Controlling Weeds in Your Farm Pond

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Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

It seems that of all the questions that are received in our local AgriLife Extension office, from cattle to hay meadows, home gardens and landscapes, questions on how to manage a farm pond is the most common. Then when you break up the questions into different management groups, getting rid of excess vegetation is at the top of that list.

Before we start, please understand that some vegetation is good and even needed in most ponds. Vegetation provides cover for fish and helps add oxygen. But as we can imagine, too much vegetation can get in the way and lead to problems.

There are four kinds of aquatic vegetation: submerged (mostly underwater), emergent (having dry vegetative matter existing above the surface), floating (with no roots holding it in place), and algae. In simple terms, these pond weeds are either all below the water, above the water, float around on top, or “pond scum”.

Thankfully, pond owners have more than a few herbicide management options. Let us be very clear, herbicides approved for aquatic weeds, when applied according to the label, are an effective, safe, and environmentally responsible method of eliminating excess vegetation.

Timing is important when applying aquatic herbicides. Pond owners will find a poor response to any treatment that is made in the dead of winter when vegetation is not actively growing. Fall is too late to do much good and summer may find aquatic weeds that are already causing too many problems to get ahead of.

That leaves spring as the ideal time to stay ahead of excessive pond weed growth. True, you can treat for weeds in the summer months, but you will have more total vegetation to contend with and you could run into a low oxygen problem if you try and treat too much of your pond’s weeds at once.

How do we have an oxygen problem that kills fish after killing weeds in the summer? First, warm water holds less oxygen than cold water. There is some exacting science behind this fact. Specifically, “The solubility of oxygen and other gases decreases as the water temperature increases.” This means that colder water can hold more dissolved oxygen available to fish than warmer waters.

Additionally, when something is killed and begins to rot, the decomposition process consumes oxygen. A dead squirrel on the side of the road will not cause any serious lack of oxygen, but the drop in oxygen in a pond full of decomposing vegetation is not easily replenished.

So, if you combine warmer pond water in the summer with a large mass of decomposing vegetation, you certainly can have fish die from lack of oxygen. But do not blame the product used. The blame is all on the timing and actions of the applicator.

One may ask how to avoid any fish die-off in the summer months or during any time of the year. The solution is to treat no more than a third of the pond at a time so that fish have oxygen available.

With the large number of products on the market, there is not enough room in this article to cover them all. Proper identification is a must for any success. The Texas A&M AgriLife Extension Service has a tremendous website that will help with weed identification and treatment. Go to aquaplant.tamu.edu for a world of valuable information.

Let us say it again, herbicides approved for use in ponds will not kill fish. If they did kill fish, then I cannot imagine how they would have ever been approved.

It is also important to state that there are products commonly used in ponds that have no business being used. If the label does not state that it can be used in a pond with specific instructions on how to apply it, then it is not legal to use in a pond. No, I don’t care what your neighbor said, if it is not labeled, then it is out of bounds. If you wish to dive deep into farm pond management, go to the website fisheries.tamu.edu/pond-management/. This site has an abundance of pond and sport fish management resources and links. Also feel free to call your local county Extension agent with the Texas A&M AgriLife Extension Service.


Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu .

Educational programs of the Texas A&M AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, national origin, genetic information or veteran status. The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.

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