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Controlling Excessive Plant Growth

Plant management options are detailed in a comprehensive and very readable manual prepared jointly by the North American Lake Management Society and the Aquatic Plant Management Society. A manual specific to plant control in Pennsylvania is also available through Penn State University. Aquatic plants may be removed from undesired areas by 1) physical removal, 2) habitat modification designed to reduce growth rates, 3) chemical treatment, 4) biological control, and 5) winter drawdowns.

Physical removal from small ponds is usually done by hand, using rakes, shears or cutter bars dragged along the bottom. As with the physical removal of metaphyton, a big advantage of this approach is the concurrent removal of stored nutrients, and harvesting should therefore take place before the plants decompose (mid- to late summer is a good time for most species). The plant material should be transported away from the shoreline so that nutrients don't reenter the pond, and can be composted for use in gardens.

Habitat modification usually seeks to 1) reduce light penetration to the sediments, either by adding colorants (dyes) to the water column (see Section I above), or 2) deepen the pond by dredging, again effectively reducing light penetration to the bottom and also removing nutrient-rich organic sediments. Dredging in particular has wide-ranging system-wide effects (see Section H above).

Chemical treatment with herbicides is a commonly used management tool, but should be considered with caution, with an understanding of which species are likely to be affected by treatment, how rapidly they act, and the longevity of the chemical within the pond. A general list of commonly used herbicides is provided in the Table below. Homeowners who choose to apply chemical treatments themselves must apply to the Commonwealth for an "Application and Permit for Use of an Algicide, Herbicide, or Fish Control Chemical in Waters of the Commonwealth" usually the service of a licensed applicator is recommended. The guide Aquatic Plant Management in Lakes and Reservoirs provides an excellent overview of the many options available.

Herbicides commonly used to control aquatic plants. For mode of action,SYS = systemic, CON = contact; for selectivity, SEL = selective, BR = broad spectrum.
Herbicide Common Brands Mode of Action Selectivity Half Life (wks)
2,4-D 2,4-D Ester SYS SEL 1-7
Copper Cutrine Plus CON BR (incl. algae) very long
Diquat Weedtrine CON BR 1-2
Endothall Aquathol Endothal CON BR 1-2
Fluridone Sonar SYS BR 3-15
Glyphosate Rodeo SYS BR 2

Herbicides that are considered "broad spectrum" are typically used to remove all aquatic plants, regardless of species. Others are more "selective" , targeting particular plants. Most modern herbicides have relatively short durations of activity, breaking down into less harmful constituents. Exceptions are formulations of copper, which do not readily break down and may accumulate in the sediments.

Herbicides differ in their general modes of action. Some are termed "contact" herbicides, and rather rapidly cause the death of those plant tissues with which they come in direct contact. They have little effect, however, on other plant parts, so regrowth from the roots is likely and may necessitate repeated treatment during the growing season. Commonly used contact herbicides are copper, Diquat and Endothall.

In contrast, "systemic" herbicides generally produce visible effects more slowly, but are taken up and transported to all tissues, with the ultimate result of killing the entire plant. Their long-term effectiveness can thus be much greater than for contact herbicides. Systemic herbicides include 2,4-D, Fluridone and Glyphosate.

Plant control with herbicides should also be considered with caution because of fish kills and increased algae that may result. The rapid death of massive amounts of plant biomass can often lead to high rates of bacterial decomposition and consequent oxygen sags, resulting in the death of fish and other pond organisms (slower-acting systemic herbicides may thus be more appropriate if plant densities are high). Decomposition also has the added unfortunate side effect of releasing nitrogen and phosphorus, previously stored in plant tissues, in forms that are directly usable by rapidly growing phytoplankton. Thus, herbicide use may be viewed as indirectly creating a problem with excessive algae. As stated at the beginning of this section, and explained in greater detail in Volume 2 Section S, aquatic plants are key components of healthy pond ecosystems, and long-term ecological problems caused by their removal are likely to outweigh any short-term benefits.

Biological control may be used to reduce plant abundance. The introduction of triploid grass carp (Ctenopharyngodon idella) can provide effective control of many aquatic plants. The fish are typically introduced at a size (approximately 12" in length) to avoid their immediate consumption by piscivorous fish (e.g., largemouth bass), and at a density sufficient to impact the plant species of concern. Most studies have indicated that grass carp do not prefer filamentous algae. Increased densities of either phytoplankton or metaphyton thus can often occur within a few years of grass carp introduction. A permit is required to introduce grass carp in Pennsylvania. Pond owners should contact the Triploid Grass Carp Coordinator, PA Fish and Boat Commission for an application and suggestions regarding stocking procedures. A number of commercial hatcheries have been designated by the Commonwealth to acquire and sell grass carp.

Triploid carp

Triploid grass carp, at Kurtz Fish Farm, Elverson.

Like grass carp, many species of ducks and swans can be effective herbivores on aquatic plants. Unlike grass carp, however, numbers cannot be carefully controlled, and waterfowl are usually considered more of a problem than a management tool.

Winter drawdowns are a useful management tool in ponds with bottom drains. Pond levels are dropped during winter to expose the root systems of aquatic plants to freezing, thus clearing much of the shallower water of plants the following spring. Some species of plants, however, are relatively resistant to winter freezing, and these species may become increasingly dominant in the pond over time if drawdowns are repeated each winter.