How Can I Keep Lake Weeds From Taking Over My Pond?

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The best control is prevention. Preventing lake weeds from infesting your pond or lake is the single most important part of lake weed control. The methods are simple, and straightforward. One is a matter of vigilance, and the other a matter of good management practices. Most of our lake weed problems are with weeds that are not native to our area, therefore, the best method of controling them by prevention is to avoid introducing them into your pond or lake. Prevent non native lake weed problems through vigilance This is a simple matter of making sure that anything that might have been in an infested pond or lake does not make it into your reservoir without a good inspectin and cleaning. This includes boats and fishing tackle, trailers and anything else that went into or near the water. Sometimes animals, and even runoff from other water bodies may be a problem. There is not a lot you can do about natural causes, but they account for only a very small fraction of invasive lake weeds. Most lake infestations come from human carelessness. Lake weed prevention through good management Have you ever noticed how lake weeds start in the shallows? There is a reason. It is because light can reach the bottom. Lake weeds cannot grow without sunlight. It is as simple as that. If you can prevent the sunlight from reaching a depth of more than about 20 inches, you may still have problems in areas that are very shallow, but you will be able to prevent lake weeds from starting in other areas. The method is pretty simple. Here are the steps:
  1. Determine the pH. I put this first, because it is something you should do to make the rest of this exercise success. It should be between 6.8, and 8.6. If it is not, correct it by using lime as described in the lake weed control section of this site.
  2. Determine sechi depth. This is the depth that light can penetrate water. This will tell you what your next step should be. If the sechi depth is under 1 foot, your lake is too fertile, and you need to do something to reduce it's fertility, like planting a vegetative barrier, reducing fertilization around the surrounding area, or increasing water exchange. You can find the answers you need in this section.
  3. Fertilization. If the depth is more than 24 inches, you need to fertilize. Fertility will cause algae bloom, which will result in reduction of light penetration, which will reduce the chances that lake weeds will spring up. The information you need to accomplish this can be found here.
Follow these steps, and you may not avoid all your lake weed problems, but you will reduce them to a manageable level. Remember, You do not want to eliminate all vegetation, just the non native invasives, and excessive natives. About 20% of your lake area should contain vegetation.

Biological Lake Weed Control

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Biological lake weed control involves introducing a species into the lake to destroy the weeds. At the present, there are only a few biological controls available to the lake manager, and they have their drawbacks. Biological controls are popular because they do not involve chemicals, and are perceived as being “earth friendly”, but this may not in fact be the case.

Problems with biological lake weed control

Remember, that the need for the control is probably because a non native species was introduced into the mix to begin with, introducing another to control the first hardly seems wise.

In cases where the “grass carp” has been introduced into environments other than it’s native habitat, the results have been mixed, ranging from achieving the desired control, to lakes completely devoid of vegetation and almost all other forms of aquatic life. Other issues are escape, which can produce disastrous results downstream, and the potential for the accidental release of non sterile, reproductive capable grass carp into the environment.

There are a few other biological controls available. Basically, these are “bugs” that dine on specific species. The same issues exist with them as with the grass carp.

In general, we do not recommend the use of biological control measures for these reasons.

A better form of biological lake weed control

Natural, native biological control can be used safely by simply following good cultural practices for your pond or lake. This simply involves providing for proper pH, proper fertility, and then allowing nature to do the rest. A properly fertilized pond or lake will produce the right amount of phytoplankton to keep the water tinted, and the weeds at bay, as well as producing healthy fish.

See also: How To Fertilize A Pond Or Lake

Lake Mowers Lake Mowing

Lake mowing, that's right, I said lake mowing! Mowing lakes is a form of controling lake weeds without using chemicals or other severe methods. There are 4 types of lake weed management: Prevention, which is preferable, and involves keeping the weeds out, or having the right conditions to keep them from getting started, chemical, which involves the use of herbicides, biological which involves introducing a living species into the water, and mechanical, which is the physical removal of all or part of the weed. Lake mowing falls into the mechanical category.

Lake weed management progression

I would consider this to be the normal progression for most people who find non native, invasive vegetation in their ponds or lakes: Attempt to erradicate, and failing that for long enough, attempt to control and work toward erradication. The best control is prevention, but it is now beyond that point, and the weeds are already multiplying. The next best control, and this should have been done earlier and would have been a part of the prevention program, is to produce enough phytoplankton to tint the water for the purpose of feeding fish, and preventing sunlight from reaching the lake bed. Now, however, you must control the weeds before you can do it. Even if you already have problems with lake weeds, and especially non native invasive lake weeds, this is a step that should be taken as soon as the weeds are controlled. The final step is resignation to the fact that you cannot erradicte the lake weeds, and that continued control is a fact of life. In such a case, you have to make a few choices: Chemical, biological, or mechanical.

Mowing lakes

Mowing lakes is not something that most people think about when mowing is mentioned, but it can be an effective means of maintaining a pond or lake. Admitting this is a bit of a problem for me, you see, from the perspective of a vegetation manager with many years of experience in the field, resigning myself to the fact that we can not get rid of all the non native invasive weeds in a forest or lake is almost tantamount to admitting defeat, and everybody hates losing! After a few rounds with chemicals, and perhaps, a few rounds with biological controls like grass carp, most lake owners have resigned themselves to the fact that treatment and control will be an ongoing process with gains and losses each year. It is obviously best to get rid of invasive lake weeds if possible, but sometimes it becomes more than a manager can handle using the standard methods, so lake mowing begins to look like a good option. In such cases, when a lake or pond manager has reached the end of the proverbial rope, our next best advice is to give mowing a try. The mower to be used should cut smoothly, and with as little vibration and fragmentation as possible, because many aquatic weeds can reproduce from small fragments at a disheartening rate. Less vibration means less fragmentation. The floating remains of the pond weeds should then be gathered and removed from the water for the same reason. If you are resigned to the fact that you will have to have several chemical applications a year, or buy some foriegn fish of questionable reputation, mowing your lake is realy a less expensive option.

Lake mowers

There is one tool on the market which is head and shoulders above the rest, because it cuts cleanly with little vibration, and is user freindly. This is not a product review, it is a product recommendation, because if you are going to have to do it, you should do it with a machine that will not cause further problems.

Lake Weed Control: The Best Method

The best lake weed control is smart cultural practices

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  • The best lake weed control is not aquatic herbicides or lake weed killers.
  • The best lake weed control is not with mechanical harvesting devices or lake weed cutters.
  • The best lake weed control is not with hand cutters and rakes.
  • The best lake weed control is not with biological controls like Asian fish and strange foreign beetles.

Lake weed prevention

The very best lake weed control is to never give them a chance to become a problem in the first place! The best lake weed control, like the best disease control, or the best fire control, is prevention!

How do you prevent lake weeds? Simple, do not give them a start, and do not give them a place to start. If you never allow the introduction of exotic lake weeds, you will never have an exotic lake weed problem. Other types of lake weeds may grow naturally in your area, and that is where the “not giving them a place to start” part comes in. I am not talking about getting rid of your pond or lake, but about getting rid of the conditions which favor a lake weed control disaster.

Lake, lake weeds, lake environment

For any organism to take hold, it must first have a host, and second, a proper environment. If the environment is unfavorable, or no organism or host present, the chance of having a serious problem is just not present either. Think of your pond or lake as the host, lake weeds as the organism, and the condition of your pond as the environment. Obviously you don’t want to do away with the host, that is your lake. The organism, that is, the lake weeds are ever present, so you can’t do much about that, the only thing left is to deal with the environment, that is, the conditions of the pond or lake.

Modify the lake environment

Your goal as a pond or lake manager practicing preventive lake weed control, is to modify the lakes environment to favor the health of the lake, and to help it to become resistant to the invading weeds. This is done by using good cultural practices.

Lake fertility

Lake weed control problems are a result of one of two things. Either the lake is overly fertile, and the weeds emerge as a result of the excessive fertility, or the lake is under fertilized, and the weeds grow as a result of the lake beds exposure to sunlight. In both cases, shallow water is usually a factor. Fertility, out of balance, is always a potential problem. In the majority of cases, lack of fertility is the problem.

Why is this? When sunlight reaches the bed of a pond or lake, it warms the soil, and activates photosynthesis in plant life existing in a more or less dormant stage. This encourages growth. If sunlight does not reach the bottom, growth is prevented. Pond fertility, when in balance, produces phytoplankton which shades the bottom by coloring the water.

To solve the lake weed problem, to prevent it from recurring, the fertility must be kept in balance. Monitoring and maintaining lake fertility is the primary tool for lake weed prevention.

See also: How To Fertilize A Pond Or Lake

Aquatic Weed Killer: Aquatic Herbicide List

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Aquatic weed killer, or lake weed killers are aquatic herbicides. Aquatic herbicides are chemical products that kill lake weeds. This page includes a description of the various types available to the pond and lake owner for lake weed control.

Most aquatic herbicide products have counterparts with the same or similar chemical make up in agriculture or horticulture. Glyphosate products like Rodeo, Aquamaster, Eraser AQ, Touchdown Pro, and AquaNeat are related to other commercial terrestrial products like Roundup, but Roundup should not be used for lake weeds because it is not labeled for aquatic use, because it contains a surfactant that is dangerous to aquatic wildlife. This is just one of the many reasons why label directions are important. Remember, the label  is the law!

There are 2 types of aquatic herbicides: Contact and systemic.

Systemic aquatic herbicides work by getting inside the plant and killing it to the root. Contact aquatic herbicides kill only the plant material they touch. Both have their place as aquatic weed killers, both must be applied correctly for optimum results, and neither will solve an aquatic weed problem unless there is proper follow up.

Systemic aquatic herbicides

Systemic herbicides move from one part of a plant to another untill the plant is dead, that is, if  they are applied properly. Keep in mind that too much of a systemic herbicide will cause it to behave like a contact herbicide, killing the top of the plant before the chemical has a chance to trans-locate to the root of the plant. This makes the systemic herbicide no more effective than a contact herbicide for long term control.

Complete control is sometimes difficult even with systemic herbicides, so, if the conditions which allowed for the plants growth initially are not changed, it, or something like it, will return.

Systemic aquatic herbicide list:

  • Butoxyethyl ester of 2,4-D. 2,4-D (granular form): Navigate and Aqua-Kleen
  • Triclopyr: Renovate
  • Glyphosate: Rodeo, Aquamaster, Eraser AQ, Touchdown Pro, and AquaNeat
  • Imazapyr: Habitat
  • Fluridone: Sonar and Avast

Contact aquatic herbicides

Contact herbicides tend to work rapidly killing only the plant material that they contact. Keep in mind that this is the sole extent of the herbicidal activity. If you apply this to one part of a single plant, it will kill only that part. You can expect the plant to return at some point if it is peranial, or if good growing conditions exist.

Contact aquatic herbicide list:

  • Alkylamine Salt of Endothall: Hydrothol 191
  • Compound copper: Cutrine Plus, K-Tea, Captain, Clearigate, Nautique, Komeen
  • Copper Sulfate: Sold under many names. Sold mostly as a crystal
  • Dipotassium Salts of Endothal: Aquathol, Aquathol K, and Aquathol Super K
  • Diquat: Reward
  • Sodium Carbonate Peroxyhydrate: GreenClean, PAK27, and Phycomycin

All products are not equal! Lake weeds should be properly identified before choosing an aquatic weed killer, and then, and only then can a proper course of treatment be undertaken. There may be restrictions on the use of a particular product in your area.

Aquatic Invasive Species Control

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Too many weeds can spoil a lake, and make fishing difficult, if not impossible. There is one thing that is worse: Too many of one type of weed. When this begins to happen, there is a danger of having this pond or lake become a monoculture. Some weeds tend toward domination. Many of the most dominant weeds in U.S. ponds and lakes are exotic aquatic invasive species like Hydrilla and Giant Salvinia, and most of them are a serious threat to the necessary biodiversity of our ponds and lakes in America.

Aquatic biodiversity

Why is aquatic biodiversity so important? The culture of a lake or pond is complex, with one species dependent on another for food, cover, and a variety of other natural products and services. Removing any one natural species from the mix can unbalance the habitat, and break important links in the biological chain. Think of it as a domino effect. The introduction of aquatic invasive species in a pond or lake will remove and replace some of the native species on which other native species depend.

Exotic lake weed threat is real and present

Aquatic invasive species are already a serious threat to fishing in some public lakes. The list of public lakes and large areas of public lakes which are closed for fishing, at least for a time, grows every year.  Because of this, the threat of continued and accelerate spreading is ever present, and measures should be taken by every private pond and lake owner to avoid introduction into their own ponds or lakes.

Aquatic invasive species control

The best method for dealing with aquatic invasive species, is the best method for dealing with any problem: Prevention! Preventing exotic lake weeds, like preventing auto accidents, disease, or forest fires is much better than treating the aftermath.

Preventing aquatic invasive species: Lake condition

The lake condition prevention method for exotic lake weeds is the same as for native lake weeds. Decrease the likelihood of sunlight reaching the bottom of the pond where most lake weeds get their start by raising the water level, dredging, or producing algae bloom to color the water. Unfortunately, due to the extremely aggressive nature of some exotic lake weeds, and the fact that there are some exotic lake weeds that float on the water surface and therefore cannot be completely controlled or prevented in this way. Aggressive efforts should be taken to keep these invaders out to begin with.

Preventing aquatic invasive species: Keeping exotic weeds out

Keeping exotic lake weeds out of your pond or lake is a matter of vigilance. It is that simple. If you have used your boat, recreational watercraft's like jet skis, fishing equipment, and even recreational equipment like 4 wheelers that might have traveled through the edge of another lake, you should give anything that went on or near the water a complete and thorough inspection and cleaning before you hit the road. Check everything visually before reintroducing your equipment into your own lake. Even a small piece of Hydrilla, Giant Salvinia, or many other exotics can be enough to start an invasion that can eat your lake! Vegetative reproduction rates can be as high as 51%.  Some exotic lake weeds can go from 1 plant to 60,000 in less than a month, so pay close attention to cleanup.

Why Aquatic Weeds And Filamentous Algae Occur


This is the basic sequence of events leading up to pond scum and lake weed problems. It should be noted that not all problems arise from these causes. There are problems related to the over fertilization of a body of water which can lead to some of the same results. There are tell tale signs which can be easily seen by checking water clarity. If the water is a virtual soup of green suspended algae, the water is too fertile, and other measures, like adding vegetation to the intake side to filter some of the nutrients should be taken. While this does happen, it occurs so infrequently that I have seen only one or two cases in my life. Sequence of events resulting in pond scum and lake weed problems:
  • There is not enough fertility
  • Water becomes too clear
  • Sunlight penetrates to the bottom
  • Sunlight warms the bottom
  • Weed and algae germination and growth is initiated
  • Weeds quickly grow through the water profile
  • Filamentous algae begins to rise from the bottom
  • Weeds and pond scum become the dominate feature of the lake or pond
  • Fishing and recreational use becomes difficult
  • Mosquitoes arrive and decide they like the surroundings
  • Water quality worsens
  • Dissolved oxygen is low in the evenings
  • Fish rise to the top to gulp oxygen close to the surface
  • Without plankton, the life cycle is slowed and the fish become hungry and skinny
The last problem listed in this sequence has probably been a problem for a while. Phytoplankton is needed in the water to:

Color the water for sunlight control

By coloring the pond or lake water, sunlight is prevented from reaching the lake bed where weeds and filamentous algae get their start. This provides lake weed and pond scum prevention.

Shade the water for heat control

By shading the water, the temperatures are moderated, providing better, more comfortable conditions for aquatic life.

Produce oxygen throughout the water profile

With higher oxygen production in the water, fish are able to breathe easy, and concentrate on avoiding predation, and food intake.

Provide food for the life chain

Because phytoplankton is the beginning of the "circle of life", food for fish is produced as a natural result. Producing phytoplankton, or algae bloom is pretty easy to do, but there are a few preliminaries. You can read more about them: Pond Scum Prevention Pond Algae

How The Parts Of Ponds And Lakes Interact


All of the parts of your pond or lake interact with all of the other parts. Each problem you have is a part of other problems, probably leading back to a single problem at some point. pH, lake fertility, dissolved oxygen levels, fish size and weight, and lake vegetation each have something to do with the other, therefore, it is important to trace any problem back through each factor, and find the primary cause. As an example, we have a 4 part series about how these interactions occur. Pond Scum Lake Weeds Mosquitoes And Skinny Fish Controlling pond scum, lake weeds and mosquitoes and raising healthy fish may seem complex and difficult. There is no miracle cure, there is no magic bullet. Pond Scum And Lake Weeds Most of the questions we receive about lake weeds revolve around how to get rid of them. As with pond scum, most lake weeds get their start at the bottom of the pond when sunlight reaches them. When conditions are right, they may grow out of control causing problems for the pond owner. Mosquitoes And Skinny Fish Mosquitoes on ponds and lakes may seem to be unrelated to lake weeds and filamentous algae. The truth however, is that mosquitoes become a problem when there is too much vegetation in the pond... If the water has too little of one type of algae, and too much of another, the ponds life cycle is interrupted, and the fish go hungry, resulting in skinny fish. Pond Scum And Lake Weeds The Real Problem The problem is sunlight. You can’t do much about that. Specifically, the problem is sunlight reaching the pond bottom, which you can do something about!

Pond Scum And Lake Weeds The Real Problem

Part 4 in our series on lake and pond interactions

The real pond scum and lake weed problem


The problem is sunlight. You can’t do much about that. Specifically, the problem is sunlight reaching the pond bottom, which you can do something about! The common issue for pond scum and lake weeds is sunlight penetrating the water to the bottom of the pond or lake. When this happens, a process begins which will allow lake weeds to grow unchecked, pond scum to develop and start to rise to the top, and the secondary result will be mosquitoes, and even though the plants and algae will produce oxygen, they will at some point cause conditions which will deplete oxygen in the water, and all the while, your fish will be starving. All of this because sunlight reaches the pond bottom!

Preventing lake weeds and pond scum by preventing light penetration

There are a number of ways to prevent light from reaching a pond or lake bottom. You can raise the water level, lower the bottom through dredging, or color the water in some way. Raising the water level can produce good results in some cases, but may not be practical, and it still doesn’t deal with the issue of skinny fish. If the water is deep enough the sunlight cannot penetrate, but something needs to be done to feed the fish if the water is clear to a depth of more than 24 inches, so even if you can raise the water level, or dredge the shallow areas, more is needed. The water needs color to shade the bottom. This can be done with dyes and colorants, but, once again, you still have the issue of skinny fish. There is a product that can resolve both the light penetration issue, and the food supply issue.


Plankton is a single cell algae which is suspended in the water of healthy ponds and lakes. It provides oxygen to the water, food for the tiny invertebrates that feed the larger aquatic life, which ultimately feeds the fish in the lake, and provides color for the water helping to prevent the excessive growth of unwanted vegetation and pond scum. This is sometimes called algae bloom.

Producing algae bloom

The reason that plankton does not grow in a pond or lake is lack of fertility. The solution to this problem is to add fertilizer. To learn more about this process see: pond scum prevention and Lake weed prevention Pond scum is filamentous algae, and is also known as blanket weed, pond blanket weed, and a few, less desirable terms.

Pond Scum And Lake Weeds

Part 2 in our series on lake and pond interaction

Pond scum control


Most of the questions regarding pond scum, lake moss, or pond algae ask how to get rid of it. This is perfectly logical. The problem is, that the pond scum itself  is just a symptom of other problems. The first evidence we have of it’s existence is when it floats to the top and interferes with the aesthetics, our recreation or our fishing. Filamentous algae, or pond scum as it is more commonly known exists in most bodies of water, and will begin to show itself when conditions are right for it. It grows from the bottom, and sunlight allows photosynthesis when it penetrates to the bottom. The photosynthesis process allows the algae to take in carbon dioxide, and exhale oxygen, some of the oxygen, which is good for fish, will be released under the algae mat, and gradually cause it to float to the top, where it becomes a nuisance.

Lake weed control

Most of the questions we receive about lake weeds revolve around how to get rid of them. As with pond scum, most lake weeds get their start at the bottom of the pond when sunlight reaches them. When conditions are right, they may grow out of control causing problems for the pond owner. Some aquatic weeds are capable of doubling in mass in a week or less, leaving the pond owner with a huge aquatic weed control problem. If measures are taken to kill the weeds with aquatic herbicides, the weeds will die, and drop to the bottom where they will decay over time. This can produce a slimy, and high organic fertility on the pond bottom, perfect to supply nutrients to the next generation of weeds!

Mechanical lake weed control

Mechanical controls and mechanical lake weed harvesting may produce less slime at the bottom, but care must be taken when using a lake weed cutter to collect all the fragments, as aquatic weeds are notorious for their ability to reproduce from fragments. One plant chopped into 100 pieces may turn into 50 new plants in a matter of days. See: The relationship between excessive aquatic vegetation, Mosquitoes And Skinny Fish