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Although pond plant growth is not exactly the same each year; if you had problems using your pond last year, chances are that similar problems will exist this year. A healthy pond needs aquatic plant habitat, but too much of a good thing can inhibit the pond’s use and cause imbalances in the fish population. In an existing pond, mechanical and chemical vegetation control are your best options for immediate control and are generally most effective if you start early, when plant growth is not at its maximum level. If you wait until growths are thick, control will be difficult, take longer and could result in water quality issues that harm fish.
Mechanical methods include bottom blanketing, shading, removal, and water draw-down. Temporarily blanket the bottom of a private pond and leave in place for 30 days early in the season to inhibit plant growth in fishing or swimming areas. Use weighted weed barrier or thick black plastic (punctured to allow gases to escape) or commercially available bottom barriers.
To shade a pond, apply a pond dye in April to reduce the amount of light needed for plant growth. This is most effective in ponds with a small watershed. Many blue and black pond dye products are available for pond beautification; Aquashade® or Admiral®, both blue dyes, are the only products labeled by the EPA for plant suppression. Re-application at a reduced rate is essential throughout the spring and summer as the product is diluted by inflowing water or decomposed by ultra-violet light.
Removal is a low-cost method to take out plants from high-use areas. It can be done by hand or with rakes and cutters (purchase from retail outlets). Draw-down the pond’s water level this spring (or winter) to expose aquatic plants to drying (or freezing) conditions to limit their growth in future months.
Herbicides will control pond vegetation chemically, though retreatment may be necessary to get season-long control. Follow these five steps with any herbicide application: 1) correctly identify the plant to be controlled; 2) measure the area to be treated (surface area and average depth); 3) read the herbicide label to determine the correct timing and amount to apply; 4) identify potential restrictions on uses of the water (e.g., irrigation or watering animals) and 5) apply according to label directions. All aquatic herbicides produce the best result if applied on a calm, sunny morning.
Granular herbicides are attractive because of their ease of application, but many granular products are 2,4-D-based and are only effective on broadleaf plants. Most aquatic plants are in the grass family, so the 2,4-D products will not be effective.
Most underwater plants can be controlled with any diquat-based product. Algae and a few underwater plants can be treated with copper-based herbicides; some of these products are granular. Application of liquid herbicides to underwater plants requires that the liquid be applied underwater to be in contact with the plants (unless otherwise specified on the label). It kills by contact, and is not as effective if sprayed on the water’s surface or (for diquat) applied in muddy water. Active ingredients that do well in muddy water are endothall or copper complexes.
Use of more than one of the above methods may be needed to provide the desired level of plant control. All of these methods work best if used early in the season, before plants reach the pond’s surface.
More information on aquatic plant identification and some of these methods can be found on the Department of Natural Resources website (go to www.iowadnr.gov and search “Pond Plants”).