The Centre for Aquatic Plant Management (previously the Aquatic Weeds Research Unit) published an information sheet on the practical application of straw. They have been involved in the research on straw for about 15 years.

  A new method was developed by the Centre for Aquatic Plant Management for controlling algae. It uses barley straw in the water and has been tested in a large variety of conditions. They have documented a high rate of success with no side effects.

  Putting barley straw into water, starts it to rot. A chemical is released which inhibits the growth of algae. They say it can take about 6-8 weeks for straw to start working at temperatures below 50°f. 
  When the water is over 70°f the straw may start working in 1-2 weeks.

  The straw can function for approximately six months, afterwards gradually decreasing in its effectiveness.

Calculating the Amount of Straw

  The Center has found that the most important measurement in calculating the amount of straw is the surface area of the water. The volume of the water does not seam to have an affect on the efficiency of the straw. This may be because the majority of algae takes place on the surface of the water.

  For most instances approximately 1 ounce / 3ft2 to 3 1/2 ounces/3ft,2 should do the job of controlling algae. After the algae is under control you can reduce the amount of straw in the water and use it to prevent a recurring algae bloom.

How to Use the Straw In garden ponds.

  Only a couple of ounces of straw are needed. The straw can be put into a net bag, nylon stocking or tied into a bundle with string. This can be attached to a stone, or brick, to keep the straw in place and dropped into the pond. However, as the straw becomes water logged, it can sink to the bottom, and will not work as well as it does near the surface. 
  It is advisable to include some form of float in the net, 
i.e., corks, Styrofoam or small plastic bottles with tops.

  Some pond supply businesses may have small bales of the barley straw already in netting. Follow the directions supplied with the straw or just 
anchor and attach to a float as described above.

SUMMARY

  When algae occur in water from garden ponds to lakes and rivers, barley straw offers an environmentally acceptable way of controlling the algae.

  Apply it twice each year. (Spring and Fall)
The straw should be in a loose form so water can pass through it easily.
Straw should not go any deeper than 3 feet. Use floats to keep it up in the water.

  If the straw starts to smell then it is not working and should be removed.
This is caused by too much straw in too little water.