Controlling Nuisance Aquatic Plants and Algae in Farm Ponds

Aquatic plants and algae are a vital part of the pond ecosystem, but sometimes they can grow so abundantly that they cause issues like clogged pumps, limited access or pond use, or even just make the pond look disagreeable.  Nuisance levels of aquatic plants and algae growth are typically viewed as a problem; yet, more often than not, they are a symptom of a larger problem such as sediment or nutrient pollution, shallow water, stagnant water, or natural pond aging.  While managing a pond to prevent these problematic conditions will always be preferred over treating the plant itself, sometimes controlling the plant is the only option.  Treating aquatic plants and algae can be simplified into a three-step process: first, identify the problematic plant.  Second, explore treatment options.  Third, enact your treatment plan. 

Types of Aquatic Plants

Proper plant identification is crucial for deciding what path to take.  There are dozens of different aquatic plant and algae species in Pennsylvania and some are easier to identify than others.  The first step to plant identification is identifying which category the plant fits in: algae, submerged plants, floating plants, or emergent plants.  Additional help identifying aquatic plants can be found in the Field Guide to Common Aquatic Plants of Pennsylvania or online using Texas A&M’s AquaPlant website.

Algae

Algae are the simplest type of aquatic plant and are the basis of the aquatic food chain, which means that they occur in every pond and frequently at nuisance levels.  Algae can be further broken down into planktonic algae, filamentous algae, and branched algae.  Planktonic algae are single-celled organisms that live suspended in the water column and give pond water it’s green, brown, or reddish color (Figure 1).

Filamentous algae are the most common algae complaint.  These stringy algae begins growing on the bottoms of ponds in the shallow water and as they grow, they trap oxygen in the filaments, causing the algae to float to the surface as large slimy mats (Figure 2).  Finally, branched algae look very much like a plant but lack true root systems (Figure 3). 

Common branched algae in Pennsylvania are Chara and Nitella. Although not true algae, blue green algae or cyanobacteria can grow abundantly during hot, dry summers and create Harmful Algae Blooms (HABs) that can release toxins into the water (Figure 4). 

HAB species look more like spilled paint or pea soup on the surface of the water, rather than just making the water look murky green.  Although cyanobacteria can be treated in the same way as algae, it is best to avoid water that you think may be experiencing a HAB.

Submerged Plants

Submerged Plants grow under water and typically have flexible and buoyant stems and leaves.  Typically, they grow in less than three feet of water, but some plant may have the ability to grow in deeper water depending on water clarity and individual light requirements.  Submerged plants are the most diverse group of aquatic plants and can have many different types and shapes of leaves, diverse roots, and varied substrate preferences.  They also have a variety of reproductive methods such as reproducing through seeds, spreading through roots or rhizomes, propagating through fragments, or continual growth all year long.  There are too many submerged aquatic plants to name them all in this article, but some common submerged plants include over two dozen different species of pondweeds (Figure 5), elodea (Figure 6), coontail, naiad, and bladderwort (Figure 7).  In addition to our native submerged aquatic plants, some ponds may experience issues with invasive species such as hydrilla, curly leaf pondweed (Figure 8), and Eurasian milfoil. 

Floating Plants

Floating Plants are those that grow with most or all of their leaves floating on the pond surface.  Some floating plants are rooted, while others are free-floating.  Rooted plants will get most of their nutrients from the pond bottom while free-floating plants will get all of their nutrients from the water itself.  There are several common native floating aquatic plants in Pennsylvania and they tend to be easier to identify than the submerged plants.  Water lilies typically grow in water less than five feet deep and have a flexible stem and large, round leathery leaves.  Native species will have large, white flowers.  A similar species is water shield which has smaller, elliptical leaves that have a gelatinous purple underside and small purple flowers (Figure 9). 

Duckweed and watermeal are free-floating species that may be mistaken for algae at a distance because they can grow so abundantly to cover the entire surface of a pond.  Duckweed has small floating leaves with a tiny root extending from the bottom of the leaf structures.  Watermeal is smaller, has a grainy texture, and sometimes is said to look like grass seed on the water surface (Figure 10).  Other native floating plants include the American Lotus and spatterdock.  Invasive species to look out for include yellow floating heart (Figure 11) and water chestnut (Figure 12).

Emergent Plants

Emergent Plants are those that grow at the edge of the pond and look similar to terrestrial plants because while their roots grow in saturated soil or in very shallow water, their stems and leaves grow above the water line.  Emergent plants provide excellent fish and wildlife habitat as well as some protection from bank erosion.  The most common emergent plant are cattails, which have ribbonlike leaves that taper to a point and dark brown cigar-like flowers and can grow up to six feet tall (Figure 13).  Other emergent plants include rushes, sedges, and reeds as well as species such as arrowhead, pickerelweed (Figure 14), and smartweed.  Examples of invasive emergent plants include purple loosestrife and phragmites.

Treatment Options

It is important to recognize that reacting to overabundant plant growth after it has occurred can be considered a band-aid approach.  Essentially, as long as the growing conditions are right, aquatic plants will continue to grow.  Consequently, doing our best to prevent nutrient pollution and the spread of aquatic invasive species should always be considered.  Even if plant growth has reached overabundant levels, reducing nutrient inputs can still lead to a gradual, long-term reduction in plant and algae growth.  However, when a quicker although potentially shorter-lasting solution is needed, there are many different physical/mechanical, biological, or chemical control options.

Physical Control

Physical control strategies are those that use devices, machines, or actions to control the nuisance plant.  Many physical control strategies are labor intensive but may have longer-term benefits than chemical control because physically removing a plant or accumulated sediment from the pond also removes the nutrients tied up within that plant or sediment, meaning that there are fewer nutrients in the overall pond ecosystem. 

Harvesting

Harvesting is the most straightforward control strategy – essentially cutting, mowing, raking, hand-pulling, digging, or skimming the unwanted plant from the pond either by hand or using tools or machines.  Harvesting will often need to be repeated several times until either the seed bed or root system is exhausted.  A caution is that harvesting should not be used to remove plants that spread through fragmentation because attempts to cut and harvest these plants can lead to increased growth.

Aeration

Aeration can be used either to improve the overall health of some ponds but also to reduce plant and algae growth.  Aeration works by oxygenating the water in the pond, leading to the precipitation of phosphorus, making it unavailable for plant growth.  Diffuse aerators are more effective than fountain aerators, although fountain aerators can also have effective control at reducing growth of some floating plants such as water lilies and duckweed, which prefer stagnant waters.

Drawdown

Drawdown is the partial draining of a pond to expose the shallow pond bottom to freezing temperatures during the winter.  Drawdown can have varying effectiveness based on the plant species and the level of exposure to below-freezing temperatures.  Plant species that reproduce by seeds tend to be less affected by drawdown but those that utilize rhizomes, tubers, or turions will have a varying level of susceptibility to freezing temperatures.  It can be challenging to maintain a lower level of water in a pond for an extended period of time, so this approach tends to be more common in larger lakes and reservoirs.  A permit is needed to use drawdown on a pond larger than an acre in Pennsylvania.

Dredging

Dredging a pond involves removing the sediment and organic debris that has accumulated in a pond over time.  It helps to control nuisance plants and algae by restoring a pond to its original, deeper condition, therefore reducing the total area where aquatic plants can grow.  Dredging will also remove nutrients that have accumulated in the pond bottom.  Steps should be taken to identify the source of sediment and determine how to reduce or prevent sediment delivery into the pond so that future sediment accumulation can be minimized.  Dredging a pond may require a permit both for the associated drawdown as well as the sediment removal.    

Biological Control 

Biological control strategies are those that use living things to control the nuisance plants or algae. These control options are less physically demanding than physical controls and are often less damaging to the overall pond ecosystem compared to chemical controls.

Triploid grass carp

Triploid grass carp can be stocked in ponds and lakes to provide control of submerged aquatic plants. These plant-eating fish are nonnative, so a permit is required to stock them in Pennsylvania and they must be purchased from an approved supplier to ensure that they are sterile prior to stocking. Care should be taken to identify the plants of concern and the extent to which they cover your pond to ensure that grass carp will be an effective control and that they are stocked at the proper rate. Grass carp can live up to fifteen years, however their ability to control aquatic plants declines after about ten years.

Biological additives

Biological additives contain microbes or enzymes that consume nutrients, making them unavailable for plant growth.  Multiple applications may be needed, but a permit is not required to use these products. 

Barley straw

Barley straw has been used with varying degrees of success to prevent the growth of filamentous algae. Approximately three to five bales of barley straw can be used per acre of pond area.  The bales should be broken apart, placed in cages or nets, and submerged underwater to allow water to move through the straw.  As the straw decays, it prevents the growth of algae. Therefore, in order to be effective, it needs to be applied several weeks before algae growth begins. 

Chemical Control

Chemical control involves using aquatic herbicides to kill nuisance plants or algae.  When used correctly, aquatic herbicides are effective and usually provide a quick solution.  However, herbicides may need to be applied year after year in order to achieve continual control.  Using aquatic herbicides should be considered after all other options have been investigated.  Prior to applying an herbicide, it is crucial that the nuisance plant be accurately identified and the area and/or volume of the pond be properly determined so that an appropriate herbicide may be selected and the correct dosage may be calculated.  Please note, that any application of an herbicide to a pond in Pennsylvania requires obtaining a Permit for Use of an Algaecide, Herbicide, or Fish Control Chemical in Waters of the Commonwealth.  This Guide for Completing the State Application for Aquatic Herbicide Use provides additional guidance. 

As with any pesticide application, you must read and follow the product label very carefully to ensure proper application.  Applicators are legally obligated to follow all label instructions.  Pay close attention to requirements for protective clothing, mixing instructions, application dosage/rates, water use restrictions, timing of applications, allowable application methods, and cleanup and storage requirements.  Aside from label requirements, it is also important to take note of the extent to which the nuisance vegetation is covering the pond.  It is always best to treat nuisance plants and algae early in the season, before they overtake a pond.  With abundant plant growth, treating only one-third to one-half of the pond at a time will help prevent an accidental fish kill due to oxygen depletion of too many aquatic plants dying and decaying.  If you do not have the appropriate equipment or expertise, professional herbicide applicators working for pond and lake consulting firms can be hired to do the application. 

Not every aquatic herbicide will effectively treat every aquatic plant.  Active ingredients can range in effectiveness from no control to some control to good control to excellent control.  Applying the product that provides the best control on the aquatic plant that you are trying to manage is often the logical place to start when narrowing down the list of choices to ones that can achieve the desired outcome (Table 1).

Table 1. Effectiveness of various active ingredients on common aquatic plants and algae in Pennsylvania ponds.

+++ excellent control
++ good control
+ some control
-- unknown or little/no control

All ratings assume that the product is used early in the growing season to suppress growth. Mid- to late summer use is not recommended

Note: This table includes some of the more common herbicide active ingredients used in Pennsylvania, but it is not a comprehensive list of all available products

¹ Includes copper sulfate and elemental copper products for algae

There are several additional things to consider when selecting the appropriate herbicide.  While the plants in need of control will be the primary deciding factor, oftentimes there are several herbicides that could be used. The following are some additional considerations when selecting an herbicide:   

• Contact time of the herbicide in comparison to your pond's residence time. The mode of action for aquatic herbicides may be either contact or systemic. Contact herbicides tend to work more quickly, but they must directly touch the plant to be effective.  Systemic herbicides are absorbed through the leaves and roots and then transported within the treated plant, so these may take longer to control the pest. Some herbicides may state on the label what the required contact time is.  It is important to ensure that the required contact time of the herbicide is less than the residence time of your pond, or the average length of time water spends in your pond.  More information on calculating pond measurements can be found in Pond Measurements: Area, Volume and Residence Time.
• Precautions related to life in your pond. Depending on the primary uses of your pond and where your pond outlets, it may restrict what herbicides you can use based on their toxicity to aquatic life.  For example, herbicides with the active ingredient of copper sulfate, elemental copper, or flumioxazin can be toxic to fish, especially sensitive fish populations including trout, grass carp, koi, and channel catfish.  Consult the label for more detailed information.
• Water quality of the pond. Hardness and suspended sediment can impact the effectiveness of some aquatic herbicides.  For example, copper sulfate will be less effective when applied to pond water with a hardness greater than 50 ppm.  Diquat has a high soil binding potential, so may be less effective when applied to muddy pond waters.  Test your pond water and consult product labels prior to applying.
• Water use restrictions related to your pond uses. For farm uses like irrigation or livestock drinking water (and human consumption as well), herbicide labels may restrict how soon the water can be used again after applying the herbicide to the pond.  Some herbicides do not have any water use restrictions while others range from several days to several months or simply state that the product should not be used on water bodies used for irrigation, animal watering, or human consumption. 

Conclusion

Nuisance plant and algae problems are the number one issue that pond owners encounter in Pennsylvania.  Oftentimes pond owners are most anxious to take action when the problems look the worst; however, thoughtful planning will help ensure that you come up with a treatment plan that works for you.  As was mentioned throughout this article all of these control measures require some homework such as identifying the plant and measuring your pond, and many will require work in the off season, such as obtaining permits or applying barley straw.  For additional guidance about the various pond permits and regulatory agencies, refer to Pond Agencies and Permits in Pennsylvania.  Penn State Extension also has a variety of educational resources as well as Water Resources Educators who are available to help you evaluate treatment options and select a management plan.  

This article has been adapted from the publication, Management of Aquatic Plants.

Additional Sources

Seldon, George, and Scott Jones. “Winter Drawdowns for Aquatic Weed Control and Pond Management .” University of Arkansas Cooperative Extension Service. Accessed July 29, 2024.

Whitford, Fred, Donald Penner, Bill Johnson, Larry Bledsoe, Norm Wagoner, John Garr, Kiersten Wise, John Obermeyer, and Arlene Blessing. “The Impact of Water Quality on Pesticide Performance.” Purdue Extension. November 2009.