The War Against Pond Algae

Arm yourself with the knowledge and skills to keep algae out of your farm pond.

by John Morgan
PHOTO: Chesapeake Bay Program/Flickr

A farm without a pond is like a shoe without laces: functional but incomplete. Ponds can be critical for watering livestock, supporting irrigation, offering recreation and painting the picturesque scene that is the farm. Just like every aspect of the hobby farm, ponds need thoughtful planning and maintenance to maximize their value and function.

Aquatic vegetation can be beneficial to a pond in moderation, particularly for fish and wildlife habitat, but when left unmanaged, water-loving plants wage war, robbing dissolved oxygen through decomposition and shading fish, which can result in kills. Shorelines become impenetrable walls, making the pond inaccessible, and plant transpiration can minimize water-storage function. Pea soup-colored water? Well, it’s just plain ugly.

Get Your Pond Defense On

The green armies invade in many forms. Vascular plants, including submergent (e.g., pondweed, coontail, milfoil), emergent (e.g., cattail, phragmites) and floating (e.g., duckweed, water hyacinth) types, are the first that come to mind. Nonvascular plants include algae, such as microscopic phytoplankton and filamentous algae. Despite the myriad plant invaders, your defenses are equally as broad when deployed with the proper intelligence.

For starters, ponds can be designed to create a home-field advantage. Water depth along the shoreline is the critical first line of defense when building or renovating a farm pond. A depth of 2 to 3 feet of water at the pond edge hampers light penetration to the bottom. Plants struggle without their photosynthetic engine firing on all cylinders.

Maintaining shoreline depth over time is a must. Avoid allowing livestock direct access to your pond; instead use pipeline and tanks to water animals away from the fenced pond. Trampling will cause banks to collapse and set the stage for a green invasion. The infestation might not be limited to the bank either. Animal waste will enrich nitrogen levels, promoting algae blooms across the pond. Maintaining an upland vegetative buffer around your ponds will also help keep excessive nutrients away. Finally, keep a watchful eye on sedimentation over time. Shallow shorelines equal problem shorelines.

A savvy pond designer will install a water-control structure supporting a winter drawdown. By exposing 5 to 10 feet of bottom around the pond, Ol’ Man Winter can freeze and kill unwanted aquatic vegetation. It’s wise to sow a cover crop to prevent erosion when using a drawdown—winter wheat is an inexpensive and easy choice.

Subscribe now

Despite good intentions, it’s not unusual to inherit a problem pond or have one poorly designed for controlling aquatic vegetation. If you’re looking to save money, manual labor is a viable option. Catching plants early in the growing season will minimize plant material and density. Hand pulling, raking and cutting can be an early spring project. Get as many plant fragments as possible because many species can reproduce as cuttings. Toss the aquatic plant material on your compost pile—aquatic plants are great sources of nitrogen and phosphorus for the garden.

Another fairly inexpensive technique is sabotaging the photosynthetic process by blocking sunlight. Instead of using water depth, stop at the hardware store and purchase heavy black plastic. The plastic can float on the water, but staking it to the bottom guarantees success without having to reposition plastic moved by wind. Puncture holes throughout your sun shield so gas bubbles don’t force it back to the surface. After a month or so, green plants are reduced to brown mush. Dyes that color the water can also be purchased to block the sun’s rays.

Natural Pond Management

Thoughtful land stewards always seek natural techniques to accomplish pond-management objectives. Ponds are dynamic systems with relationships between living and non-living things. Animal waste and dead vegetation are consumed by bacteria requiring oxygen, plants oxygenate water and consume nutrients, and fish, especially fry, eat plants and use oxygen. Managing the balance of plants and fish can generate desirable plant communities.

The first approach is pitting plant against plant. Undesirable vascular plants can be controlled by managing a phytoplankton bloom in late spring and early summer. Deploy a water-soluble, liquid fertilizer at a rate of 1 pound nitrogen, 2 pounds phosphorous and 1 pound potash per acre when water temperature reaches 60 degrees F in the spring. The goal of your managed bloom is the development of a phytoplankton layer between 18 and 24 inches of water depth. The algal layer inhibits light penetration in the water column stifling other plants.

To assess your bloom, build a transparency stick. Paint a coffee can lid black on one half and white on the other, and attach it to the end of a yardstick. Two weeks after your first fertilization, vertically submerge your yardstick until the white and black colors are indistinguishable, noting the depth on the yardstick. If the colors are distinguishable beyond 24 inches deep, then fertilize again. If it is less than 24 inches, wait one week and check it again. Continue this weekly program through the first half of the growing season. (Mid-summer fertilization fosters undesirable algae and warmer water could lead to dissolved oxygen problems for fish.) Not only will this method shade out unwanted vascular pond plants, but it will foster accelerated fish growth.

If green-colored water is not your cup of tea, another natural approach is pitting fish against plant. An Asian triploid (sterile) grass carp can “graze” your overabundant aquatic plants. State-agency fishery biologists recommend a stocking rate of five fish per acre for minor weed problems, 10 to 15 fish per acre for moderate infestations and more than 15 for severe green infestations. If largemouth bass are in the pond, it’s prudent to stock fish nearly 1 foot in length to avoid losing stock to hungry bass. Be patient with your grass-carp program as it might take a year or longer for carp to be large enough to adequately control vegetation. They also favor warmer water, so feeding increases when the temperature exceeds 60 degrees F.

As an added bonus, grass carp can be caught by fishing with worms. Landed carp should be quickly released back to the pond, as prompt release will minimize stress to the fish and maximize survival. Grass carp live long lives (15 years or more), and their effectiveness can wane as they age. Supplemental stockings might be needed if vegetation begins to expand after several years of good control.

Chemical Pond Management

When you want a quick fix and don’t mind spending some money, chemical control is an option. However, be sensitive to the pond’s purpose—herbicides in a pond supporting irrigation would be counter-productive, killing off your garden crops in the process. As with any farm chemicals, care should be taken with the exposure of chemical compounds when human food consumption is involved. It’s imperative for hobby farmers to read the labels of chemicals used for controlling vegetation on their farms. They will provide information on exposure to humans and animals. If fish are in the pond, be cautious about treating the entire pond. Excessive dead vegetation can cause a dramatic decline in dissolved oxygen, resulting in a fish kill.

Start by identifying the plant species you’re targeting, and work with your farm store to identify the best chemical. There are a number of products on the market, so some expertise is needed. Additionally, some chemicals require application by certified professionals. Any time a chemical is used, read the label on the product completely and stay within the identified parameters.

No matter the approach, a green invasion on the farm pond is best dealt with promptly. Allowing unwanted aquatic plants to establish and expand can turn a manageable problem into a war on all fronts. Consider biological controls to create long-lasting solutions and gain benefits of better environmental stewardship and healthy fish populations. The focal point of a pond should be the water, and it takes a little work to keep it that way.

This article originally ran in the November/December 2013 issue of Hobby Farms.

Leave a Reply

Your email address will not be published. Required fields are marked *