How it works.
Barley straw does not kill existing algae, but it inhibits the new growth of algae. The exact mechanism is bad understood, but it seems that straw barley, when they are exposed to
the light of the Sun and in the presence of oxygen, produce a chemical which inhibits the growth of algae.
Barley straw does not reduce the growth of other aquatic organisms plants. In fact, in some cases, the growth of aquatic plants has increased after the applications of barley straw, because algae are longer there to compete with aquatic plants.
The use of barley straw as an algae control method began in England during the early 1990s. In addition to ponds I England, barley straw is used in large reservoirs and canals. Laboratory research conducted in England suggests the presence of rotting barley straw in water inhibits the growth of several planktonic and filamentous algae. However, limited research studies in the U.S. have had mixed results. In general, water clarity will improve in ponds treated with barley straw due to a reduction in planktonic algae growth, but thus far evidence suggests no affect on filamentous algae.
The exact mechanism in which barley straw inhibits algae growth is not known. It is thought that as fungi decompose the straw in water a chemical(s) is released that prevents the growth of algae. Two suggested chemicals are oxidized polyphenolics and hydrogen peroxide, but it is not clear if these chemicals come from the barley straw itself or are metabolic byproducts produced by the fungi. Either way, barley straws inhibits algae growth; it does not kill already existing algae.