If you go into a garden center and look at the products sold for the maintenance of garden ponds, you may see a small bale of straw. This small bale is not to feed small cows, it's a bale of barley straw. Since the early 1990s, barley straw has been used to control the growth of algae in ponds, lakes and reservoirs. Studies have been contradictory as to the reasons why this happens. However, field evidence seems to indicate that using barley straw will reduce algae and increase water clarity over time.
Most theories as to why barley straw kills algae in bodies of water point to chemical reactions that stem from the decomposition of the barley straw. In the water, fungi decompose the submerged barley, which releases chemicals. These chemicals, once released in the water, prevent the growth of algae. However, the exact chemical that slows the growth of the algae is unknown. The two most likely candidates are oxidized polyphenolics and hydrogen peroxide. It is also not known if the chemicals come from the decomposing barley or the fungus. The process also does not kill existing algae but simply prevents new algae from growing.
The straw should be unbundled and packed loosely in mesh bags then placed so that water can flow freely through it. The ideal place for this is near a waterfall or underwater current so that as the straw decomposes the byproducts are carried away to the rest of the pond. The straw is anchored to keep it in place and tied to floats to keep it from sinking as it becomes waterlogged.
The amount of straw to apply is based on pond surface area. The general calculation is 0.025 pounds of straw per square yard of pond surface. A 1-acre pond may require 107 pounds of straw, which amounts to two or three standard bales. Ponds with high initial algae problems may require more straw initially, but less straw as the algae problem becomes controllable.
For best results, the straw should be placed in the pond around April, just before algae reproduction begins. Although straw may take four to five months to decompose, algae reproduction may continue throughout the summer. For this reason, old straw should be replaced with new in midsummer.
Use of barley straw can promote the growth of aquatic plants such as pondweed. Another disadvantage is that it's possible to "overdose" a pond with barley straw. Decaying vegetation like barley straw or algae removes oxygen, and if too much oxygen is removed, it can kill fish in the pond. The pond owner may wish to install an aerator to reduce this risk.