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Combating algae with barley straw

Sometimes algae, such as blanket weed (filamentous, or 'string', algae), can take hold in garden ponds to such an extent, that sitting back and letting nature take its course might not be the best option: nature's course may well result in a wet hole permanently dominated by algae.

Why does algae take hold?

If you have a problem with algae in your pond, the root cause will almost certainly be a surfeit of nutrients in the waterWhen blanket weed and other algae 'bloom' in a pond, they do so because the conditions suit them very well. What are those conditions? Well there's light of course but, more significantly a plentiful, perhaps unnaturally high, supply of nutrients. If you have a problem with algae in your pond, the root cause will almost certainly be a surfeit of nutrients in the water. Unfortunately in the garden environment, there are many ways that nutrients can get into your pond:

This list was derived from that found in Problem Pond Plants.

Combating algae

We must be careful not to label all algae as 'bad'. In fact, algae are a vital component of the ecosystem in any pond: snails, tadpoles and whole host of other animals depend on it for their survival. Algae perform a useful oxygenating role within the pond. Algae is only a problem when it proliferates to such an extent that it is detrimental to other plants and animalsAlgae is only a problem when it proliferates to such an extent that it is detrimental to other plants and animals (and offensive to your eyes). Clearly, the long term solution to combating excess algae in the pond is to keep nutrient levels down (see Living with duckweed), but given that this is always likely to be difficult in a garden pond, other techniques may also have their place.

You could try to control algae in your pond with algaecides, but in accordance with wildlife and organic gardening principles, you should have a healthy scepticism of fighting these sorts of problems by adding more chemicals. This feature is about a low-tech method for controlling algae: barley straw. The story goes that after leaving a bale of barley straw in a pond where it had fallen, a farmer noticed much less filamentous algae in the water than usualafter leaving a bale of barley straw in a pond where it had fallen, a farmer noticed much less filamentous algae in the water than usual. This information found its way to the Centre of Aquatic Plant Management, and there studies were undertaken which confirmed that barley straw can inhibit the growth of many types of algae.

Exactly how barley straw works is not fully understood (indeed other research has not even found the same effects; see Algae control with barley straw). However, it is thought that as the barley straw begins to decompose in the water, materials from the cell walls are released into the water and oxidise into humic acid. Combined with sunlight, the humic acid may cause low levels of hydrogen peroxide to form and this appears to interfere with the growth of the algae cellsCombined with sunlight, the humic acid may cause low levels of hydrogen peroxide to form and this appears to interfere with the growth of the algae cells. The low levels of hydrogen peroxide (which is also very short lived) do not appear to have any deleterious affects on higher plants or animals of any sort. Indeed other plants and invertebrates have been observed to do very well after barely straw is added to the water.

How to use barley straw

Commercially produced barley straw bales can be obtained from most large garden or pond centres. The bales I bought (illustrated below) weigh 225 grams each and a single bale of this size is supposed to treat a 1000 galon (4550 litre) pond for six months. (This is, coincidentally, about the size of my pond, so a pack of two bales should last me about a year.)

It is recommended that bales are changed in early Spring, before the growing season begins, and in AutumnIt is recommended that bales are changed in early Spring, before the growing season begins, and in Autumn. For maximum effectiveness, the bale should be placed in the most oxygenated water, e.g. near a fountain, but in a pond without such a water feature (like mine), all you can do is make sure that the bale stays close to the relatively well oxygenated surface of the water. When you change bales, you should leave the old one in the water next to the new for about a month or so. This helps 'activate' the decay of the new bale by allowing the decomposing fauna/flora from the old bale to migrate to the new. If the bale begins to smell, you may have too great a dose for the size of your pond - remove some and see if it settles down.

Note that barley straw has been most effective in inhibiting the growth of new algae rather than reducing the amount of algae that's already there, so if you have a bad infection, you should remove as much algae as you can mechanically (i.e. by hand) before applying the barley straw.

The life cycle of the barley straw bale

These pictures illustrate the visible stages in the life cycle of a barley straw bale added to my pond. Of course all the real action goes on under the pond surface.

This is a barley straw bale commercially produced for the garden pond owner. I bought this as one of a pair from a local garden centre.

The bale is placed into a fine-meshed bag which is supplied with with the straw and then the binding string is cut to release the straw into the bag.

Here the bale is placed into position in the pond and tethered to the side with some string. Initially the bale floats, but after about a week, it sinks under the surface (like the old bale which you can just about make out to the left). Note that the green you can see in this picture is duckweed, not algae.

Here's the previous bale removed after about a month of being in the pond next to the new bale. I've split it open to allow any creatures to escape and make their way back into the pond (it should be left like this for a day or so before being added to the compost heap).

The story in my pond

Within a few weeks of creating my pond, there was a pronounced algal bloom - all the water in the pond became very green. This was almost certainly due to the high levels of nutrients in the tap water I initially used to fill the pond. After a few months, I also noted very strong growth of blanket weed in the shallow 'beach' area of the pond. I bought a pack of two barley straw bales and introduced one into the pond (near the beach end) in the autumn.

Over the course of the winter I did not notice any change in the amount of blanket weed, though it did not appear to get any worse. In actual fact, this fits with what is known about barley straw; it is more effective as an inhibitor rather than a cure for a bad infectionit is more effective as an inhibitor rather than a cure for a bad infection. However a strange thing happened in later winter/very early spring. One day I noticed that the water in the pond appeared to have cleared a little. Then I started to notice that the blanket weed was receding. It did so at such a dramatic rate that I could the difference from one day to the next. About two weeks after initially noticing the decline, the blanket weed had all but disappearedAbout two weeks after initially noticing the decline, the blanket weed had all but disappeared.

It seems clear to me that something had happened to the water chemistry in my pond to affect this decline; but was the barley straw responsible? There are a couple of things which make it hard for me to conclude that the barley straw was definitely responsible. Firstly, the decline occurred before the weather had really warmed up and the growing season kicked off. Secondly the change was so sudden and dramatic that it didn't point obviously to the barley straw which had been there for months. I wondered to myself 'did anything else change in the pond before the onset of the decline?' and I realised that something hadI wondered to myself 'did anything else change in the pond before the onset of the decline?' and I realised that something had. Just before the blanket weed disappeared I had introduced a log into the pond which had been submerged in my old pond. But this was just a small log (under a metre long and around 5cm in diameter) and I could not believe that it had anything to do with the decline of the blanket weed.

Recently however, I read the following interesting statement in the publication Problem Pond Plants; 'Interestingly, some new information shows that rotting tree bark may be as good as barley straw at getting rid of algae (tannins from bark may be the controlling chemical) - a good reason for leaving fallen branches in the water. Willow is particularly effective.' When I read this I immediately recalled the 'coincidence' of the decline in my blanket weed and the introduction of the log. Interestingly though, the log was completely bark-less! There is a strong possibility that it might be willow (I cut down an old willow tree a few years back and kept the wood), though it could equally well be beech or cotoneaster.

So what did for the blanket weed: was it the barley straw, the log or something else completely? The truth is, I don't knowSo what did for the blanket weed: was it the barley straw, the log or something else completely? The truth is, I don't know. Barley straw and logs/bark are both cheap techniques to try and in a wildlife pond, even if they turn out to be ineffective against blanket weed, they will certainly enrich the habitat for wildlife. So if you are in need, go ahead and try; what have you got to lose?

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