MECHANICAL HARVEST OF NUISANCE POND AND LAKE WEEDS

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Mechanical weed harvest is the physical removal of vegetation. At its simplest, you can uproot plants by hand, as in weeding a garden. Large-scale harvest, on the other hand, can involve highly mechanized equipment costing $50,000-100,000.

An over-abundance of aquatic plants is caused by "eutrophication," where excess nutrients, usually from human activity, causes excessive weed growth. The only long-lasting remedy is to bring waste discharges, disruptive land uses and other nutrient sources under control. But the necessary alteration of the surrounding human community can take many years. In the meantime, mechanical removal is a temporary approach to consider in contending with the weed problem.

ADVANTAGES OF MECHANICAL HARVEST:
The advantages of mechanical control include:

• Cutting at times of major growth can weaken the plants. Re-growth after harvest is slower than normal.
  
• Repeated harvest in one year may give even greater reduction of regrowth in subsequent years.
  
• The treatment is flexible. Equipment and methods can be selected to fit the type and size of the problem. Openings can be cut in weed beds, leaving a vegetation edge attractive to fish and beneficial plants can be protected.
  
• Removal of the cut material eliminates accumulation of decaying plants which, as in some other plant killing methods, could deplete the water's dissolved oxygen, causing a decline of fish and other animals.
  
• Nutrients will be removed with the harvested plant matter, rather than becoming available again in the lake bed or water. Removing the lake's store of nutrients, especially phosphorus, is often cited as a major benefit of weed harvesting. However, it probably can make a significant difference on phosphorus availability only where phosphorus input is low or where the amount of plants is great in proportion to water volume, as in some very shallow bays or ponds.

DISADVANTAGES OF MECHANICAL HARVEST

The disadvantages include:

• High initial cost of equipment or services where large machines are needed. Often the cost of a weed harvest service will result in under-harvesting for maximum control .
  
• High fuel costs and pollution from heavy exhaust discharges where motorized gear is used.
  
• Spread of certain types of plants by harvesting (mylfoil, curly leaf pond weed).
  
• Harvested plant materials must be disposed of properly.

COMPARISONS WITH AQUATIC HERBICIDAL TREATMENT

Currently the choice in temporary management is chiefly between mechanical harvest and the use of herbicidal chemicals (although there are other alternatives such as nutrient-precipitating chemicals, lake aeration and water level manipulation). Use of aquatic herbicides is convenient, but it has the following drawbacks compared to mechanical removal:

• Herbicides kill plants without removing them from the water. The material sinks after dying. Its decay consumes oxygen and releases nutrients for new plant growth.
  
• The poisoned plants disappear only slowly from the treated area. Several weeks to a month may pass before the nuisance plants sink away.
  
• Beneficial plants are killed, as well as the nuisance plants. Each herbicide kills several or many kinds of plants.
  
• Herbicides drift beyond the point(s) of application. Even lakes that appear placid have currents making it impossible to confine herbicides to a localized area of a lake without barriers that tend to be cumbersome. In lakes with significant currents, chemicals may be diluted before desired treatment effects occur.
  
• Lake owners and users seem increasingly concerned with finding alternatives that avoid the possible risks of herbicides.
  
• Areas may be closed to swimming, fishing or other uses for a few days or weeks depending on the chemical used.

There are certain disadvantages common to mechanical removal and chemical herbicides:

• The killed or removed plants are often replaced by other undesirable vegetation, annoyingly soon in some cases. Less than a month after cutting or poisoning rooted plants, the area may become clogged with masses of stringy algae. As long as light, warmth and nutrients exist in a lake, nature will strive to fill the water with vegetation.
  
• Treatment costs and efforts are recurring, in many cases annually or even more often. The expenditures must be repeated for as many years as control is desired--for centuries if need be--until the nutrient sources of the problem are abated by some other means. Costs for both treatments are substantial. Comparisons of costs will vary depending on many factors.

HOW IS HARVESTING DONE?

Where the plant problem extends less than about 15-20 feet out from the shore or pier, a hand rake with extra-long handle can be used. Where the problem area is somewhat larger, as in a pond or between piers, a log wrapped with barbed wire or an old set of bed springs can be dragged through the weed bed by a tractor driven along the shore.

Mechanized harvesters are manufactured in a wide range of designs. Small models, which resemble the cutter bars of hay mowers, cost several hundred dollars and can be mounted on a rowboat. These units can weeds to a depth of about 4 feet. After the weeds are cut, they float and must be raked to a removal point on shore. Other weed cutters, which cost a few thousand dollars, can be used in small lakes and bays to harvest weeds in very shallow water as well as depths up to five feet. Large harvesters, cutting at depths of 6-8 feet and lifting the plants by conveyor belt to a storage hopper, may cost $50,000-100,000. These have significant operating costs, need large launching sites and require many hours of harvesting to effectively remove plants from large, weed-clogged lakes.

Harvesting is best done during or summer when it will result in the maximum amount of actively growing plant material removed and still allow full recreational use of the lake. Such timing depends on knowledge of the growth characteristics of the species in question and on experience with the lake's recreational pattern.

DISPOSAL OF HARVESTED MATERIAL:

Harvested plants make good garden mulch, soil conditioner and composting material, since the thin cell walls of aquatic plants break down rapidly. Some lakeside communities make harvested materials available to farmers, landscapers and homeowners. This eliminates the costs to dispose of the materials in a landfill.

Links to other pond pages:

Chemical Control of Nuisance Pond and Lake Weeds

Pond Construction
Pond and Lake Resources

Pond Permits

RESOURCES:

Handouts/booklets available from MSU Extension:

- "Managing Michigan Ponds for Sportfishing" describes construction, fish stocking, weed control.

MSU Extension, Genesee County: (810) 732-1470
NRCS (Natural Resource Conservation Service): (810) 766-5192
Soil Conservation District: (810) 230-8781
MDNR (Shiawassee District): (517) 625-4600
MDNR (Permits): (517) 373-9244
MDNR (Information): (517) 373-9400

Michigan State University Extension programs and materials are open to allwithout regard to race, color, national origin, gender, religion, age,disability, political beliefs, sexual orientation, marital status, or family status. MSUE-Genesee County is funded jointly by Michigan State University, the U.S. Department of Agriculture and the Genesee County Board of Commissioners. MSU is an affirmative-action equal opportunity institution.

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