Although pond plant growth is not always the same each year; if you had problems using your pond last year, you could have similar problems this year. A healthy pond needs aquatic plant habitat, but too many plants can limit the pond’s use and cause imbalances in the fish population.
In an existing pond, mechanical and chemical vegetation control are the best options for immediate control. They usually work best 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 cause 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 it in place for 30 days early in the season to limit plant growth in fishing or swimming areas. Use weighted commercial weed barrier products or thick black plastic (punctured to allow gases to escape).
To shade a pond, apply a pond dye in April to reduce the amount of light needed for plant growth. This works best 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 to limit plant growth. Re-apply the product at a reduced rate throughout the spring and summer since inflowing water can dilute it and ultra-violet light can decompose it.
Removal is a low-cost way 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 is another method of control. To use this method, let water out of the pond this spring (or winter) to expose aquatic plants to drying (or freezing) conditions to limit their growth in future months after water is allowed to refill the pond.
Herbicides will control pond vegetation chemically, though retreatment may be needed 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 possible restrictions on uses of the water (e.g., irrigation or watering animals) and 5) apply according to label directions. All aquatic herbicides work best if applied on a calm, sunny morning.
Find more information on aquatic plant identification and control on the DNR website at www.iowadnr.gov/ponds.