The process is not well understood, but it is thought that as the straw decays in the presence of oxygen (and possibly sunlight), chemicals are released or produced that inhibit algae growth. It does not directly kill existing algae, but as old algae dies, it’s consumed or otherwise removed, it is not quickly replaced by new growth.
How much should be used?
If used proactively before the pond is overrun with algae, a common recommendation is 10 grams per square meter of pond surface area. In ponds with a history of heavy algae growth, two or three times that amount may be required at first, and even ten times that concentration could be used.
Can it be overdone?
The straw is not known to be directly hazardous, but anything that decays in water in large quantities will reduce dissolved oxygen levels. This is not a likely problem unless the barley is massively overdosed (more than ten times normal) and the pond is already oxygen-limited by overstocking fish, medication or the decomposition of other organic materials such as leaves.
How is straw applied?
Barley straw should be used unbundled or packed loosely in mesh or in a wire cage so that water flows freely through the straw. It should be placed where moving water will oxygenate it and carry its byproducts to the rest of the pond. It’s best to anchor it by weight and line, and then to add some sort or float to keep if from sinking to less oxygenated water as it becomes waterlogged.
How long before improvement can be seen?
This depends on several factors, including temperature, quantity of straw, and how well it was applied, and the types of algae. Once the water temperature is in the 60’s (Fahrenheit), the straw should begin fermenting within three to eight weeks, and its effect should become noticeable.
How often should it be added?
The first dose is best applied early in the spring, before algae has a chance to get a foothold. A second dose can be applied when the first straw appears to be mostly biodegraded, or when algae appears to be returning-usually in midsummer. Here in the northern half of the U.S., it is rarely necessary to dose more than twice
What reduces barley straw’s effectiveness?
Since aerated (moving) water and sunlight are required for the proper decomposition to occur, these must be given adequate attention. In particular, a heavy growth of algae or “blanket week” needs to be removed to allow water circulation and sunlight penetration. The beneficial chemicals are also rapidly absorbed or inactivated by the presence of mud and other organic debris.
Is straw harmful to fish, invertebrates or desired plants?
There are noreport of harm to either fish or invertebrates, other than the overdose/deoxygenation precautions mentioned above. Higher plants appear to also be unaffected, in fact they may well benefit from not having to compete with algae for nutrients.