Many different aquatic plants from algae, which drifts suspended in the water, to plants floating on the water surface or rooted in the pond bottom can grow in ponds. Rooted plants grow either entirely under the water, have floating leaves, or grow with stems above the water surface. Some have both underwater and floating leaves. Both algae and rooted plants will grow in all ponds. Keeping a balance is sometimes difficult. Any plant can become a nuisance with the right conditions.
Single-celled algae, usually not visible, form the base of the food chain and make much of the oxygen needed for other life in the pond. Filamentous algae, sometimes called moss or grass, is more visible and most easily becomes a nuisance. Sudden growth of either type of algae is called a “bloom”.
Rooted aquatic plants are important to the overall health of ponds and lakes. They stabilize the shoreline and pond bottom, tie up plant nutrients thus reducing algae blooms, help the water to clear faster after a rain, produce oxygen and provide food and habitat for the many forms of life that live in and around a pond. Plants also provide nursery habitat for many fish and moderate levels are important for good growth, condition, and abundance of sport fish. Research shows that lakes without rooted plants may not have balanced, desirable fish populations.
Rooted plants that stick above the water's surface (emergent), like cattails, are the easiest to see. There are also rooted plants that are completely under the water (submergent), as well as free-floating plants like duckweed.
Algae are the smallest, these can be in cottony filaments that float freely or attach to the bottom as well as single-celled algae that make the water look green and cannot be seen with the naked eye.
Rooted plants are necessary for a healthy pond, but too many can upset a fishing pond’s balance and become a nuisance. Everyone has different tolerances to pond plant life. Most biologists agree that once the plants cover over 30% of the surface area of a pond, they are over-abundant. There are four plant control options: preventative, mechanical, biological and chemical.
List items for Pond Plant Control Techniques
Managing the watershed and deepening the shoreline are preventative and are easiest and most effective to use before the pond is built. Conservation practices like silt retention structures, wetlands, buffer strips and grass waterways in the pond’s watershed keep soil and plant nutrients from reaching the water.
Deepening shorelines create smaller areas where sunlight can reach the pond bottom, reducing the area that rooted plants will grow. Any number of livestock in a pond’s watershed can overload a pond with plant nutrients over time; steps should be taken to divert or contain animal waste products.
When preventative measures are not practical, mechanical methods like hand removal, bottom blanketing, shading, and water draw-down can be effective for short-term control. Removal can be done any time of the year. Remove plants by hand, with a rake or by dragging a light wire lattice or steel frame. Many aquatic plants can root from fragments, so be careful to remove all plant stems that float up. Once plants are removed from high-use areas, blanket the pond bottom with sand or gravel to slow plant re-growth. Small areas can be blanketed for 30 days with thick black plastic (punctured to allow gasses to escape) or commercial weed barrier products held with a frame or weighted at the corners.
Biological control with triploid grass carp (white amur) can provide effective, long-term control for underwater plants, but will not control algae or shoreline plants like cattails. Stocking too many grass carp can increase the growth of algae and nearly eliminate all aquatic vegetation. Carp are difficult to remove and can live beyond 20 years.
The automatic stocking of grass carp is not recommended for aquatic plant control in new ponds. Use a conservative stocking of 1 to 2 fish per acre for heavy plant growth. These fish should be 10 inches or longer to avoid being eaten by largemouth bass. Be patient, vegetation control may not be noticed for 2-3 growing seasons after stocking. Additional stocking may be needed if no control is seen, or if plant growth returns to nuisance levels.
Iowa DNR does not stock grass carp into public waters and does not recommend them for stocking private ponds. Stocking grass carp often results in increased algal growth and near elimination of all aquatic vegetation.
Although Iowa DNR does not encourage stocking grass carp, they can legally be stocked into private ponds in Iowa. If grass carp are stocked into a private pond, Iowa DNR recommends a conservative stocking of no more than 1-2 ten-inch fish per acre. Pond owners who opt to go this route should be patient as vegetation control may not be seen for 2-3 growing seasons.
Restocking should not occur until rooted aquatics become a problem. Often, additional fish are stocked because vegetation control is not apparent immediately. When the happens, the pond ends up with too many grass carp resulting in too little vegetation, poor water quality, and ultimately sport fishing suffers.
Chemical control with herbicides is another method for seasonal control of aquatic plants. Follow these five steps with any herbicide application:
Use a pond plant identification guide to help you correctly identify the plants you want to control. if this basic guide does not meet your needs, the Texas Agrilife Extension Service has an interactive website that is useful for identifying aquatic vegetation.
Measure the area to be treated (surface area and depth).
Read the herbicide label to decide the best time and how much to apply.
Identify possible restrictions on uses of the water, e.g., irrigation or watering animals.
Apply according to label directions. applying herbicides or dyes in excess can put you at risk of violating npdes general permit #7.
There are several drawbacks to using herbicides to control plants. control can be ineffective if treatment is applied at the wrong time of year (read the product label for best application times). When there is a large area of vegetation to kill, the decay of all these plants can use much of the oxygen in the water and cause a fish kill.
Divide large areas of vegetation into smaller areas to prevent a fish kill. Treat the smaller areas several weeks apart, especially in summer months when plants decay faster and the water holds less oxygen due to warmer water.
Control can be temporary and expensive and repeated applications may become necessary. some herbicides require that selected pond uses stop for a period of time after application. Find an excellent overview of which herbicides will work for different plant types at the aquatic ecosystem restoration foundation website.
Depending upon the level of plant growth, the types of plants present and the intended use of your pond, it may be helpful to use several control methods to get the desired results. For example, hand removal or herbicide control with bottom blanketing in high-use areas with a light stocking of grass carp may work better than any one of these methods alone.
Pond dye products that reduce light penetration can shade the whole pond. Apply these products early in the growing season; they often need to be re-applied later in the season. Winter drawdown methods expose these dewatered areas to freezing and drying for several weeks through the winter to kill underwater plant roots. Leave at least eight feet of water depth in the pond to avoid killing fish over a long, snowy winter.
