Barley straw has been used to combat algae in ponds, lakes and waterways. The exact mechanism is poorly understood but it seems that barley straw, when exposed to sunlight and oxygen, produces a chemical that inhibits algae growth, without reducing the growth of other aquatic plants or affecting the fish. Although it will not kill existing algae, it will inhibit new growth.
Barley straw is most effective when applied early in the year prior to the appearance of algae (autumn through early spring). When applied to cold water (less than 10°C), it may take 6 - 8 weeks for the straw to begin working. If the straw is applied to warmer water (above 21°C), it may become effective in as little as 1 - 2 weeks. It will slowly decompose and remain effective for approximately 6 months after application.
The most common amount is 2 - 3 bales per surface acre of pond. The depth of water in the pond is not important. Since most of us have ponds a lot smaller than that, you can see only a little is required for the average backyard pond. A pond 3,000 - 4,500 litres should only need about 230g of barley straw. Less than 3,000 litres use around 120g.
In ponds that are frequently muddy, or those that have a history of heavy algae growth, two or three times this recommended dose may be required for the initial treatment. However, overdosing the pond with barley straw may kill the fish because straw deoxygenates the water as it decays. This is especially a problem if the pond is overdosed with straw during a prolonged warm spell.
To use, tie a string to each bale (or put smaller amounts into mesh bags or netting) and attach a float at the other end (it sinks as it decomposes) and place it in your pond with the straw partially out of the water. It needs continuous exposure to both water and oxygen. You can also add it to the top of an waterfall unit or anywhere else there is good water circulation.
Those living in warm temperatures year round, can add another "dose" of straw after about 5 months and remove the old ones about 2 months after that, thus overlapping doses. This will ensure a continual treatment of organic algaecide from your barley straw and you won't have to wait before the new bales become active. Those who live in colder areas should put the barley straw in the pond once the water temperature is above 10ºC. Make sure that the barley straw you get has been organically grown and free of harmful chemicals, pesticides, insecticides, etc.
Another point to make sure of is that you get barley straw and not hay, wheat, or other straws. Barley straw is the most effective of the straws and hay will actually increase algae growth and deoxygenate the water as it decomposes rapidly.