Why Sow a Crop You Do Not Want? To Look After the One You Do

Certain valuable plants establish slowly, spending their first weeks putting down roots while producing very little above ground, which leaves the soil exposed and the seedlings vulnerable. A fast-germinating species sown with them covers the ground within days and provides several things at once. It shades the soil surface, which reduces evaporation and keeps temperature stable. It occupies the space that weeds would otherwise take. Its roots hold the soil against rain and wind. And on sloping or exposed ground it reduces the physical battering that young seedlings receive. It is then cut, grazed or harvested before it starts competing seriously.

The applications share the same shape across very different scales:

• •Cereals sown with a new grass and clover ley, harvested in the first year
• •Fast annual grasses sown with slow wildflower meadow mixtures
• •Cover species sown with young tree seedlings on bare ground
• •Quick-growing shrubs planted to shelter woodland restoration
• •Oats or barley sown over autumn-planted vegetables
• •Sacrificial plantings on eroding slopes and spoil heaps

The technique fails when the companion stops being a nurse and becomes a competitor, which is the central risk and depends entirely on timing and on the season. In a dry year the fast species takes the available water and the crop it was meant to protect is worse off than if nothing had been sown, which is the commonest failure. Sowing the companion too thickly shades the slower plant rather than merely shading the soil. Leaving it too long lets it set seed and become a weed in its own right for years afterwards. And a vigorous companion left standing after harvest leaves a dense stubble that smothers what is underneath.

Getting rid of the sheltering species at the right moment is the part that decides whether the technique works, and the methods differ by situation. Cutting for hay or silage before the seed sets is the traditional approach with a cereal sown over a new grass ley, and it produces a saleable crop from the operation. Grazing achieves the same thing and adds manure, at the risk of the animals damaging the young plants underneath. Mowing and leaving the material where it falls returns organic matter but can smother what is below if the cut is heavy. Winter killing is used where a frost-tender species is chosen deliberately so that the cold removes it without any work at all.

The practice copies something that happens naturally, and the ecological version is well studied under the heading of facilitation. Bare ground colonised by pioneer plants becomes hospitable to species that could not have established there directly, because the pioneers stabilise the surface, add organic matter, moderate temperature and in some cases add nitrogen. Shrubs in deserts and in alpine ground shelter seedlings of trees that will eventually overtop and kill them, which is called a nurse plant relationship and is easily seen where a young tree grows from the middle of a shrub. Restoration ecology uses the principle deliberately, planting tough pioneers first rather than attempting the target species immediately.

A fast species sown with a slow one shades the soil, suppresses weeds, holds the surface and shelters seedlings through establishment, and is then removed before it competes. Sown too thickly or left too long it takes the water and light instead, which is the usual failure and is worst in a dry year. The same relationship occurs naturally, with pioneer shrubs sheltering tree seedlings that eventually kill them.