Many readers have called recently about weeds in ponds, both for agricultural use and for aesthetics. Many of those weeds can be problems in both situations.

The one “weed” I get the most questions about is not actually a weed, but algae — a very aggressive and bulky one. Filamentous algae is the one that looks like a thick carpet on the surface of the water. Without roots, it can free float about the whole body of water, often close to the bank.

It can be very difficult to control, especially if you have fish in the pond and want them to survive. It can be controlled and relatively cheap, if you opt for a chemical solution.

If you can catch it just as it is starting to invade the pond, you can remove it by hand or rake. This is not as easy to do as it sounds, because it has already grown undetected under the surface and is massive before you know it is there.

In winter, it might sink below the surface to ride out the cold weather, to resurface the next spring.

Even though it is not a plant, it still needs the sunshine to survive, and that fact can be used to your advantahge.

You might have seen the pretty but unnatural-looking ponds of a blue-green color. That color was created by a product intended to  block sunshine from entering the water and providing sustenance to the weeds in the pond; it does not harm fish at all and can help control weeds that are rooted and stay submerged.

Another option for controlling filamentous algae is copper sulfate, a powder-like substance sprinkled on the surface of the water. 

While it is very effective, if not managed properly, it can cause massive fish kills by eliminating the oxygen in the water. This happens after massive die-offs of algae during decomposition.

I recommend applying it by sections of the pond to reduce the chance of killing fish. This technique leaves escape areas that still have free oxygen for the fish to breath. It works for nonmoving ponds, which are ponds not fed by a spring and those without overflow pipes that are constantly active.

Types of aquatic weeds

The actual weeds in your pond are of four types: emergent plants, submerged plants, rooted floating plants and free-floating plants.

Emergent plants are shallow-water plants with roots, but most of their vegetative parts grow above the surface, such as cattails, rushes and willow trees.

Submerged plants dwell below the surface of the water but still rooted into the pond bottom soil. Sometimes these are attached so deep they are difficult to see. However, they still need sunlight, so their leafy parts will be as close to the surface as they need to be to get it. These include milfoil, coontail and naiad, as well as others.

Rooted floating have roots in the pond bottom but the plant floats on the surface and is attached by long tendrils. This include water lilies, and water lotus, among others.

The free-floating plants can be among the most difficult to control. Among this group are duckweed and watermeal. They are resistant to many aquatic weed control herbicides, and the ones they are susceptible to can be expensive — as much as $300 per gallon.

Now is not the best time to treat for most of these aquatic weeds. Treatment should be applied in spring, as with many yard and field weeds.

For more information, call your local extension office. In Boyd County, the phone number is (606) 739-5184.

LYNDALL HARNED is Boyd County Extension agent for agriculture and natural resources.

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