What Lies Beneath

Aquatic plants play important roles in ponds and lakes by providing oxygen and habitat for fish and other aquatic organisms. In landscaped areas, aquatic plants, such as water lilies, lotus and ornamental sedges, are attractive and colorful additions to the water garden or pond. However, the excessive growth of aquatic vegetation in ponds makes a landscaped area look unkempt and unappealing.

The major approaches to controlling aquatic vegetation are preventive, mechanical, physical, biological and chemical. The key to the success of any of these approaches is correctly identifying the problem plant. The two major botanical groups found in water are algae and flowering plants. Algae are simple organisms, and the most common types are planktonic and mat-forming. The planktonic algae are microscopic and cause green or red colors in the water. The mat-formers form large masses of green “string-like” growths. They begin growth along the edges or on the bottom of the pond in the spring and float to the surface as the summer progresses.


Preventing the introduction of algae and aquatic plants into a body of water is often overlooked as an important step in control. When planting ornamental aquatic plants, clean the plants and roots of attached algae, remove foreign plants and discard them away from the site. Do not allow homeowners to dump their aquarium plants into the water. When working on a series of ponds, clean boats, boat trailers and other equipment before moving them.


Hand pulling, raking and the use of hand-held cutting tools are techniques that have been used for years. They can provide temporary relief, but plants and algae tend to resprout from underground propagules. Because many aquatic plants can resprout from a few surviving fragments, you must harvest and remove the cut vegetation from the area so that it can't be washed back into the water.


This approach is based on methods that change the physical conditions of the environment. Probably the most common is the use of blue dyes, which are added directly to the water. Blue dyes are most effective on submersed plants and you should apply them early in the season before the plants reach the surface. The dye concentration must be maintained throughout the growing season, so additional applications are often needed. This method is not appropriate in waters that have significant outflows.

Another important control is to reduce the amount of nutrients (nitrogen and phosphorus) entering the water. Algae and aquatic plants require nutrients, and the appearance of aquatic vegetation, particularly algae, in shallow water and coves is a result of the direct runoff of fertilizer. Discontinue fertilizing surrounding grounds within 10 to 20 feet of the water's edge, use phosphorus-free fertilizer in areas that drain into the water and apply phosphorus only when a soil test suggests that it is needed. Plant native emergent vegetation along waterways or around the edges of ponds. Not only can they provide an attractive cover but they absorb nutrients that might enter the water.


Using organisms to control pests has been beneficial. In aquatics, the most common biological control is the grass carp. This fish, originally from northern China, eats primarily submersed vegetation. It is weak on mat-forming algae and free-floating vegetation. Check with your state's regulatory agencies for permits and restrictions.


Algicides and aquatic herbicides are very widely used to control aquatic vegetation. Consider the following tips prior to using these products.

  • Identify the target species

    Algicides, such as the copper products, are only effective on algae. The aquatic herbicides are effective on flowering plants, but identify these plants too, in order to ensure that you are using the right product for the right species.

  • Note the water use restrictions

    This is an essential consideration for a groundskeeper. The use of treated water for irrigation and other purposes (e.g. fishing, watering livestock, drinking) varies with the product. Read the label carefully for this information.

  • Consider timing and temperature

    Most aquatic treatments are best applied in the middle to late spring when the plants are growing but still young. Mid- to late-season treatments, particularly when the pond is heavily infested, can cause fish kills. If too much vegetation is killed, the decomposition process removes oxygen from the water. Water temperatures at the time of treatment should be around 60°F in the area of treatment.

  • Take the time to calculate area and volume

    Algicide and aquatic herbicide labels provide rates based on surface acres or water volume. The amount of a product, like glyphosate for emergent plants, is calculated on a square-foot or surface-acre basis. Algae and submersed plant control is often based on volume, which is usually described in acre-feet. To determine this value, measure both the area (in acres) and the average depth (in feet). A pond that was first built 20 years ago may have been 20 feet deep then, but today, because of sedimentation and plant debris, the pond may be only 10 feet deep.

  • Sterilization is not a goal

    Unfortunately, there is no chemical that, when used once, will control weeds in that body of water forever. Algae control, usually requires periodic treatment as the growths reappear from spores or surviving cells. The same is true for duckweed and watermeal when using a burndown treatment. On the other hand, you can apply fluridone (for free-floating and submersed flowering plants) once (or as a split treatment) and it can result in yearlong and sometimes two years of control.

  • Use only products that are labeled for aquatic sites

    A good example is glyphosate, which is the active ingredient in Roundup. Roundup does not have an aquatic label. To use glyphosate around water for emergent plant control, you must purchase glyphosate formulations that are labeled for use in or around water. In most cases, you should purchase a wetting agent separately and add it to the formulation (Rodeo, AquaNeat, AquaPro, Eagre and many others) to ensure penetration into the vegetation. However, one formulation (GlyphoMate 41) does include a wetting agent in the container.

Controlling aquatic vegetation doesn't have to be difficult. As long as you follow these control methods, your body of water will remain healthy and aesthetically pleasing to your clients.

Carole Lembi is professor of botany at Purdue University (West Lafayette, Ind.).

Want to use this article? Click here for options!
© 2020 Penton Media Inc.

Interactive Products

Equipment Blue Book

Used Equipment Valuation Guide

Riding mowers, lawn tractors, snow throwers, golf carts


Grounds Maintenance Jobs

search our jobs database, upload your resume