Wednesday 15 December 2010

Dig or no-dig?

I’ve long been seduced by the idea of no-dig vegetable gardening. I like the idea of recycling cardboard and newspaper, I like the idea of letting nature get on with managing the soil ecosystem itself, and most of all I like the idea of not having to dig over beds at this time of the year.

Around four years ago, I began gradually introducing no-dig cultivation on some of the beds at the allotment. I started with one area of about 8sqm which had been cleared but never cultivated, which I planned to use for strawberries – an obvious crop to try this with as you won't be deep digging around strawberry plants each year anyway.

No-dig gardening practice starts with digging. Not everyone will tell you that. At least, it starts with clearing the ground, and the best way to do that is to dig out the existing vegetation. The Garden Organic booklet for Schools, for instance, says “Dig over the ground as usual taking out as many perennial weeds as possible or cut down the vegetation and then cover with a light excluding mulch which will kill the weeds”

Now this is fine if your perennial weed population consists of a few fat hens, a dandelion and a spot of couch grass at the edges, but when you are taking over a 10 pole plot completely overgrown with brambles, ash saplings, nettles and bindweed (and horsetail, as I later discovered), then strimming followed by digging is going to be required.

Once the site is clear of weeds, lay down cardboard, in a single layer, but leaving no gaps, over the whole of the bed. Tesco, Homebase and B&Q are your friends here, as you can plunder their old cardboard boxes to give you enough for this. I then like to paper over the cracks – literally – spreading newspaper sheets over the cardboard to ensure that all the joins are covered. I have found out the hard way that you need to do this on a day when there is no wind.

The next layer – you can see why this is sometimes called lasagne gardening – is the top mulch. Pretty much any organic matter goes here: compost, manure, leaf mould, composted bark, grass cuttings, straw, wood chip, or a mixture of all or some of them. With wood chip, be aware that until it has fully composted it will deplete the nitrogen in your soil rather than adding to it. If you just spread a layer over the top, it won’t affect the nitrogen content further down and will still do all the good weed suppressant, moisture conservation work. 

After that, I consider the bed finished. I know that a top dressing of straw and a thorough soaking to prevent the mulch being blown away are also recommended. The final straw layer is useful in that it does seem to deter cats and foxes from digging up your carefully layered mulch and cardboard. The wetting part of it I leave to the rain.

The bed is then left until planting time. You can plant through the cardboard, by cutting a small hole in the cardboard and inserting the seedling in the soil beneath. On onion beds in particular, I have made a very deep top layer of mulch and simply planted the alliums in that.

In theory, you only ever hand-weed after that, and top up the surface mulch with a 5cm layer annually, while the soil gradually breaks down into dark crumbly richness. In practice, it isn’t quite like that.

FIrst of all, do not imagine – as I did – that ‘no dig’ is somehow synonymous with ‘no effort’. With an initial digging out to start the process, plus the layering stages, starting a no-dig system is considerably more laborious. It can also be quite a challenge to find enough OM for the mulch layer. If like me you have no vehicular access to your plot then everything has to be bagged up and carried in. However, it encourages you to think creatively about the OM that you use. Buying in enough bagged commercial compost would be insanely expensive, so you do start thinking about recycling compost, homemade compost, straw, cuttings, etc. And the wider the mix, the better it probably is for the soil.

But I seriously question how suitable the system is in its purest form if you have perennial weeds that spread horizontally: bindweed and brambles for instance. Even if you manage to dig out every scrap of perennial weed root from the bed (and that in itself is quite a tall order), they will invade again from the outside. With these, and horsetail, you will probably have to settle for managing them rather than eliminating them. And at some stage this will require further digging.

For the first year, however, it was lovely. The top mulch settled down and provided a fertile environment for crops. The card and newspapers did their light-excluding barrier job and there was very little weed regrowth. At the end of the season, the mulch and cardboard had rotted down into a lovely rich dark mix – although with all that OM piled on to it at the beginning of the year, I felt it damn well should have done.

In year two, the bindweed reappeared. Attempts to hand-weed it out demonstrated that while some roots had come in over the original cardboard layer and could therefore be easily peeled back to the surrounding bank, plenty had encroached from under the cardboard, and wouldn’t come out in one piece without snapping. Unless I set to with the fork and dug it out. Which is not how a no-dig bed is supposed to work.

In year three the returning bindweed was accompanied by nettles and couch grass. By year four I decided a complete re-dig would be necessary and found that underneath the top layer of mulch a complex, dense network of bindweed and couch grass root had established itself – certainly much more than could ever have been handweeded out. However, in contrast to my initial struggle to extricate roots from thick clay, it was relatively easy to lift them out – the soil really had improved that much in a comparatively short space of time. But it was disconcerting to find quite so many perennial weed roots lurking in the soil.

In a no-dig bed I started two years ago, I planted sweetcorn last summer, and when pulling up the corn plants this autumn I disturbed a number of chafer grubs – enough of them to make me get the spade out to dig the bed over again and see how many of the grubs we had. This does seem to me to be the strongest argument against the no-dig system: you can’t see what’s happening under the soil surface. Pests can build up very quickly without you knowing anything about it, as can an underground root system.

Four years on from my initial forays in no-digging, nearly all of my beds have now had the big dig/cardboard and mulch layering treatment, followed by a year or more of hand-weeding. The bed at the front corner of the plot where there is no surrounding bindweed, brambles or nettles, hasn’t been dug  out since 2008; the bed at the other end, which was created out of a nettle forest, has had two cardboard and mulch treatments in three years. 

I think this can best be described as a minimal dig system, and I'm happy to settle for that. I like not feeling obliged to dig out at the end of the season, but at the same time, bindweed/horsetail can't be eliminated by mulch alone, and I do like to know what's going on under the soil surface from time to time.