Pest Alert – Weeds in Ponds

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pond with algae
Weeds and algae in ponds are occurring more frequently in western North Carolina. There are several reasons for this problem. First, winters have not gotten as cold as they have in the past; ponds once froze in the winters. Secondly, most ponds and lakes are old with very few new ponds having been built in the last twenty years. These old lakes have filled with silt and are now shallower. That means water temperatures are higher. Finally, water fowl are much more common today than in the past, their populations having been decimated by DDT, habitat change, hunting by the 1970s. Waterfowl populations are now recovered and in some cases such as Canada geese, they have become a pest! These birds spread weeds and algae between water bodies.
Learn about algae in this previous article. Some algae control products available locally are Cutrine Plus, Flumigard, Copper Sulfate, Seclear and Captain.
Pond Weeds
When ponds are too clear or if lakes are too shallow (mountain lakes often silt up over time), then sunlight penetrates to the soil on the bottom of the pond and weeds grow. Lots of heavy rains can flush ponds of their murky phytoplankton rich waters. Phytoplankton are tiny plants that make water cloudy. Clear water is often not a good thing for ponds. Fertilizing and liming ponds can improve phytoplankton populations. Learn more by reading the NC State Pond Management Guide.
Another problem is that the winters here are not as cold as they have been in the past. Ponds once froze over killing lots of weeds. A severely cold winter has not happened in a long time.
Finally, new people are moving here from all over the world and are spreading weeds and algae via boats and careless dumping of aquariums.
Pond  Weed Control
Deepening the pond can alleviate weeds. You can drain the entire pond and dig it deeper or you can dredge it with the water intact. These methods are costly. In some cases lowering the water level and deepening the pond edges can help. This method is less expensive.
Winter drawdown helps control aquatic weeds by exposing them to drying and freezing. Lower the pond three to four feet in late fall. Reduce surface acreage by at least one-third but not more than one-half. Allow the pond to remain low until February or early March. To be effective, use a drawdown for at least two successive years. The water source for the pond must be sufficient for complete refilling by early summer.
Grass carp are often a good long-term solution for pond weed management. Triploid grass carp are fish that are bred to be sterile so they won’t reproduce. They control tender vegetation such as submersed weeds and floating duckweeds and some algae. Most people buy the fish from this fish farm.
Herbicides are used in ponds in a similar way that we treat lawn weeds. In fact, the active ingredients are often the same, only the formulations are different. DO NOT USE TERRESTRIAL/LAWN HERBICIDES IN AQUATIC ENVIRONMENTS. READ AND FOLLOW ALL LABEL INSTRUCTIONS FOR APPLICATION AND PERSONAL SAFETY EQUIPMENT.
Locally available aquatic herbicides are Navigate (aquatic 2,4-D ), other 2,4-D products, Sonar, Flumigard and aquatic versions of glyphosate.
Aquacide sells lake weed control products online. Their website is very informative and helpful in identifying weeds.

photo by Master Gardener Volunteer RM

*Inclusion of product names neither implies effectiveness, nor endorsement by the author or NC State University. Not all products described may be locally available, and the active ingredient content may be changed by manufacturers. Read the label CAREFULLY and follow all label instructions whenever using any pesticides as required by the EPA.