Clockwise from top left: Jodi Roebuck planting bean seedlings with a measuring stick; Spacing plants for maximum growth; Compost is essential in double-digging – get the best quality you can, or make it yourself; Digging the first trench; Forking the base of the trench to break up the subsoil; and Double-dig diagram. Images by Kirsten Bradley
There are many ways to start a garden bed: no-dig to double-dig. Each suits different situations, people and budgets; how you do it is up to you. Double-digging – the biointensive growing technique – is excellent for anyone wanting to grow a large quantity of organic vegetables in a small space, in a small timeframe. But you need to work for it.
This method is all about building beautiful deep, friable topsoil that can grow an intensive amount of good food, quickly. But to do this you need to create that soil structure yourself rather than waiting for the roots of well-chosen and tended plants, and the soil food web, to do that for you.
Tools And Materials
Once you’ve designed and positioned your beds, get your tools together. You’ll need: a good spade, preferably a posthole shovel or similar; a good quality garden fork; a big board to lay over the bed as you work; and a good rake. And don’t forget a big bottle of water, and the gumption to get the job done.
You’ll also need to source some good compost to add to your bed: about four buckets per square metre is good.
The aim of double-digging is twofold, to create: a friable, nutrient rich topsoil to plant into; and a well loosened subsoil which allows plant roots to reach far down for water, stability and food.
Start at one end of your bed, and work across it in strips, one shovel wide: dig a trench to the depth of your shovel all the way across the bed. Turf the soil from that first trench onto the ground beside the bed, or straight into a wheelbarrow (you’ll need it later).
Then work along that same trench with your garden fork, loosening the subsoil at the bottom of the trench to the depth of your fork, and working from behind the fork so that you don’t tread on the loosened soil. Everyone has their own way of forking a bed, but standing on the fork and pivoting it back and forward until it sinks to its full depth works well.
Repeat that sequence, shovelling the top half of your second trench into your first trench, mixing it with some compost as you go and breaking up any large clods. Move down your bed in this way, flipping each new trench’s topsoil into the previous trench. When you get to the last trench, you’ll need the first trench’s topsoil to complete the bed.
At the end of this process you will have a lumpy bed of goodness. It’s time to clean things up. Take your rake (or start with your garden fork if the clods are large) and break up the surface of the bed until it’s flat, and looks like a great seedbed for your plants. Stand on your big board as you move down the bed, if your bed is too wide to do this comfortably from the side; the board will distribute your weight and reduce the impact on the bed.
And then – you’re done! Proceed to planting or, if that’s not possible immediately, cover the bed with either hessian or seventy per cent shadecloth until you’re ready to plant. This will protect the soil from sun and pests, and also from heavy rain washing away your precious and hard-won soil tilth.
The aim of biointensive growing is to maximise growing space and minimise bare soil; planting at the minimum spacing for good growth. Because of the extra-deep tilth of the soil, plants can reach down into the soil easily for nutrients and water, which is what makes the close spacing possible. Spacings for each type of common vegetable are different.
A general rule of thumb is planting in an offset triangular pattern: each plant ends up in a hexagon with six plants around it, maximising the space and the harvest at the same time.
For more information about biointensive growing, see John Jeavon’s website www.growBiointensive.org which is full of films, resources, books, tools, planting plans and how to’s. Milkwoods hold courses, south of Sydney with Jodi Roebuck, twice a year – see www.milkwood.net.