UNIVERSITY PARK, Pa. -- Homeowners and farmers with ponds, streams or waterways on their property don't necessarily have to apply pesticides if aquatic plants become a nuisance, according to an expert in Penn State's College of Agricultural Sciences.

"Most bodies of water support some kind of plant life," explains Winand Hock, professor of plant pathology and director of the Pesticide Education Program. "Aquatic plants become weeds when they impair or prevent use of the pond or waterway."

Mark Hartle, biologist for the Pennsylvania Fish and Boat Commission, says aquatic plants vary according to the environment they are placed in, and control measures are dictated by what type of plant life is choking off the pond. Surrounding land use also can influence aquatic plant growth, either by acting as an effective filter for excess water and nutrients, which limits growth, or by failing to filter nutrients, which can cause excessive aquatic plant growth.

Scientists divide aquatic plants into four groups:

• Algae. The most widely distributed of all aquatic plants, algae can be subdivided into three types.
• Plankton: Also called phytoplankton, these microscopic single-cell plants cause water to appear pea-soup green or reddish brown.
• Filamentous: These plants, often mistakenly described as moss or slime, use filaments of plant material to form dense mats of growth on rocks or other underwater objects.
• Attached branched algae: Known as Chara and Nitella, these two varieties resemble flowering plants, but they have no root system. They have a skunky odor when crushed.
• Submerged Aquatic Vegetation. Most plants in this category grow in deeper water and are rooted on the bottom. Most of their leaves remain underwater until flowering. Common types include coontail, pondweeds, common elodea and milfoil.

--Emergent Plants. These varieties grow along the edges of the water body, with just a short portion of their stems under water. Most of these plants do not cause problems for water users. Problem emergent plants include cattails, purple loosestrife and common reeds.

--Floaters. Most of the structure of these plants float on the water surface. Most are rooted plants such as water lilies and spatterdock. Other varieties, such as duckweed, absorb nutrients by dangling roots in the water.

Experts say aquatic weed control can be approached using three methods, singly or in combination.

Physical or Mechanical Control

"This technique means cutting, mowing, raking, digging or pulling vegetation out of the water," Hartle says. "Most aquatic plants can reproduce by fragmentation, so cleanup will have to be repeated several times to eliminate new growth."

Hartle explains that physical removal is effective only for small quantities of shoreline plants but reduces the rate of regrowth as well as the availability of nutrients.

Mechanical plant harvesting is used only on large lakes.

Landowners also can alter a pond's environment to control vegetation. Lowering the water level, called a "drawdown," exposes sediments and plants, which can be killed if temperatures dip below freezing. Hartle says drawing down any pond larger than an acre requires a permit.

In small ponds, aeration can partially control the growth of blue-green algae, but is ineffective on other plants.

Biological Control

Introducing vegetation-eating fish into a pond system can be very effective, particularly for submerged plants. Filamentous algae can be reduced somewhat as well.

Hartle says the triploid grass carp, a fish genetically altered so it cannot reproduce, is the only effective biological vegetation control. He adds that pond owners must get permits from the Fish and Boat Commission before stocking the triploid grass carp. The pond outlet also must be screened to prevent escape. Koi, common carp and Israeli carp are not exclusive herbivores, and fish experts do not recommend their use.

"The drawback to the grass carp is the defecation of consumed plant material, which recycles nutrients back into the water," Hartle says.

Chemical Control

Herbicides are effective controls, but Hock warns that mishandled chemicals will cause many more problems than they solve. Hartle also says algae is the most common problem Pennsylvania pond owners encounter, and control of nutrients entering the pond must accompany chemical treatments to effectively control algae.

"Choose the correct chemical for the problem plant," Hock says. "Make precise water volume and chemical measurements, and read the product label directions carefully. Over-application can damage other plants and wildlife."

Chemical applications also can kill fish indirectly because too much oxygen is consumed by the rapid decay of dead plants. Hock suggests treating just one-third or half the pond at a time. "Start applications in shallow water and work out to deeper water," Hock says. "This will let fish escape the chemical."

All aquatic pesticides require a permit, which must be approved by the Fish and Boat Commission and the Pennsylvania Department of Environmental Protection before the pesticide is used. For more information on this subject, visit the Web site http://www.pested.psu.edu and read the publication "Pond Management & Aquatic Plant Control."

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EDITORS: For more information, contact Winand Hock at 814-863-0264. To contact Mark Hartle, please call 814-359-5116.

Contacts: John Wall jtw3@psu.edu 814-863-2719 814-865-1068 fax