Harvesting - An Aquatic Plant Management Method

Harvesters

The Western Aquatic Plant Management Society

Description of Method

Mechanical harvesters are large machines which cut and collect aquatic plants. Cut plants are removed from the water by a conveyor belt system and stored on the harvester until disposal. A barge stationed near the harvesting site for temporary plant storage is an efficient disposal method; alternatively the harvester carries cut weeds to shore. The shore station equipment is usually a shore conveyor that mates to the harvester and lifts the cut plants into a dump truck. Harvested weeds are disposed of in landfills, used as compost, or in reclaiming spent gravel pits or similar sites.

Harvesting is usually performed in late spring, summer, and early fall when aquatic plants have reached or are close to the water's surface. Harvesters can cut and collect up to two acres per day depending on weed type, plant density, storage capacity of the equipment, and distance between the offloading site and the harvesting site. Depending on the equipment used, the plants are cut from five to ten feet below the water's surface in a swath 6 to 20 feet wide. Because of machine size and high costs, harvesting is most efficient in lakes larger than a few acres.

Advantages

Harvesting results in immediate open areas of water.
Removing plants from the water removes the plant nutrients, such as nitrogen and phosphorus, from the system.
Harvesting as aquatic plants are dying back for the winter can remove organic material and help slow the sedimentation rate in a waterbody.
Since the lower part of the plant remains after harvest, habitat for fish and other organisms is not eliminated.
Harvesting can be targeted to specific locations, protecting designated conservancy areas from treatment.

Disadvantages

Harvesting is similar to mowing a lawn; the plant grows back and may need to be harvested several times during the growing season.
There is little or no reduction in plant density with mechanical harvesting.
Off-loading sites and disposal areas for cut plants must be available. On heavily developed shorelines, suitable off-loading sites may be few and require long trips by the harvester.
Some large harvesters are not easily maneuverable in shallow water or around docks or other obstructions.
Harvesters may leak fuel or hydraulic fluid into the water.
Many small fish and insects are often collected and killed by the harvester.
Harvesting creates plant fragments which may increase the spread of invasive plant species such as Eurasian watermilfoil.
Plant fragments may accumulate and decompose on shore.
If equipment is moved from waterbody to waterbody, the harvester must be thoroughly cleaned so as to not transport invasive plants or animals.

Permits

Permits are required for many types of projects in lakes and streams. Check with your state and local jurisdictions before proceeding with a harvesting program.

Costs

Costs per acre vary with numbers of acres harvested, accessibility of disposal sites to the harvested areas, density and species of the harvested plants, and whether a private contractor or public entity does the work. Costs as low as $250 per acre have been reported. Private contractors generally charge $500 to $800 per acre. The purchase price of harvesters ranges from $35,000 to $110,000.