Herbicides have the potential to be an excellent way to control weeds in ponds, but they can also be quite dangerous, especially if you are interested in having fish in the pond. While nuisance pond weeds can be a big problem, you must take the time to understand the various herbicides and what they are meant to do. Also, knowing the most opportune time to apply the pond herbicide is a key to making sure they are effective at controlling weeds, using the least amount of chemical possible.
There are four basic types of plants that different herbicides will help control. Emergent plants are those that grow above the pond's service, such as cattails. Submerged plants stay below the water. Floating plants remain on the water's surface. And algae can cover the bottom or sides of the pond, or perhaps also float on the water's surface. In each case, the herbicide may be different.
For controlling emergent plants, use a granular 2,4D herbicide. The best time to use this is when the plant is reaching its peak flowering or seeding stage, as that is when the root system is most active. Contact herbicides, such as diquat herbicides, can be used but it will not kill the root system, and therefore is only a temporary solution.
One the most popular herbicides for submerged plants is fluridone. This treatment will help control species such as coontail, Eurasian watermilfoil, curlyleaf and many others. Fluridone should be applied in the spring before the plants have reached peak biomass. This reduces the amount of chemical to be used and offers the most effective options.
Weeds in this category include duckweeds and watermeal. As with the previous application, the time to take care of this weed problem is as soon as it is noticed, which most likely will be in the shallower areas of the pond. The weed is best controlled with a liquid, amine 2,4D herbicide.
Algae is often a very beneficial form of aquatic life for a pond, but too much can not only be a nuisance but very dangerous to the pond once it dies. Use a treatment before the algae reaches its peak growth, but after the pond has been at a temperature of at least 60 degrees F for at least a week. Herbicides used to control algae include copper sulfate, chelated copper and endothall.
The amount to use will vary depending on the product and size and type of pond. Be sure to follow all label instructions, but here are a few common formulas. If you have a larger or natural pond, the following formulas may help. To determine the amount of herbicide by pond surface area, multiply the area in acres by the recommended product per acre. For acre-foot treatments (treatment of an acre per foot of water), multiply the area by average depth, then multiply by the recommended treatment.
Remember not to use too much herbicide in a pond, as elevated concentrations of most herbicides can also be toxic to fish. Also, if your pond is not self-contained, such as those having an outlet to a stream, river or other body of water, use of a herbicide could damage the natural environment and could even be against local laws.