How to Control Pond & Lake Weeds (Best Methods)

As a pond owner, you’ve likely encountered (or will eventually) various pond weeds and algae popping up in your pond. While a few of these aren’t always a bad thing, keeping them under control is important to ensure that they don’t overtake your pond, limit sunlight, or use up vital dissolved oxygen in the water. Once established, they can be quite difficult to get rid of, depending on the species.

For example, cattails develop thick, extensive rhizome mats in the ground that are similar to roots. New cattails sprout from these even if the old ones have been cut, making this particular foe a difficult adversary if allowed to grow too long and establish widespread rhizome mats.

Others, like algae, are necessary in small quantities due to being at the base of the aquatic food chain, but if left unchecked can cloud the water, lead to oxygen deprivation (also known as hypoxia), or potentially kill organisms in extreme cases. “Bad” algae in particular (such as cyanobacteria, which is not actually algae but looks and behaves very similarly) feeds on any excess nutrients and can establish itself in a relatively short span of time. Phragmites, a tall reed grass, is invasive and able to exceedingly efficiently overtake large areas quickly, partially due to them also possessing rhizome mats.

If not controlled, these various organisms can choke out your pond, ultimately harming your fish, other plants, and water quality. They can even spread beyond your pond, and move into natural rivers and lakes where they often lead to the collapse of ecosystems when not managed properly.

How To Get Rid of Pond & Lake Weeds

The best method of controlling pond weeds depends entirely on the type of pond weed you’re facing off against. Below we’ll cover the main groups of pond weeds, some species within them, and the best ways to control and/or eliminate them.

Weed 1) Algae

As mentioned previously, while the mention of algae is a source of anxiety for many pond owners, the presence of some algae is critical for proper ecosystem functioning. However, some species can get out of control. The eight main phyla of algae are: blue-green algae, which is more accurately known as cyanobacteria; diatoms; chlorophyta, or green algae; chrysophyta, which is most commonly known as golden algae; euglenophyta; dinoflagellate; phaeophyta, often termed brown algae; and rhodophyta, or red algae.

Algae comes in two forms within these groups: planktonic and filamentous, or string, algae. Overall, if you see a bit of algae in your pond, don’t panic! They provide a vital food source for insects, fish, and waterfowl, and as primary producers they generate oxygen and nutrients that is then utilized by other organisms that cannot produce their own. Test your water quality regularly, and so long as everything is within healthy parameters, any algae present poses little to no threat to your pond or its inhabitants.

Best Algae Control Methods:

1) UV Clarification

Free-swimming algae can be controlled mechanically with the use of UV Clarifiers, which are very effective at smaller scale (but may not be sufficient for lakes). Clarifiers are safe for fish, wildlife, and micro-organisms,  so are the preferred method for controlling green water algae in most situations.

2) Shading & Pond Dyes

Another approach that has been shown to have some impact – pond dyes can help to limit the amount of sunlight available to the algae, thus reducing its numbers. However, this can have adverse impacts on other plants in your pond that also depend on sunlight.

3) Vacuuming, Raking & Manual Removal

String algae is typically a fair bit easier to control. Some species, like chara, more closely resemble plants than stereotypical algae and can therefore be cut, raked, vacuumed or simply pulled by hand. Generally, string algae resembles strings and often collect and intertwine with one another to form mats. These are straightforward to remove, again using a rake or just your hands if the mats aren’t too heavy or broad.

4) Draining Water

If manual or chemical control methods haven’t proven to be particularly effective in removing or controlling green water algae; your best bet is to drain your pond, clean it, and fill it with clean, fresh water. This will remove the vast majority of algae spores and you can implement another control method, such as a UV clarifier, to maintain clear water.

5) Algae Herbicides 

In terms of herbicides, you can make use of certain products, such as copper sulfate or cutrine plus. Make sure to pay close attention to the label of the product, so that you apply it properly and in the correct dosage. Only treat half of your pond at one time, and remove any dead algae as soon as possible.  You can alternatively utilize naturally occurring microbes to break down the algae without harming your overall water quality, though you will still have to remove the algae after it dies to prevent its decomposition from using up oxygen.

Weed 2) Emergent Weeds

Emergent weeds are those that typically grow in only a couple of inches of water, rather than being purely aquatic or purely terrestrial. Common emergent weeds include some cattail species, phragmites and purple loosestrife (both of with are invasive species, to boot), bulrush, and alligator weed.

These plants commonly grow along shorelines and lake embankments, and if left unchecked can densely cover the entire shoreline, making access for both humans and wildlife difficult. They also spread incredibly quickly and across large distances due to their rapid reproduction cycle.

Best Emergent Weed Control Methods:

1) Seasonal Cutting & Pulling 

Most emergent weeds reproduce via rhizomes, which are thick, matted root-like structures from which new organisms are able to sprout and grow. Because of this, simply cutting these plants is not enough, as the rhizomes will still be present and continue producing new plants. You’ll need to either pull these weeds up, rhizomes and all, which can be quite a daunting and intensive task depending on how established and large the rhizomes are, or utilize herbicides. If done with the right timing, cutting can be an effective approach. Don’t cut them prior to July, as during this time they are devoting a great deal of energy to spring growth and will be able to effortlessly resprout. Wait until July, August, or even September, when they won’t have energy available to resprout. You’ll have to repeat this for a few years, but each year you should notice fewer and fewer plants, at least in the case of cattails and phragmites.

Take special care to properly dispose of the plants, so that the seeds aren’t able to spread from the cuttings. Keep in mind that the seeds will also remain in the ground (or seed bank) for many years, so this will likely be an ongoing battle for a while.

2) Emergent Weed Herbicides

While using chemicals shouldn’t be your first option because of the potential to cause harm to other creatures like your fish, in some cases herbicides are necessary. For plants such as cattails, phragmites, and bulrush, you’ll need to utilize a systemic herbicide, which is applied directly to the plant (and thereby is less likely to impact other organisms), and works its way through the entire body of the plant, rhizomes and all.

Glyphosate in particular is affordable and efficient at treating emergent weeds, though depending on your location, the exact herbicide brand, and what you’re trying to control, you may need to either obtain a permit to apply the herbicide or contact a professional to do so for you. To prevent rhizome establishment altogether, you can install a thick pond liner or several inches of gravel. The downside of this is not having the same substrate that some other plants rely on.

Weed 3) Floating Weeds

Floating weeds are generally found in shallow water a few meters deep or less, but can occasionally be found in deeper water as well. They are able to spread across the water’s surface, potentially totally blocking light and oxygen from reaching anything below them.

Common nuisance floating weeds include water lilies, duckweed, water lettuce, water hyacinth, spatterdock, and waterweed. Unlike emergent weeds, floating weeds are often much easier to control as their rhizomes are less extensive and more sensitive to manual and chemical removal methods.

Best Floating Weed Control Methods:

1) Manual Removal

Some floating weeds, like water lilies and spatterdocks, have rhizomes so they will need to be treated like the rhizomous plants mentioned above, either by pulling them or cutting and applying herbicide. Water lettuce grows atop the water without an extensive root system, so it can usually be removed by simply raking or skimming it from the water’s surface. Duckweed is very small, and thus can be raked, vacuumed, or skimmed from the water’s surface without much effort. Simply adding more aeration to your pond to promote more water movement will also help prevent some floating weeds from being able to establish, as they typically prefer still water.

2) Floating Weed Herbicides

If weeds are more established, either a contact or systemic herbicide may need to be used. Some contact herbicides, like Ultra PondWeed Defense, work quickly on a variety of emergent, submerged, and floating weeds, but have an added perk of not remaining in the water system for long before breaking down. This means there’s less risk of harm to your fish or other organisms.

Weed 4) Submerged Weeds

Submerged weeds are found, predictably, beneath the water’s surface, though the upper portions of them may float limply on the surface. They have soft stems, rendering them too weak to rise above the surface, but this also makes them easier to cut, rake, or remove vacuum

Familiar problematic species include coontail, several pondweed species, bladderwort, hydrilla, and some species of water milfoil, such as Eurasian water milfoil (other species of milfoil are native and much less aggressive).

Best Submerged Weed Control Methods:

1) Manual Removal 

Due to their soft stems, submerged weeds are among the easiest to remove by hand pulling, raking, cutting, or skimming. Trees could also be planted about the pond to help shade it and deter overgrowth – try to limit shading to only about 50 to 60% of your pond so that you don’t stunt the growth of native, desired plants.

2) Pond Dyes

Alternatively, you could utilize pond dyes to temporarily shade the pond long enough to kill off established weeds and prevent their regrowth. Eurasian milfoil is more aggressive than other submerged weeds, able to grow from even tiny portions of the plant that has broken off, so you’ll have to be very thorough when removing this particular species.

3) Submerged Weed Herbicides 

Though these species are easily manually removed, you can also utilize herbicides if they’re well-established and presenting a particularly persistent threat. There are seasonal herbicides available, such as Sonar, that can be applied once and will treat submerged weeds and some floating weeds for the duration of the season. This does, however, pose a potential threat as this means that the chemical persists in your pond for an extended period of time. You can also use a broad spectrum herbicide contact herbicide, like Ultra PondWeed Defense mentioned above.

Considerations & Alternatives When Controlling Aquatic Weeds

The control method that you use will depend entirely on your location, pond size and overall ecosystem, and the nuisance species that are present. Texas A & M University has a website with an extensive database to help identify a plethora of aquatic weed species.

In some areas, herbicides may be considered illegal or you may need to obtain a permit or hire a professional to apply them. In addition, herbicides should always be your last resort if no other options are working or available to you.

In regards to all of the categories covered in this article, fish species can be implemented to help control problematic weeds. For example, koi, goldfish, and grass carp are known to munch on water lilies, water hyacinth, and water lettuce, among others. This method won’t work on ponds that are overgrown with weeds, but is most effective to continually help keep the plants under control as time goes on after their initial eradication, either by manual, chemical, or biological means.