How does it work?Barley straw does not kill existing algae, but it inhibits the new growth of algae. The exact mechanism is poorly understood, but it seems that barley straw, when exposed to sunlight and in the presence of oxygen, produces a chemical that inhibits algae growth.Barley straw does not reduce the growth of other aquatic plants. In fact, in some cases aquatic plant growth has increased after barley straw applications because algae are no longer present to compete with the aquatic plants.When should it be applied?Barley straw is most effective when applied early in the year prior to the appearance of algae (fall through early spring). When applied to cold water (less than 50¡F), it may take six to eight weeks for the straw to begin producing the active chemicals that inhibit algae growth.If the straw is applied to warmer water (above 70¡F), it may become effective in as little as one to two weeks. In any case, barley straw remains effective for approximately six months after application.