Methods to Control Weeds in Your Lake

In any lake or pond, nuisance aquatic plants should be controlled through an Integrated Pest Management (IPM) approach. The IPM approach includes prevention that include biological, mechanical, and chemical control. The best prevention is constructing the pond with rapid slopes to 2.5-3 feet and following a good fertilization program. In addition, stocking with grass carp at five to seven fish per surface acre before any weed problem appears can also prevent many submerged weeds from establishing. It is a very good idea to start weed management programs before the noxious plants cover large sections (20%+) of the pond.

Biological control of submerged weeds is possible through the use of triploid grass carp, but grass carp are regulated fish species. Permits for triploid grass carp must be obtained from your state fish and wildlife department. Stocking rates for grass carp should be based on the species of weed to be controlled and the percent or amount of the pond coverage . Triploid grass carp are usually effective at controlling submerged weeds for five to seven years but usually live to be 10 to 12 years old. Therefore, they have to be restocked periodically to maintain control.

Mechanical control is expensive, time consuming, and not often practiced for these reasons. If weeds are mechanically cut they usually need to be removed from the pond so the decomposition will not cause oxygen depletions. Mechanical control is best practiced immediately when weeds first invade the pond. Physically removing the first pioneering cattail plant or willow tree stops their immediate spread and saves time and expense on future treatments.

Chemical control through the use of EPA registered herbicides is species specific and requires careful attention to application techniques and water use restrictions. Chemical control also requires an applicators license or paying a trained professional to do it for you. It can be expensive, but in many cases it is the most viable option. Often when herbicides are not applied correctly, the massive amount of weeds killed leads to rapid decomposition and oxygen depletions. Oxygen depletions  usually result in fish kills or stress that leads to parasite and disease outbreaks.

Chemical treatment of algae, floating, and submerged plants is best done in the early spring before the plants have time to build up large biomass and while water temperatures are cool and dissolved oxygen levels high. In contrast, chemical treatments of emergent plants are often best in the fall before plants go dormant for the winter. Where possible, most chemical treatments of heavy weed infestations should be applied in only a portion of the pond and
not over the entire area. Treating one-quarter to one-third of the pond at a time, then allowing two weeks for decomposition before another treatment, will reduce the chance of an oxygen depletion and fish kill.

The key to aquatic weed control is correct identification of the noxious weed species and selection of the best management option for that individual plant or alga and specific pond conditions.

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