Aquatic plants are necessary parts of the ecosystem of a pond. Plants produce nutrients through photosynthesis, in the process giving off oxygen that fish require. However, forms of plankton or pond scum and filamentous algae, also called string algae, can overwhelm a pond, making it unattractive and a poor environment for fish. There are ways to treat ponds for weeds, depending on the type of weeds and use of the pond.
Types of Treatment
There are essentially five ways of treating ponds for weeds. Weeds can be physically cut, pulled or skimmed. Adding or cutting back on nitrogen will kill some of them. Draining the pond and letting it dry before refilling it will kill them. Plant-eating fish may get rid of them. Finally, they can be poisoned with herbicides. All these treatments have advantages and disadvantages.
The best way to treat ponds for weeds is to physically remove them. Duckweed blown against the shore by the wind can be netted. Cattails and weeds growing around the edges of a pond can be pulled by hand or cut with a scythe. Booms and barriers prevent floating weeds from entering by incoming streams. Mechanical harvesters range from 5-horsepower hydraulic sickle-bar cutters mounted on a pontoon boat to harvesters with a 10,000-lb. capacity that scoop weeds on board and dump them on the shore.
Weeds growing on the bottom can be controlled by increasing the amount of phytoplankton in the water, thus blocking the light reaching the weeds. This is done by adding nitrogen fertilizer to the water. However, hydrilla and some other bottom-growing weeds will consume more nitrogen than the phytoplankton, and the weed problem will get worse. If nitrogen is added to stimulate food production for aquaculture, weed-eating fish should be added along with herbicides.
Decreasing the amount of nitrogen and other nutrients can reduce the growth of algae and floating weeds. Nitrogen can come from food given to fish, plus fertilizer and manures washed into the ponds from fields, livestock and ducks.
Draining the Pond
A pond can be drained to kill weeds by exposing them to the sun. The pond must be kept drained long enough to kill exposed weeds or some of them will grow back more vigorously than before. Draining ponds used for aquaculture can improve areas that fish use for spawning and nursing. Also, some herbicides used to control pond weeds are labeled for drained ponds. Herbicides applied to the bottom of a drained pond can control aquatic weeds for several years.
Adding Plant-Eating Fish
The grass carp (Ctenopharyngodon idella) can provide good control for submerged weeds, filamentous algae and small floating weeds, but it eats all plants in the water, not just weeds. Some states require the use of the triploid grass carp that has three sets of chromosomes rather than two and so is sterile, avoiding the invasive spread of the fish outside of the pond. The University of Florida suggests that pond owners remember that it is easier to add grass carp than it is to remove them.
Treating with Herbicides
Horticulturists at the University of Missouri recommend copper-based algaecides or Aquathal to control filamentous algae or chara that cause pond scum. They recommend Diquat or Sonar to control floating weeds, including duckweed watermeal. Aquathol, Diquat, 2-4-D and Sonar are useful against coontail, watermilfoil and other submerged weeds. Rodeo or 2,4-D are useful against smartweeds, cattails and other weeds that grow on the edges of ponds.