from Plant & Pest Diagnostic Laboratory, April 25, 1997, Number 112

Control of aquatic weeds begins with prevention. For ponds, maintain good sod and grass cover around the perimeter. This will help prevent runoff and erosion. Keep livestock out of the pond. If the pond is used to water livestock, fence the pond and water the animals from a stock tank below and outside the fence. For larger sites, a good watershed program is essential to reduce nutrient input. Eliminating, when possible, upstream sites of fertilizer inputs, septic tank discharge, or lawn fertilizer applications that drain into feeder streams is essential to maintaining good water quality.

Aquatic weeds can be pulled out by hand or with rakes. But most people will use one of two methods: the grass carp or chemicals. The grass carp, which eats weeds, is recommended for most sites at 15 fish per vegetated acre. A higher stocking rate can be used in ponds that are solely intended to serve aesthetic purposes, like a pond on a golf course. Under no circumstances should grass carp be introduced into natural lakes, rivers, wetlands, or other natural areas. The fish must be purchased from a licensed vendor, who will actually stock the site. The site must not have an outflow, or the outflow must be screened, so that fish cannot escape. It may take quite a while (more: than one year) to see the effects of the grass carp if the pond is heavily vegetated. In addition, grass carp have their taste preferences just like we do. They will eat some species (like the pondweeds) in preference to others (like Eurasian water-milfoil). Therefore, they may not start eating the species that is causing the problem until they have removed everything else. Fish may have to be restocked after five years or so because as they get older and larger, they tend to eat less. Finally, effects of the grass carp on water quality are uncertain. In some cases, the water in a pond will become quite murky; in others, it may stay clear.

The grass carp is an alternative for people who have been using chemicals over the years with little success, who are simply tired of spending the money that chemicals cost, or who do not wish to introduce chemicals into their pond at all. Chemicals are still being used because they usually control the vegetation quickly and because they can be used selectively. For example, they can be used to control invasive stands of Eurasian water-milfoil but leave desirable native species that serve as valuable fish habitat, Chemicals are sometimes the only alternative (other than nutrient reduction and mechanical harvesting) for aquatic weed problems in large lakes.

For those who choose to use chemicals, the best time to treat aquatic weeds is during late spring, when plants are young and actively growing and most susceptible to herbicides. Do not wait until July or August, Waiting until late summer to treat the pond could result in fish kills. Bv that time. the plant growth is thick.

Killing off all the vegetation will quickly deplete the water of oxygen, causing a fish kill. If you must treat during the summer, treat only a portion of the pond at a time. Just as with weeds growing in a corn or soybean field, aquatic weeds must first be correctly identified before selecting the proper herbicide for control. Consult your local county Extension office for help in identifying aquatic weeds or send a sample to the Plant and Pest Diagnostic Lab at Purdue University. Also you can refer to the publication, WS-21 Aquatic Plant Management, available at your local county Extension office.

Algae (a.k.a. moss, scum) is controlled with Copper sulfate. Copper sulfate can be applied by dragging the crytals in a burlap sack behind the boat. The best method of. Application for floating mats is to dissolve the crystals or powder in water and spray directly on the mats. Copper will corrode metal fittings and nozzles, so wash equipment thoroughly after use. Chelated copper products such as Cutrine are less corrosive but more expensive.

Submerged (underwater rooted) plants such as the · pondweeds, naiads, coontail, and Eurasian watermilfoil (depending on the species) can be controlled using Aquathai, Diquat (also sold as Reward), or Sonar. Sonar is more expensive than the other products; however, it can provide two or more seasons of control. The Reward restrictions on water use after treatment were recently relaxed. The maximum waiting restriction on drinking water is three days, on livestock consumption, one day, and on irrigation, five days.

Free-floating plants like duckweed and watermeal are extremely difficult to control with herbicides. The Sonar AS formulation is effective on duckweed. Sonar can also be used for watermeal, but only at the highest rates and would probably require multiple treatments. This product is not for everyone. A single treatment for a surface acre with a depth of five feet or more would require 1.5 quarts, and one quart costs about $400! The only other alternative chemical is Reward, and this must be applied continuously during the season. The best treatment for the duckweeds is to remove sources of nutrient input, such as fertilizer runoff or leaf litter.