Information Possibly Outdated
The information presented on this page was originally released on February 26, 2016. It may not be outdated, but please search our site for more current information. If you plan to quote or reference this information in a publication, please check with the Extension specialist or author before proceeding.
Start early to get the upper hand on pondweeds
STARKVILLE, Miss. -- Having too many weeds in a pond is the bane of many Mississippi pond owners.
Pond weeds start growing early, as soon as day length and water temperatures allow. Don’t let them get out of hand! Start a weed management program before they become a problem to keep your pond picture perfect.
Prevention is the best way to avoid weed problems. Shallow areas where light reaches the pond bottom are ideal for the growth of rooted aquatic weeds. You can reduce weeds by deepening the pond edges so that the water depth quickly reaches 3 feet. For safety, make the slope 3:1 -- 1 foot increase in depth for every 3 feet farther from shore. Another way to help prevent weeds from becoming established is to stock triploid grass carp at about five fish per acre in new or weed-free ponds.
Too many or not enough nutrients in the pond can lead to weed growth. Excess nutrients from livestock or other sources can run off into a pond and lead to weed problems, especially algae. Duckweed and watermeal also thrive in nutrient-rich waters, especially in dry winters when ponds are not flushed out by rainwater.
In clear ponds, sunlight can penetrate to the bottom and stimulate the growth of rooted plants. One way to prevent this problem is to fertilize the water to stimulate the production of microscopic plants that shade the pond bottom. Ponds with existing weed problems should not be fertilized, as this will only stimulate the growth of the weeds. Fertilization is not a good option for stock watering ponds.
Prevention is always the best approach, but if weeds do become a problem, the first step in controlling them is properly identifying the problem plant(s). The most effective control measures vary with the kind of weed, so be sure to identify the plant accurately. Your local Extension Service office can help with identification, and there are several good online resources you can use.
Once you know the weed, there are four forms of weed control: physical, mechanical, biological and chemical. These control measures are usually most effective when combined.
Physical control involves using pond dyes to shade out plants, adding liners to prevent rooting or deepening the pond edges to make it harder for weeds to get sunlight. Pond dyes often are not effective, and liners can be prohibitively expensive. Pond deepening, however, is a good, long-term solution to many weeds problems.
Mechanical control by cutting or pulling plants is possible for small ponds or isolated patches of weeds. Weeds that are cut often grow back and have to be cut again. Also, some plants can spread from small fragments, so disturbing them may make matters worse. Floating weeds often are blown into a corner of the pond, where they can be scooped out with a fine mesh net.
Biological control, such as grass carp, is effective for some types of aquatic weeds. Grass carp prefer tender, succulent vegetation submerged in the water, such as hydrilla, pondweed and naiad. Carp will not control tough, fibrous plants that grow up out of the water, such as alligator weed and cattails. Grass carp may or may not eat other types of weeds, depending on how hungry the fish become, and results may not be predictable.
Chemical control of aquatic weeds should generally be considered the last resort in weed management. Spot treatments of weedy areas usually can be done without problems, but when the whole pond needs treating, it is important to accurately measure the pond area. It is amazingly difficult to visually estimate the area of a pond, even for “experts,” but it is necessary to get an accurate estimate so that you know how much herbicide to use.
The best time to treat aquatic weeds with herbicide is during the spring when the plants are growing rapidly and water temperatures are around 70 to 80 degrees. Extension Publication 1532, “Weed Control Guidelines for Mississippi - 2015” provides the latest information on approved herbicides, which plant species they control and restrictions on using the chemicals.
Decomposition of weeds killed by herbicides removes oxygen from the water and can even result in a fish kill, especially in the summer. When using a fast-acting herbicide, treating only a section, up to a third of the pond area, at a time will reduce the chances of oxygen problems. Unless the herbicide is intended for whole pond application -- for example, fluridone -- treating only a portion of the weeds at a time allows affected weeds to decompose before the next application.
Editor’s Note: Extension Outdoors is a column authored by several different experts in the Mississippi State University Extension Service.