Using cardboard to control allotment weeds

John Walker
Sunday, 1st June 1997

Fresh from a trip to Australia and having seen the video 'In Grave Danger of Falling Food - The Permaculture Concept', John Walker found himself severely bitten by the 'Mollison bug'. He began turning a derelict Ipswich allotment into a productive, organic food garden in less than six months...

Watching 'In Grave Danger of Falling Food' had prompted me to think deeply about forests and natural vegetation systems in general, alongside all I'd ever been taught about digging and other forms of soil cultivation.

Being a professional horticulturist at heart, I was surprised at how easily, after a traditional education bestowing the virtues of agriculture/horticulture, I warmed to the many lessons permaculture began to teach me. It seemed to make perfect sense, natural vegetation systems don't dig then plant themselves, so why shouldn't our gardens and allotments mimick the successes of Nature? Of course there's bound to come a time when we might need to reach for the spade, but if the means justifies the end, then I'll meet Nature halfway! My whole approach to growing plants was, and still is changing. A revolution in my thinking had begun which is still going strong today.

The Big Cover Up

Once I'd chosen my plot (neglected but not too overgrown - I avoided any with 8ft high brambles), the great cover-up began. Firstly I cleared any rubbish, tough woody waste, and large bricks/stones. I then 'flattened' the whole area, both by using a grass hook, and by simply trampling any weeds flat. Next, I scattered a generous helping of pelleted chicken manure over the entire area, both to encourage breakdown of the soon-to-die weed layer and generally give the soil bacteria a bit of a treat. The next step was the most satisfying - laying the cardboard. Avoiding windy days, I simply laid the sheets out like tiles, overlapping their edges by no less than 6in (15cm). I found that the thicker cardboard was best, and promised maximum weed suppression. The other key factor is to use large sheets, as there's less chance of weeds popping through, and you cover a larger area more quickly. You might think that what I did next was in some sense 'cheating', but I felt it was vital that the cardboard layer should be weighed down in some way, to keep it firmly in place, especially with an established layer of weeds lurking below.

After weighing up all the options, I decided to treat myself to some mushroom compost, supplied rather fortuitously by a local organic mushroom farm. I've received a fair bit of criticism for this, ranging from the 'how can I afford to do that' argument to 'what about the fossil fuel used to get it there?'. These are very good points, but the best advice I can give, is to do what you know you can easily achieve and afford; use garden compost, leaf mould, well-rotted manure, even topsoil if you can get it. This season I'm using manure, and I don't mind paying a few quid for it.

I spread the mushroom compost out in a layer 4in (10cm) deep over the top of the cardboard, then soaked it thoroughly. This also moistened the cardboard, which started going mouldy after just a few days. A year later there's hardly a trace of cardboard left, just a crumbly surface enriched with the mushroom compost that's gradually being incorporated by the worms. My final act in the cover-up was to cover the whole area with a 6in (15cm) deep layer of straw, to act as a water-saving, attractive and weed-blocking mulch, and this too was given a good soaking to keep it in place.

It was around this time that I was first asked, 'When are the donkeys arriving?'. To my fellow allotment holders, the layer of fresh straw meant one of two things; either I was about to set up a donkey sanctuary (complete with free rides for the kids), or I had gone into commercial mushroom production.

Seed Setbacks

After allowing a few weeks for everything to 'settle in', it was time to start sowing and planting, and at this point my cardboard revolution suffered its first big setback; the layer of mushroom compost was just too wet, and the cardboard still too intact. My makeshift solution was make a slit through the cardboard, down into the soil below, and then mix the soil with the mushroom compost, sowing the seeds onto this. Smaller seeds posed more of a problem than say peas or beans, and I still found that germination was very poor in places, which I put down to both less than ideal conditions (it was an exceptionally cold spring in 1996) and various 'damping off' fungi which I suspect were lurking in the mushroom compost (where else!). Later in the season, and after some experimentation, I found that the best results were achieved by taking out a 'slit' in the mushroom compost, filling the base with fine, crumbly soil, sowing the seed, then covering with more fine soil.

On reflection, if I were repeating the same exercise I would leave an area free of the cardboard 'treatment' for the first season, even if it needed digging (a little compromise won't hurt at this stage) as some clear ground is essential for crops like carrots, parsnips and most with small seeds, and certainly any that cannot be raised elsewhere and transplanted into mulched areas. A year after starting all you need to do is draw back the straw layer (which itself is breaking down nicely), level the soil and sow as usual. The straw can be put back just as soon as the young plants have some decent-sized foliage.

The Insulation Effect

Planting ready-grown transplants straight through the straw/mushroom compost/cardboard layer posed few problems, and this is definitely the technique to go for when you start from scratch. All you need to do is draw back the straw, part the mushroom compost, make a hole in the cardboard, break up the soil below, work the young plants' roots well in, and water. I did all this, but nothing happened. The plants sat there quite happily, but that's all they did, and I was very puzzled, until it dawned on me just what was happening. The straw layer (which covered the entire area), was acting as an 'insulating' layer, preventing the suns rays from hitting the dark soil, so stopping it from warming up. Growth did eventually take off, but only when actual air temperatures rose noticeably. This season I drew the straw back off the beds in early spring to let the sun do its job, then got it back on again as plants developed, but before any serious drying from the surface started to take place.

Weed Wars

As you would expect, virtually no annual weeds came through the cardboard layer, instead they lay rotting and dying below, making a valuable contribution to the organic matter in the soil, their breakdown aided by the super-charged pelleted chicken manure scattered months before. But three weeds in particular made it through; creeping thistle (pull out with as much root as possible), docks (loosen with a fork and ease out the deep tap root), and hedge bindweed - the one with the big white flowers that smother hedges (and garden plants) in summer, which proved the biggest challenge. It's vigorous underground stems go off in all directions in search of light, looking for the tiniest flaw in your cardboard layer. The solution? Diligence, patience and that smug feeling when you manage to extract a 4ft (1.2m) long runner, and trace it back to its point of origin!

Couch grass also rose to the challenge, but a double-thickness of cardboard, from experience, should see it off. Through the season the straw layer developed its own community, and was the stage for many unseen battles; spiders loved it, slugs loved it, beetles adored it (mostly the beneficial kinds) and birds loved to peck about in it! This latter activity might not give you the neatest of beds, but I'm sure that whatever the birds were after, they certainly found them.

The heat of the day

Suffolk, and especially Ipswich, which seemed to be able to dodge even the passing shower, was exceptionally dry last summer. Although in the heat of the day my crops flagged just like everyone else's, they generally had a much better 'look' to them, and recovered from the midday heat much quicker. The straw mulch of course played a great part; by reducing surface evaporation moisture loss was greatly reduced, and it also prevented the soil from getting overheated (the straw's insulating properties now having a positive effect), keeping the roots cooler. When I did water, I pulled the straw back first, soaked the soil around the plants, then fitted it back around the stems.

A Real Revolution?

I really believe that the 'cardboard revolution' could be the start of something big on Britain's allotment fields. It's here that so much enthusiasm is so easily quashed by the sight of overgrown plots, brambles and a waving sea of weeds. But it doesn't have to be that way. Using cardboard (I find plenty in those big skips behind electrical superstores), some kind of organic material, and a mulch like straw, you really can convert a bare, unproductive plot into a vibrant, healthy no-dig food garden in one growing season. The best bit is that people will come and talk to you about it, and perhaps some of them will try it for themselves.

John Walker is a freelance writer with an interest in permaculture and other environmentally-friendly gardening matters, and would like to hear from fellow no-diggers on their experiences. Email: johnwalker@onetel.com

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