Answer: There are several methods and products that come to mind. These options are summarized below, along with pest management considerations with respect to certified organic production.
First, a background note. If you are certified organic, it is rather important to verify the approved, restricted, or prohibited status of pest control products for use in organic production.
The Organic Materials Review Institute (OMRI) publishes a prominent listing of approved crop production inputs geared to certified organic production.
The OMRI Products List—a Directory of OMRI Listed® Products
Crop Products Sorted by:
Crop Supplier Contact Information
2.1 Barley Straw
One of the "natural" control methods that comes to mind is barley straw technology, and for greenhouse applications, barley extract.
Barley straw has gained considerable attention among farmers as a control of algae in farm ponds. Some of the initial research and information on this technology was developed at the Centre for Aquatic Plant Management in the UK, which is part of a national institute known as Centre for Ecology and Hydrology.
The following excerpts from the Centre's information sheet on barley straw explain the biological and chemical mechanisms that control algae as barley straw undergoes decomposition.
Information Sheet 1: Control of Algae with Barley Straw
Centre for Aquatic Plant Management, Centre for Ecology and Hydrology (UK)
The details of the exact mechanism by which straw controls algae has not been fully investigated, but it is generally accepted that the process may occur as set out below. Only a few of the compounds released from straw have been identified, the majority are known to be non-toxic while some may have slight toxic effects at the concentrations detected. It is likely that a combination of factors results in the anti-algal activity generated from decomposing straw.
When straw is first placed in water, the soluble components of the straw are washed out, causing water to turn a brown colour. These compounds have not been identified, but they are likely to be a mixture of carbohydrates and hemicelluloses. Bacteria are the most dominant micro-organism at this stage.
After about two weeks the dominant micro-flora change to fungi. This is when decomposition of lignin and other cell wall components starts to occur.
When straw rots, the cell wall components decompose at different rates. Lignin is very persistent and is likely to remain and be released into the water as the other components decay. Decomposition of lignin leads to the production of a form of soluble lignin and other decomposition products. These decomposition products are likely to be transformed by bacterial and fungal enzyme activity before being released in to the surrounding water. This mixture of compounds is transformed into fulvic and humic acids.
These humic substances are more easily referred to as Dissolved Organic Carbon or DOC. DOC is a natural component of many freshwater and marine ecosystems. When light shines onto water which contains humic substances, in the presence of dissolved oxygen, hydrogen peroxide is eventually formed.
High molecular weight DOC absorbs sunlight energy and can pass this energy to dissolved oxygen molecules. The dissolved oxygen becomes unstable and decomposes into two singlet oxygen radicals. These are very short lived, of the order of 1 micro-second, but extremely reactive molecules. The singlet oxygen radicals form superoxide radicals and these from hydrogen peroxide in water. The hydrogen peroxide is slightly more stable and persists for approximately 2 days in freshwater. The presence of a continuous supply of the right form of DOC creates conditions whereby hydrogen peroxide and the other oxidising agents can be continuously produced.
Concentration of hydrogen peroxide of only 2 ppm peroxide have been demonstrated to inhibit the growth of algae. Experiments have shown that sustained low concentrations of hydrogen peroxide can have a very similar effect on algae to that of straw.
There are various factors which affect the performance of straw and which support this hypothesis. It is important to take these factors into account to ensure successful treatment of algal problems with straw.
Also see table:
List of Chemicals Produced by Decomposing Straw
For further references and practical tips, see the following Web sites that address the use of barley straw for algae control In the United States:
Algae Control with Barley Straw
Ohio State University
Algae Control in Ponds With Barley Bales: On-site Results
Dow AgroSciences - January 29, 2001
Aquatic Plant Management: Barley Straw for Algae Control
Barley Straw for Algae Control - Phase II
Oregon State University
The two principal hypothesized modes of action relate to the two principal components of straw: lignin and cellulose.
* The breakdown products of lignin include a host of compounds (tannins, phenolics, and hydrogen peroxide) that are known to have algistatic properties.
* The digestion of complex carbohydrates to simpler sugars provides carbon —a nutritional substrate for competing microbes and/or algae predators.
2.2 Barley Bags
Owing to the interest and success of barley straw, the aquaculture and aquarium industries now carry bags of Barley Straw.
These are on various Web sites, such as the following, as well as others:
Aqua Art Pond Specialists
2.3 Barley Extract
Owing to the interest and success of barley straw as a natural control
method for algae in farm ponds, the aquaculture and aquarium industries
now offer several products known as Barley Extract.
These are on various web sites, such as the following as well as others:
Aqua Art Pond Specialists
3.1 Hydrogen Peroxide
Hydrogen peroxide is sometimes mentioned as a household remedy for
control of algae, and in fact, hydrogen peroxide is one of the breakdown
byproducts of barley straw decomposition.
The following web sites contain home gardener recipes and discussion
on hydrogen peroxide for algae control which you may find helpful.
Hydrogen Peroxide as Algae Treatment?
The Krib: Aquaria and Tropical Fish
Algae Control Using Ultrasonic Fogger
iVillage Gardenweb Forum: Hydroponics
Whereas EPA regulatory guidelines for pesticide use state that manufacturers cannot make pest control claims unless the product is registered for that use, the employment of these general products by farmers themselves is not. The use of hydrogen peroxide (and copper and lime methods, see below) for algae control is one such example.
3.2 BiosafeSystems Products
BioSafe Systems in Glastonbury, Connecticut, carries a couple of commercial biorational products that control algae, based on peroxide chemistry. However, they are not approved for certified organic production to my knowledge. I am listing these items for your information.
P.O. Box 936
36 Commerce Street
Glastonbury, CT 06033
TerraCyte is a uniquely formulated granular algaecide/fungicide that will control moss, liverwort, algae and slimes on potted crops.
TerraCyte's mode of action is oxidation, a powerful chemical reaction that reacts with proteins and enzymes found in cell membranes of microbial organisms. It can be applied on benches and planters for ornamental plants grown in greenhouses, outdoor nurseries and landscapes.
ZeroTol is a biorational broad-spectrum algaecide/fungicide that was formulated to help growers achieve total disease control in all phases of their operations.
ZeroTol is labeled for use in greenhouses, nurseries, garden centers and interiorscapes. ZeroTol may be used as a fungicide on bedding plants, flowering plants, roses, poinsettia, ornamentals, nursery stock, trees, cut flowers, bulbs, cuttings, seedlings and seeds.
It can also be used as a fungicide and algaecide on: greenhouse structures, benches, pots, watering systems, evaporative coolers, storage rooms, ventilation equipment, floors and other equipment.
3.3 Sodium Carbonate Peroxyhydrate
For background information on the sodium carbonate peroxyhydrate
products that are used in some of BioSafe Systems products, see
the following EPA fact sheet.
Sodium Carbonate Peroxyhydrate (128860) Fact Sheet
EPA Fact Sheet
Sodium Carbonate Peroxyhydrate is a granular chemical which is the active ingredient in certain algaecide and fungicide products. The end product containing this active ingredient acts as an oxidizing agent and thus kills the target algal and fungal pests.
The product is used outdoors for treating ornamental plants, turf grasses, and terrestrial landscapes. It is used, as well, for treatments in commercial greenhouses, garden centers and plant nurseries, including their storage areas.
I. DESCRIPTION OF THE ACTIVE INGREDIENT
Sodium Carbonate Peroxyhydrate is a granular substance made by combining sodium carbonate and hydrogen peroxide. The following is its mode of action: When water is present, the compound breaks down into hydrogen peroxide and sodium carbonate. The hydrogen peroxide oxidizes and thus kills the target pests. After contact, the hydrogen peroxide breaks down harmlessly into water and oxygen.
4. Corn Meal
Howard Garrett, a well-known author of gardening books and host of a popular radio show on organic gardening in North Texas, provides the following notes of use of cornmeal for algae control on his Web site, The Dirt Doctor.
Howard Garrett, The Dirt Doctor
For floating paint-like and filamentous algae in water, use cornmeal at 5 pounds per 1,000 square feet or 150-200 pounds per surface acre.
The cellulose in the cornmeal helps to tie up the excess phosphorous
in water, balances the water chemistry and thus kills off the algae.
The organic carbon in the cornmeal enables the beneficial bacteria in the water to flourish at the expense of the algae. Then the decomposing algae provide a source of carbon for the bacteria. One or two treatments is usually enough to control the algae for several months.
Caution: any fast algae kill from any product can cause an oxygen depravation and result in fish kill.
Additional information can be obtained from the following publication:
Cornmeal – it’s not just hog feed anymore, The Peanut Farmer,
May 1996. Aquaculture Engineering (1990) 175-186.
5. Safer Moss & Algae Killer
Safer Moss & Algae Killer is a potassium based soap that kills moss,
algae, liverworts, and lichen. It is an OMRI listed organic control product.
Woodstream Corporation in Lititz, PA, is the parent corporation for
Safer® and Concern® series of natural and organic products marketed
to small farms and gardens.
Safer Moss and Algae Killer
Safer Moss & Algae Killer is also is available through various
suppliers such as Planet Natural.
Safer Moss & Algae Killer
6. Copper Fungicides
Copper is a naturally-occurring mineral that is widely used
as a fungicide and bactericide in agriculture. Various forms of
copper, especially copper sulfate, are commonly used to control
algae in farm ponds. Copper application is one of the most widely
used algal control methods in broadscale agriculture.
There are numerous fact sheets and bulletins from land-grant
colleges and universities, as well as government agricultural programs,
on the use of copper for control of algae in ponds, such as the
folllowing two items from Canada and Ohio.
Algae Control in Ponds
Alberta Agriculture, Food and Rural Development
Controlling Filamentous Algae in Ponds
Ohio State University
Farmers can mix their own copper solutions, or purchase copper-based
products. See "The OMRI Products List" for product listings and suppliers.
Lime has also been used to control algae in farm ponds. You can learn about this method by reading the following bulletin from British Columbia Ministry of Agriculture and Food, which also provides a recipe for calculating and mixing a lime slurry to spray onto surfaces.
Liming Farm Ponds for Algae Control
British Columbia Ministry of Agriculture and Food
However, I suggest that you first check with you local university to see if they know anything about the use of lime slurry for control of algae in greenhouses.
8. Sanitation and Environmental Modification
The following information sheet from the University of Connecticut IPM program offers tips on sanitation and environmental modification to prevent the growth of algae in greenhouses. It also lists commercial products that are availablefor the control of algae. Several (but not all) of these, such as the BioSafe Systems products, can be considered biorational control products. But please note that none of the products listed are OMRI approved to my knowledge, which is an important distinction if you are certified organic.
Managing Algae in the Greenhouse
University of Connecticut IPM
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