String algae is a nuisance to say the least! Adding barley extract, pellets, or straw is a great proactive approach to prevent heavy string algae outbreaks. Barley should be added in the spring when you open your pond or pondless waterfall. This will give it time to break down the barley to produce the enzyme that helps get rid of string algae. If you are using the straw or pellets you will want to check on it every two months - if it’s really broken down, it is time to add a new bag of barley straw or pellets. Leave the old bag of barley in there for about two weeks after adding the new bag. After two weeks have passed the old bag can be removed.
Now what in the barley actually prevents the growth of algae?? The technique of using barley for algae control was developed in the early 1990s in England, where it is widely used in many bodies of water, including large reservoirs and canals. In general, it is thought that fungi decompose the barley in water, which causes a chemical to be released that prevents the growth of the algae. The specific chemical(s) has not been identified (oxidized polyphenolics and hydrogen peroxide are two decomposition products that have been suggested), but it is not clear whether the chemical is exuded from the barley itself or if it is a metabolic product produced by the fungi. The property of barley straw is usually described as being algistatic (prevents new growth of algae) rather than algicidal (kills already existing algae).