond may need to be drained to effectively kill all of the aquatic weeds and do pond maintenance, such as removing sediment. I do not like to make this recommendation unless it is my last resort.

As you can see, control and prevention of aquatic weeds is a very complex topic. There are many more aspects of aquatic prevention and control that I was not able to discuss in this format. For more information on how to identify and control the weeds in your pond or lake, feel free to contact me at North Carolina Cooperative Extension – Lee County Center, 919-775-5624.

Mitch Williams is the Agriculture Agent, Field Crops and Livestock for North Carolina Cooperative Extension in Lee County.