A healthy pond needs aquatic plants, but too many plants can limit the pond’s use and cause imbalances in the fish population. If you had problems using your pond last year because of aquatic plant growth, you could have similar problems this year.
In an existing pond, mechanical and chemical control methods 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 chemical control could cause water quality issues that harm fish.
Mechanical methods include bottom blanketing, shading, removal and lowering water levels. 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, before most plants are actively growing, to reduce the amount of light needed for plants to grow. 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 special rakes and cutters (make your own or buy from retail outlets).
Lowering water levels is another way to control aquatic plants. Let water out of the pond this spring (or winter) to expose aquatic plants to drying (or freezing) conditions to limit their growth after water is allowed to refill the pond.
Herbicides will control pond plants chemically, but you may need to reapply during the summer to get season-long control. Follow these five steps when you apply any herbicide: 1) correctly identify the plant you want to control; 2) measure the area you are going to treat (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. Most aquatic herbicides work best if applied on a calm, sunny morning.
Find more information on aquatic plants in ponds on the DNR website at www.iowadnr.gov/ponds.