|MadSci Network: Botany|
Adding barley straw to ponds, reservoirs and lakes to control algae has been known for hundreds of years but why it works has only been discovered in the last decade. Chemicals toxic to algae and cyanobacteria are released from rotting barley straw, many of these chemicals are still unknown but seem to consist of various organic acids and short chain fatty acids as well as phenols and methoxyphenols. Known potential algal growth inhibitors and general phytotoxins such as p-cresol, phenol, biphenyl-2-ol and babanoic acid have all been found near rotting barley straw in water (see Everall & Lees 1996 and 1997 for details).
I have not found any scientific information on how lavender stalks control algae but I'd guess that they work in a similar way, releasing toxic chemicals into the water. The presence of oxygen and light also seem to be important as they can help transform chemicals from the barley into more toxic compounds such as hydrogen peroxide and super oxide radicals.
Another benefit of adding barley straw to water bodies is that it provides habitat for invertebrates which graze on the algae and in turn it provides habitat and food (in the form of the invertebrate animals) for ducks. There appears to be no negative impact on the water quality of adding barley straw from the published studies. The chemicals released appear only to affect plants.
I hope this answers your question. Best wishes,
Everall, NC & Lees, DR (1996) Water Research 30(2) p269-276
Everall, NC & Lees, DR (1997) Water Research 31(3) p614-620
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