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Identification is the first step in managing aquatic plants. Most control methods are aimed at specific plants or groups of plants with similar growth habits. Aquatic plants can generally be separated into four categories: algae and emergent, floating, and submersed plants.
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Algae are simple, primitive plants lacking true roots, stems, or leaves. There are two types of algae: the planktonic kind, which are microscopic and can become so plentiful they can turn a pond the color of pea soup, and the filamentous kind, which form large floating mats. |
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Emergent plants have a large portion of stems and leaves growing above (emerging from) the water surface. They are found in shallow waters (typically less than 2 feet deep) or along the shorelines. Examples are cattails, bulrushes, and arrowheads. |
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Floating plants have leaves that float on or near the water surface with root systems either connected to the leaves by long, tough stalks (rooted floating-leafed) or root systems dangling in the water, not connected to the sediment (free-floating). Examples are water lilies, duckweeds, and watermeal. |
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Submersed plants have most of their leaves and stems below the water surface, often with flowering parts projecting above the surface. They may be securely or loosely rooted to the bottom. Examples are pondweeds, coontails, and milfoils. |
Aquatic plants serve many important functions in aquatic systems. Algae form the base of the aquatic food chain. Emergent plants can help filter runoff from uplands to protect water quality. Floating plants provide shade and refuge for the nearshore animal community. Submersed plants absorb phosphorous and nitrogen over their leaf surface and through their roots, decreasing the nutrient availability to nuisance algae. All types provide countless benefits for fish and wildlife; everything from bass and bluegill to reptiles, amphibians, and waterfowl. Having a diverse community of native aquatic plants in your pond can ultimately help lead to a more balanced waterbody. These are but a few of the beneficial roles aquatic plants play. Under certain conditions, however, aquatic plants can become a problem.
Excessive growth of aquatic plants can affect beneficial uses of a waterbody, such as fish and wildlife habitat, irrigation, water storage and use, and recreational and aesthetic enjoyment. When problem plants limit waterbody uses, the solution lies in careful management. The goal is to find a remedy to nuisance aquatic plants that is effective, economically feasible, and ecologically sensitive.
The desirability of aquatic plants depends in-part on the point of view of the landowner, and management should consider all uses of a waterbody. For example, anglers may desire lily pads, plant beds, and plankton to support fish and the organisms they feed on, but only if they do not become widespread, whereas swimmers may want clean, clear water free of algae, vascular plants, and other growth. Knowledge of aquatic plants will aid the landowner as they consider approaches to meet their management objectives.
A number of aquatic plant management strategies can be considered, and the following provides just an overview. Included are references for aquatic plant identification and learning more about management, but these are just a snapshot of the information available. We recommend looking into all options before deciding which will be best for managing your pond or reservoir.
Hand-Pulling
This method involves digging out the entire plant (stem and roots) either with your hands or with a spade or long knife and disposing of the plants on shore. In shallow waters (less than 3 feet deep), no specialized gear is required. In deeper waters, hand removal may be best accomplished using snorkeling or scuba and carrying collection bags for plant disposal. Many aquatic plants can propagate through fragments, so it’s important to not leave any plant material in the water.
Hand-Cutting
This technique is also a manual method but differs from hand-pulling in that plants are cut below the water surface (roots are generally not removed). Tools used can include scythes, rakes, or specialized devices that can be pulled through the weed beds by boat or several people. Mechanized weed cutters are also available that can be operated from the surface for small-scale control. Depending on the species of plant, this can be only a short-term solution as plants will grow back quickly. However, it’s a good approach around boat docks and ramps, fishing piers, swimming beaches, and similar sites where the problem requires more immediate attention.
Bottom Barriers
Barrier material – much like landscape cloth – is laid over the pond bottom to prevent plants from growing, leaving the water clear of rooted plants. Bottom covering materials such as sand-gravel, polyethylene, polypropylene, synthetics rubber, burlap, fiberglass screens, woven polyester, and nylon film have all been used with varying degrees of success. Divers may be needed for deeper water treatments. Presence of rocks or debris do not typically impede most barrier applications, but pre-treatment clearing of the site can help. Barriers may need to be maintained as organic material deposits on top of them.
Water-Level Drawdown
Water-level drawdown involves exposing plants and root systems to prolonged freezing and drying, or hot, dry conditions to kill the plants. The mild, wet winters typical of Western Oregon may not provide adequate freezing/drying conditions to kill certain plants. Also, draining of a pond with an established ecosystem will require “starting over” as there will be impacts to any fish and wildlife supported by the pond.
Input Management
Input management involves reducing nutrients that flow or drain into the pond from outside sources, which is a primary reason for excessive vegetation. The idea is to limit, or stop, these inputs by using household yard care, agriculture, forestry, or other practices to reduce or minimize loading in the pond or reservoir. Common examples of homeowner best management practices include maintaining septic systems, using more conservative lawn and garden fertilizing practices, and disposing of yard litter by shredding or composting well away from the water’s edge.
Water Column Dyes
The application of dyes suppresses aquatic plant growth by shading the plants from sunlight. Dark-colored dyes are applied to the water, which reduces the amount of light reaching the submersed plants.
Weed Rolling
Weed rollers control aquatic vegetation by agitating and compacting the pond sediments. This method uses a power unit that drives a roller on the pond bottom. Fins on the roller detach some plants from the soil, while the rulers force other plants flat, gradually inhibiting growth. Detached plants should be removed from the water with a rake or gathered by hand to prevent them from rooting in other locations.
Harvesting
Harvesting involves cutting plants below the water surface, with or without collection of cut fragments for offshore disposal. Some harvesters combine cutting, collecting, storing, and transporting cut vegetation into one piece of machinery. Cutting machines are also available that perform only the cutting function.
Rotovation/Cultivation
Mechanical rotovation/cultivation are bottom tillage methods that dislodge and remove aquatic plant root systems. Treatments are made in an overlapping swath pattern.
Diver-Operated Suction Dredging
This technique uses a small barge or boat carrying portable dredges with suction heads operated by divers to remove individual rooted plants from the sediment. The divers initially dislodge the plants with sharp
tools. The plant/sediment slurry is then suctioned up and carried back to the barge, boat, or shoreline through hoses. Plant parts are then retained for later off-site disposal.
Application of an aquatic herbicide has been one of the most common methods used to control nuisance aquatic weeds in Oregon. While cost and target species are important considerations in choosing the appropriate product, other factors, such as potential impacts on other desired aquatic plants, recreational or other use, fish and wildlife, and the broader waterbody ecology must also be considered.
A variety of herbicides are available, with many produced for a very specific or selective use. Systemic herbicides, for example, are absorbed by plant roots and shoots and moved throughout the plant. Systemic herbicides can kill the entire plant, including roots and shoots. In contrast, contact herbicides kill the plant surface with which it comes in contact, leaving roots and unaffected leafy stems alive and capable of regrowth.
Permits may be required in Oregon for the application of aquatic herbicides. For more information.
Grass Carp
Grass carp, or white amur, (Ctenopharyngodon idella) are exotic, plant-consuming fish native to the large rivers of China and Siberia. Known for their high growth rates and wide range of plant food preference, these fish can control certain nuisance aquatic plants under optimal circumstances. Stocking rates are dependent on climate, water temperature, type and extent of plant species, and other site-specific constraints.
Grass carp are most appropriately used for elimination of submersed plants where no vegetation is desired, such as in irrigation or stock ponds or canals. Field studies have shown that the use of grass carp to maintain a desired or moderate level of vegetation has rarely been successful and they may not be a good tool in waters managed for uses other than storage and conveyance. Use of grass carp can also increase non-native plants as they can remove highly preferred native plants before consuming the non-natives.
Sterile grass carp are a controlled species in Oregon and require a Grass Carp Stocking Permit issued by ODFW before bringing into the state and stocking in a private water. The Grass Carp Stocking Permit requirements are defined in-part by Oregon Administrative Rule (OAR). Additional information regarding the stocking permit process can be found here. To inquire about the Permit Application and Management Plan for Stocking Triploid Grass Carp, contact the ODFW Grass Carp Permit Coordinator at 503-947-6249.
A Citizen’s Manual for Developing Integrated Aquatic Vegetation Management Plans. 1994. M.V. Gibbons, H.L. Gibbons, M.D. Sytsma. 50pp. (available from Kathy Hammel, Coordinator, Washington State Dept. of Ecology, Freshwater Aquatic Weeds Management Program, P.O. Box 47600, Olympia, WA 98504-7600; 360/407-6562). Online link: https://apps.ecology.wa.gov/publications/documents/93093.pdf.
A Guide to Aquatic Plants: Identification and Management. 1994. David Fink. 52pp., line drawings (available from Minnesota Dept. of Natural Resources, Attn: Steve Enger, Ecological Services Section, 500 Lafayette Rd., Box 25, St. Paul, MN 55155-4025; 612/296-2835).
A Manual of Aquatic Plants. 1957. Norman C. Fasset. 416pp., keys and line drawings (available from University of Wisconsin Press, 114 N. Murray St., Madison, WI 53715-1199; 800/829-9559).
An Aquatic Plant Identification Manual for Washington’s Freshwater Plants. Washington State Department of Ecology. Online link: https://fortress.wa.gov/ecy/gisresources/lakes/AquaticPlantGuide/index.html.
Aquatic Plant Management in Lakes and Reservoirs. 1997. North American Lake Management Society and Aquatic Plant Management Society. 103pp. (available from North American Lake Management Society, P.O. Box 5443, Madison, WI 53705-5443; 608/233-2836).
Aquatic Plants of the Pacific Northwest with Vegetative Keys. 1960. Albert N. Steward, La Rea J. Dennis, Helen M. Gilkey. 184pp. Studies in Botany Number 11. (available from Oregon State College: Corvallis, OR). Online link: https://ir.library.oregonstate.edu/downloads/3f462b44q.
Common Marsh, Underwater, and Floating-Leaved Plants of the United States and Canada. 1970 reissue. Neil Hotchkiss. 223pp., line drawings (available from Dover Publications, Inc., 31 East 2nd Street, Mineola, NY 11501-3582).
Guide for Developing Integrated Aquatic Vegetation Management Plans in Oregon. 1999. Maribeth Gibbons, Mark Rosenkranz, Harry Gibbons, Mark Sytsma. Center for Lakes and Reservoirs Publications and Presentations. Online link: https://www.researchgate.net/publication/239604579_Guide_for_Developing_Integrated_Aquatic_Vegetation_Management_Plans_in_Oregon.
How to Know the Aquatic Plants. 1980. G.W. Prescott. Pictured Key Nature Series. 158pp., keys and line drawings (available from Patricia Ledlie Bookseller, Inc., One Bean Rd., P.O. Box 90, Buckfield, ME 04220).
Lake Smarts: The First Lake Maintenance Handbook. 1993. Steve McComas. 215pp., 400 photos (available from Lake Smarts, c/o 550 Snelling Ave. South, Suite 105, St. Paul, MN 55116; 612/690-9602).
Restoration and Management of Lakes and Reservoirs. 1993. 2nd ed. G.D. Cooke, E.B. Welch, S.A. Peterson, P.R. Newroth. 548pp. Lewis Publishers, Boca Raton, FL.
The Lake and Reservoir Restoration Guidance Manual. EPA-440/4-90-006. 1990. 2nd ed. 326pp. (available from North American Lake Management Society, P.O. Box 5443, Madison, WI 53705-5443; 608/233-2836).
Weed Research and Information Center. University of California, Davis. Department of Plant Sciences. Online link: https://wric.ucdavis.edu/.
Images from Page 1: Filley, Shelby. Oregon State University. Aquatic Weed Control [Image]. Retrieved 15 September 2022, from https://extension.oregonstate.edu/sites/default/files/documents/9276/aquatic-weed-control-compatibility-mode.pdf. |
