Aquatic plant or water weed?
PONDS NEED PLANTS. But too many or the wrong kind can cause problems, from interfering with recreation to harming fish populations. At the point when aquatic plants become a nuisance, that’s when they become weeds.
“Aquatic plants are vital to maintaining a balanced ecosystem in your pond,” said Andrew Branson, fisheries programs specialist with the Missouri Department of Conservation. “They form the base of the food chain for almost all life in the pond and serve as habitat and protection for small fish and invertebrates. But if plants are getting in the way of the intended use of the pond—like fishing, swimming or boating—then it’s something you need to manage.”
Though the amount of plant life in a healthy pond will vary, Branson said, ideally no more than 10% to 20% of its surface and bottom should be covered.
“Too many aquatic plants can actually create low-oxygen fish kills,” he explained. “The plants create oxygen during the day when the sun is shining on them. But at night, when everything’s dark, the reverse happens. The plants start pulling oxygen from the water. If your pond’s already struggling due to being shallow or in the heat of the summer, you could wake up to a lot of dead fish.”
Though any kind of plant can become a nuisance if conditions are right, some are more likely to become a problem than others, Branson said. Topping that list is hydrilla, an extremely aggressive invasive weed that can clog waterways, displace native vegetation and contribute to harmful algae blooms. A plant native to Africa and southeast Asia, hydrilla infestations in the U.S. are thought to have originated from the aquarium trade in Florida.
Similar in appearance is Eurasian milfoil, another invasive aquatic plant that can threaten the health of a lake or pond. This weed forms thick mats in shallow water, quickly growing, spreading and eventually blocking sunlight, which can kill off native aquatic plants that fish and other underwater species rely on for food and shelter.
Noninvasive aquatic plants can also become nuisances if left unchecked, Branson said, pointing out cattails and water lilies as examples.
“These are found in many ponds, and people love them because they’re pretty,” he said. “But they can easily get out of control if you don’t manage them. We have all seen times where the cattails ring the pond or lily pads completely cover the surface. That’s a problem.”
Of all the nuisance plants, however, Branson said filamentous algae seems to be the No. 1 issue for Missouri pond owners. Appearing as fine green threads that form dense, floating mats, filamentous algae is often referred to as “pond moss” or “pond scum.”
“I get the most calls about filamentous algae in ponds, which can make fishing, swimming and other recreational uses nearly impossible,” Branson said. “In extreme cases, this type of algae can totally cover a pond’s surface, which restricts growth of fish and desirable plants.”
Excessive plant growth is often due to an overabundance of pond nutrients, which can be caused by agricultural runoff, livestock waste or accumulation of organic matter over time. Ponds are naturally collectors, serving as a settling basin for the land that drains into it. The older a pond gets, the more nutrients it builds.
“No matter what type of aquatic plants you have, they need three things,” Branson said. “They need lots of water. That’s no problem since they live in water. They need lots of sunlight, and that’s usually not a problem in Missouri, either. And they need fertilizer. So, if we can keep nutrients out of the pond, that’ll make a big difference.”
Having some type of barrier between agricultural fields and ponds can slow runoff and filter nutrients before they reach the water. Fencing cattle out of the pond will also help.
“If the pond is used as a watering hole in a pasture, you’re probably always going to have nuisance plants because the cattle are fertilizing it continually,” Branson said. “But if recreation is the main reason you have a pond, then you need to find another source of water for the cattle.”
While nutrient management can help prevent water weeds, pond owners will likely need to control some vegetation by mechanical, biological or chemical methods—and perhaps a combination of all three.
“Mechanical control means physically getting in there and pulling, digging, cutting or raking out the nuisance plants,” Branson said. “That is usually the first thing people start with, when they start noticing a problem encroaching or if they just want to open up a certain spot on the bank, like around a dock or a fishing spot. Aquatic plants grow fast, however, so you may have to weed the pond often to keep them under control.”
Biological control involves the addition of plant-eating fish, which, in MFA territory, is generally limited to grass carp, Branson said. A large member of the minnow family, grass carp can eat two to three times their weight in pond vegetation each day.
“They are definitely effective for certain plants, but we always recommend adding grass carp as a last resort because they don’t discriminate with what they eat,” he said. “They don’t only take out your nuisance plants, but they may also wipe out beneficial plants. MDC’s county biologists can help recommend the correct number of grass carp for each situation.”
“However, grass carp are not effective on algae,” Branson added. “They like leafy, weedy types of plants, but they won’t eat that slimy stuff.”
Herbicides, on the other hand, will help control algae and many other nuisance plants. There are no all-purpose weed killers for ponds, so selecting the right chemical is of utmost importance, he cautioned.
“While herbicides are the go-to choice for certain aquatic weeds, you must exercise extreme care,” Branson said. “Often, there’s only a small difference between dosage rates that will kill both weeds and fish. You want to make sure you use the correct, safe herbicide and in the right amount. You don’t want to overdo it. Our county biologists can help with what herbicides and amounts work best.”
Timing is critical when chemically treating ponds, Branson said. MDC recommends applying herbicides only when the water temperature is below 80 degrees, typically in the spring when the plants are actively growing.
“That means right about now, or real soon, you need to start watching the pond and hit it with chemicals at the first sign of anything growing,” he said. “Once the water warms up, it’s not able to hold as much oxygen, and you need to be done treating until the water temperature cools off again.”
The scope and frequency of herbicide use must also be carefully managed, Branson added. Only one-quarter to one-third of the vegetation should be treated at a time, leaving a week to 10 days between applications.
“When the plants are dying, they’re also pulling out more oxygen from the water as they’re decomposing,” he explained. “You want to give the pond a chance to recover before you hit it again. So don’t go out and spray the whole pond all at once.”
Before undertaking any control measures, the first step is identifying the problem plant, Branson said. The Missouri Department of Conservation offers an extensive “Nuisance Aquatic Plant ID” guide online at mdc.mo.gov, or it’s available in print form at local MDC offices.
“There are some lookalikes out there, so be sure you know what weed you’re trying to control,” Branson advised. “That’s the first step. Once you can identify it, then you can browse the list of control measures for guidance on how to control them. If you need assistance, MDC has fisheries management biologists and private land biologists for every county in Missouri. Either one of those can help as well.”
According to Branson, the aquatic weeds shown here are among those most likely to cause problems in ponds throughout MFA territory. Consult pages 38-41 of MFA’s 2023 Agronomy Guide for more information on other nuisance plants and herbicide options or visit with the crop protection experts at your local MFA or AGChoice center.
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