Know what you're dealing with before trying to get rid of (or at least control) lake and pond weeds that gross out swimmers and aggravate boaters.
Pond Weed Identification: What to Know
Updated Sep. 29, 2023
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Pond Weed Identification
All lake and pond weeds are not the same. Many provide shelter and food for fish and other wildlife. Some are native species, endemic to the region, that are vital to the pond or lake’s ecosystem. In North America, northern watermilfoil is native. Eurasian watermilfoil is a non-native.
Before you decide whether to remove lake or pond weeds, start by identifying those weeds. Study them carefully so you do not mistake them for native species.
Some algae and weeds float on the surface. Others are classified as submergent (mostly underwater) or emergent (most shoreline plants). Knowing these characteristics will help you properly identify the weeds in your lake or pond.
Local departments of natural resources, state land grant universities and extension services can be excellent resources for help with indentifying aquatic weeds. We asked Steve McComas (AKA The Lake Detective) of Blue Water Science in St. Paul, Minnesota to help us compile this list of 16 common aquatic weeds and algae and what you can do about them.
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Bulrushes
Also known as reeds or pencil reeds, bulrushes are considered a valuable part of a lake’s ecosystem. They grow in marshes and along shorelines of lakes and ponds up to eight feet deep and can rise five to 10 feet out of the water.
Bulrushes create habitats for spawning fish and provide food for waterfowl and birds. For example, redwing blackbirds will nest or roost in bulrushes. You can manually remove enough to make a channel for boat or swimming access.
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Canada Waterweed (Elodea)
This native species can be found in lake depths of up to 10 feet. It produces oxygen and habitat, but can also be dense and a nuisance. It can be pulled by hand, raked or cut, but be sure to clean up any loose fragments that can re-root into new patches. Herbicides can also be effective if used carefully.
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Cattails
Hybridized cattails can be invasive and aggressive, and spread through rhizomes. They quickly grow five to six feet tall and can be trimmed back or dug up. Native species, even the hybrids, do offer benefits — filtering water for better quality, offering habitat for birds and stabilizing sediment.
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Coontail
Similar to milfoils but with stiffer, spinier leaves, this weed (also known as hornwort) can grow underwater in depths of up to 20 feet. Because it is important to waterfowl and fish as food and habitat, it’s recommended to only clear the area needed for recreation. McComas says its weak root-like holdfasts make it fairly easy to rake out of your beach area two or three times a season.
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Curlyleaf Pondweed
This weed, with wavy-edged leaves that look like lasagna noodles, can create dense mats and crowd out other species. But it also can provide early season shelter and food for pond life. If it’s a nuisance, rake it or cut it back. It naturally dies back by mid-July.
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Duck Weed
These little round specks of green that spread from lake to lake via waterfowl can begin as a few dots and cover the surface of a pond or small lake in a matter of days. Duck weed can deprive water of oxygen, which kills fish.
Using an aerator helps, because duck weed needs still water to thrive. For expansive duck weed infestations, chemical control may be necessary.
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Eurasian Watermilfoil
This aggressive non-native species usually grows three to nine feet tall underwater, but it can be as tall as 20 feet. It easily snarls motors, and each fragment that breaks off from the plant can root and start new invasive beds.
This weed can be raked or cleared with cutting blades or treated with herbicide. Identify carefully, as it can look like native watermilfoil species or coontail.
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Flowering Rush
This reed-like weed, with clusters of pink flowers, generally spreads slowly and can be hand-pulled before it takes over an area. If there is a large lake infestation, treatment with a herbicide may be more cost-effective.
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Hydrilla
Classified federally as a noxious weed, this thick plant (once favored for aquariums!) can form dense mats that stretch from the lake bottom about 12 feet toward the surface.
Pulling, cutting, mechanical removal, open boat lanes and herbicides have all been used to try and control this hardy weed that grows and expands from every fragment left in the water. It can quickly cover large areas.
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Purple Loosestrife
This invasive shoreline weed is another one that got into the U.S. through Great Lakes shipping. It looks attractive with spikes of purple flowers, but each plant can produce up to 2.7 million seeds a year (!), quickly overwhelming native species.
If you only have a small stand of this pond weed, hand removal or digging is recommended. If there is a large area, you may need a herbicide to treat it.
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Starry Stonewort
This non-native algal-type plant from Eurasia was introduced unintentionally through Great Lakes shipping. It has the potential to overtake a lake quickly with dense mats near the water’s surface and it can crowd out native species.
If you can catch it early, hand-pull the clumps of algal growth and control it with a copper algaecide treatment. Mechanical removal or more algaecide may be needed for large infestations.
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Water Chestnuts
This non-native annual weed produces dense floating mats that hinder oxygen in ponds and lakes of New England and mid-Atlantic states. It’s important to identify and pull this plant as early as possible.
According to Penn State Extension Service, pulling a single water chestnut can prevent the growth of 140 plants the following season, if it goes to seed. It if takes over, expensive mechanical removal or herbicides may be required.
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Water Hyacinth
It may have attractive lavender flowers, but McComas says this ranks among the world’s worst aquatic plant nuisances.
Because it is a floating pond weed, it can be raked from the surface manually, but it may need to be treated with a herbicide or harvested with a mechanical harvester. Water hyacinth is a problem in waters of the Southern U.S. It can’t overwinter in Northern states.
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Water Lettuce
This little floating pond weed resemble lettuce leaves and can take over and choke canals and waterways with dense mats. Texas A&M University Extension Service recommends raking, seining with a net or treating with a herbicide or copper sulfate product.
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Blue-Green Algae
The microscopic growth of this algae (also referred to as cyanobacteria) can color the water, thicken it like pea soup or create jellylike globs on beaches and shorelines if the bloom is severe enough.
Some strains of blue-green algae can release toxic materials into the water as they decay. Those toxins can be harmful to pets or livestock. Seek an algicide to reduce nutrient runoff, such as phosphorus (a fertilizer for algae) that makes lakes vulnerable.
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Filamentous Algae
Although not usually called a weed, algae is a plant. This type of algae is also called pond moss or scum, and forms a mat across the water or gathers like green fur on submerged trees and rocks. If you leave it alone, it generally is done with its life cycle in 60 days. If you can’t tolerate it, McComas suggests removing it from the water surface with a skimmer net.
Originally Published: June 24, 2021
