Clearing an overgrown allotment or reclaiming a wild corner can be made less daunting by knocking everything flat with a line trimmer, and then putting the ground on hold with a light-excluding mulch. This starts the work for you, giving you time to deal with the area at your own pace without being overtaken by a riot of new growth.
Whack the weeds down within 5cm (2″) of ground level with a line trimmer (also known as a strimmer or weed whacker). If the overgrowth is established, including brushy growth or brambles, then don’t attempt it with a lightweight machine: buy or hire a heavy duty one. Depending on the sort of growth you are clearing, you might opt to hire a machine with a disc or chain head rather than the standard nylon line.
With unwanted growth knocked back, you’ll be much more able to see what you’re dealing with including contours and boundary features. There may be shrubs or stumps to be dug out, debris to be removed, or other surprises to contend with. You should also dig out the roots of brambles at this stage.
It might be tempting to try to tackle the whole thing at once, but there is nothing more disheartening than digging over a large section of ground, only to have it all return to weeds in a couple of months because you can’t get it planted quickly enough. Unless you’re going to use a mechanical tiller (rotavator) to do the whole thing in one go, cover the ground with a light-excluding mulch of cardboard or black plastic. As soon as the mulch goes down the weeds will begin to die, and even if it is only in place for a few weeks it will make later work much easier.
Hand digging is backbreaking work, particularly on heavy clay soils, but for smaller areas it’s extremely practical and allows you to pick out most of the perennial weed roots by hand. Divide larger plots into sections that can be dug in two weeks or less. Remove the light-excluding mulch from one section at a time, dig it over to at least a spade’s depth, and put the mulch back to supress weeds until you’re ready to plant. Don’t be surprised if a few perennials have tried to regrow when you take the mulch off. They’ll be weaker than they were before and much easier to dig out.
A rotavator (or rotary tiller/cultivator) can make short work of turning over new ground, but it is not without problems. Controlling a rotavator is notoriously difficult and exhausting work, particularly if you keep encountering objects like tree roots or buried rubble. The depth of tillage depends on the model used and the type of soil you are working on. Most seriously, rotary tillers do not stop to fish out roots of perennial weeds like bindweed or dandelion: they chop them up into tiny fragments, many of which will regrow.For that reason it is best to follow rotary cultivation with a light-excluding mulch or rotavate it again after two or three weeks to further weaken the regrowth.
Using a rotavator is much worse for soil macrofauna like worms and beetles than digging, and it can take the soil several seasons to recover. It also tends to create a hard pan just below the level of cultivation. You can minimise this by only using the machine when the soil is dry, but it can still be a problem on clay soils or if you rotavate frequently.
Leaving a light-excluding mulch in place for a whole growing season will starve out everything except extraordinarily persistent weeds like horsetail. Ordinary black plastic (sometimes sold as ‘silage sheet’) is not suitable for this because it does not allow air and water to penetrate to the soil, so much of the soil life dies. Instead, buy woven plastic membrane made for the purpose, or use a double thickness of cardboard covered with a thick layer of other organic material such as leaves, grass clippings, spoilt hay, or ‘green waste’ compost from your local council. If any particularly powerful weeds manage to push up through it, cut them to ground level and add another layer of cardboard and organic material in the same spot.
If the soil in your plot proves to be too poor or too thin to be useful, you will have to bring in topsoil from elsewhere. This is a costly business, so you might be best to confine the precious stuff to raised beds, using a permanent weed-excluding membrane elsewhere. You should cover this membrane with 7.5cm (3″) of path material as explained in our garden paths article.
Some sources recommend that the first step towards clearing new ground should be to kill all existing growth with a systemic weedkiller such as glyphosate, but even if you’re not an organic gardener this is a bad idea. Significant questions have been raised over the safety of this chemical, which is classed as ‘hazardous to the environment’ in the European Union. The claims of the original manufacturer, Monsanto, that it was ‘biodegradable’ and ‘left the soil clean’ led to a prosecution in France, and the company lost a series of appeals leaving the way open for civil suits. You have been warned!