Putting ash on the garden - should you? Shouldn't you?

For some reason, I thought I'd already written about this topic, but I received a couple of questions last week regarding using ash on the garden, and when I searched my archive - oops! Lots of articles involving Ash trees, but not a lot about using ash in the garden.

Apart from this one, which starts off with the proper disposal of  Hellebore leaves ("Burn them! Burn them!") and then segues into using the resultant ash in a sensible way.

Why is there a sudden interest in this topic? I'm guessing it's for a very simple and obvious reason: with the current increases in fuel prices, more people are turning to open fires and wood burning stoves, and therefore there is an upsurge in interest in how to get rid of the waste product, ie the ash, and can it be used as fertiliser, on the soil. 

They're asking when, where, how - all the usual questions!

So let's start with a quick look at the various types of ash -  woodburner ash, bonfire ash, and coal fire ash.

Dealing with the last one first: ash from coal fires is no use to the gardener at all. Whether it's old-fashioned coal (do they still sell it?), coke, briquettes, smokeless fuel: anything that's black and burns well, is not good for the garden. It contains a whole heap of nasty stuff, which can include iron, lead, zinc, aluminium, cadmium, even arsenic!

So don't spread that on your garden.

 

Ash from fires which have only burned wood is better: but with both wood-burning stoves, and open fires, be aware that the sort of wood you burn is what you might call "big chunks" of wood, split trunks of trees, so it contains a lot of heartwood. 

This is not much use for the garden: the heartwood is the "dead" part of the tree, so it contains very little in the way of nutrients.

Tree ash is also somewhat alkaline, so applying it to your garden will change the pH of the soil: not by much, but it might be enough to make a difference to how your plants grow, if they are finicky about the pH they require to flourish. 

In addition, too much ash in any one place - for instance, if you pile it up in one corner of the garden, until you are ready to use it - will effectively sterilise the soil. 

Rainwater will also wash any nutrients out of the ash, which has a double effect - not only will the ash then be useless as fertiliser, by the time you come to apply it, but those nutrients will have washed down into the soil, which might threaten surface water quality elsewhere, if you are near to streams or springs.

And, of course, you can only use the ash from your woodburner, or open fire, if you absolutely DO NOT also burn coal/coke etc on it.

Bonfire ash has slightly more value as it is mostly brash, ie smaller branches, twiggy stuff, etc, which contains some potassium (hence pot-ash) and it is not quite as alkaline as indoor wood fire ash.

So a little of this, on the garden, can be beneficial,  but applying too much ash to the plants can be detrimental: excessive amounts of ash can lead to nutrient toxicity and/or nutrient deficiency issues in plants.

And how much, I hear you ask, is "too much"?

On the rare occasions where I've applied it, I've taken up a fistful at a time, and scattered it quite thinly. But I have seen other people throw on great spadefuls of the stuff, which I felt was wrong, not least because a thick layer of ash can form a pan, or crust, on top of the soil. This is bad!

Bonfire ash tends to be comprised of "large" particles - it feels gritty, due to the mixed origins of the bonfire heap, which contains not just branches but perennial weeds, soil, roots, goodness knows what: so it's fairly easy to spread it thinly.

Woodburner ash, however, is soft and powdery, and tends to turn to slime, when it gets wet. Another good reason for not applying woodburner or open fire ash, to your garden!

So, getting back to "how much is too much?", I did a bit of research, and it's surprising just how little ash can safely be applied to a garden. 

I found a University site, whose horticulture department had done some studies on using ash in the garden, and they said "Applications of wood ash are generally limited to a maximum of 15 to 20 pounds (approximately a five gallon pail) per 1000 sq. ft., per year."

1000 sq ft  equates to about 90 sq m, which is an area about 9 metres by 10 meters. It was surprisingly hard to work out what that means in real terms, but a handy website offered these suggestions:

- About one third the size of a tennis court.

- Six times as big as a parking space (apparently normally 15 sq meters, although I'd debate that, looking at the way my local supermarket mark out their parking spaces, and how often cars get dinged and dented there...)

- About 85 times the size of a bath towel (!).

For comparison, a standard allotment is about 250 square meters: that's the original full size allotment, not the cut-down towny versions. So this 90 square meters area is, say, a third of an allotment.

So if you can now visualise any or all of those images, that's the area which can accept ONE BUCKET of wood ash, in ONE YEAR.

Not a lot. You can see that there's not much point saving up your bonfire ash to use on the garden!

Two other points, before we leave this topic: don't use the ash from your bonfire if you burn cardboard on it: most cardboard, especially the corrugated stuff, contains things like Boron and other chemicals in the glues, which are not good for your garden. Oh, except for Mal, and his desperate attempts to encourage his under-performing cauliflowers... (*laughs*) but generally speaking, the fewer chemicals you put on your garden, the better.

And finally, bear in mind that when wood ash combines with water it forms lye. Wood-ash lye is debatably a little less caustic than the commercial lye that is used in dyeing, and in drain and oven cleaners.... so if you leave a pile of wood ash out in the rain, you are likely to end up with a puddle of caustic lye underneath it, which might account for the whole "it will effectively sterilise the soil" thing...

..and this is definitely not something which you would want in your garden!

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