Pond life
A neighbour advised me to put a bale of barley straw into my pond to prevent weed growing. It worked, but how?
鈥� A brief explanation of the effect of barley straw by Irene Ridge was published in British Wildlife in 1992 (vol 3, p 392). There she discounts the suggestion that the effect was caused by nutrient depletion which is similar to the 鈥渘itrogen robbery鈥� that can happen when organic materials with a high ratio of carbon to nitrogen are incorporated into soils. If this were the case then the effect should be similar for other cereal straws, and this is not so. Ridge explains that when barley straw decomposes in the presence of oxygen it releases an algal inhibitor. Various studies to determine the active agents in barley have identified anti-algal activity in phenols, oxidised phenolics, quinones and esters.
Barley straw isn鈥檛 just useful in ponds. It can also get rid of cyanobacteria, diatoms and unicellular green algae in drinking water and in other still waters such as canals.
Advertisement
The questioner used a bale of barley straw: this quantity would only be suitable for a large pond, and loosely packed straw works best. Algal blooms in an average garden pond would only need, say, a small vegetable net of loosely packed barley straw. To make sure the straw has enough oxygen, the net should be kept near the surface or near an inflow or fountain. The effect takes some time to develop 鈥� up to 3 months depending on temperature 鈥� so barley straw is best used early in the season to stop the algae growing in the first place.
It does not seem to have any adverse effects on invertebrates or fish; indeed detritivores such as Asellus and Gammarus freely colonise the straw. Although barley straw is more effective than other straws, Ridge says that brown-rotted wood, for example, also works very well.
Richard Small
Biological and Earth Sciences Department,
Liverpool John Moores University, UK
鈥� Barley straw is cheap, environmentally friendly and can be used in garden ponds, streams and lakes. It works in nearly all cases. Long Ashton Research Station, near Bristol, UK, which specialises in agricultural research, has shown that under the right conditions, chemicals released by straw as it decomposes prevent the growth of new algae. All types of straw appear to work but barley straw is the most effective.
Interestingly, adding lavender straw to the barley increases its effectiveness, but it may need more oxygen to break down. It is therefore more appropriate for larger ponds or those with pumps, which help oxygenate the water.
Guy Barter
Head of Horticultural Advice at the Royal Horticultural Society
London, UK
鈥� Many readers wrote in to tell us that Rothamsted Research Centre for Aquatic Plant Management, UK, has issued a useful information sheet on the use of barley straw. You will find it at
The research centre explains that no one has yet fully investigated the way the straw works but that decomposition is thought to lead eventually to the production of hydrogen peroxide. The document states: 鈥淐oncentrations of hydrogen peroxide of only 2 parts per million peroxide have been demonstrated to inhibit the growth of algae. Experiments have shown that sustained low concentrations of hydrogen peroxide can have a very similar effect on algae to that of straw.鈥� 鈥� Ed
This week鈥檚 question
Abreast of the issue
On regular walks in a South Derbyshire opencast coal mine a few years ago, I frequently found stones that looked remarkably like human breasts, even down to their colouring (see photo). These were in stony layers usually a couple of inches thick, like a cobbled surface. The stones varied in size from a couple of centimetres to about half a metre in diameter. I鈥檝e used them as paperweights for some time 鈥� and naturally they attract comments. But what are they, and how did they form?
David Jackson
Swadlincote, Derbyshire, UK