Five to seven centimetres, over wet soil, with a gap around the stems

Mulch is the cheapest water-saving tool in a vegetable garden. A thick layer over the soil slows evaporation, keeps the surface cooler, smothers weeds and feeds the worms. It is not a substitute for watering, but it makes every watering last longer.

The common mistake is applying it too thin. A light scatter of compost looks tidy and does almost nothing.

How deep

Aim for 5 to 7.5 cm, and measure it. The RHS calls 5 cm the minimum and 7.5 cm ideal. For weed suppression on a weedy bed, go closer to 8 cm.

One cubic metre of compost covers about 15 to 20 square metres at that depth. A standard 70 litre bag covers roughly one square metre. If you are buying bags, do the sum before you go to the garden centre.

What to use

Garden compost is the best all-round mulch for a veg bed. It feeds the soil as it breaks down and you can make it at home.

Leaf mould is lower in nutrients but excellent for structure and moisture. Use it where you want to improve the soil without pushing leafy growth.

Well-rotted manure is rich and holds water well. Make sure it is genuinely rotted, not fresh, and keep it off crops you eat raw.

Straw is useful around strawberries, courgettes and tomatoes. It is loose, so apply it thicker, and keep it away from slugs’ favourite damp corners if slugs are a problem.

Wood chip is better for paths and permanent beds than for annual vegetables. The RHS suggests avoiding woody mulches on beds where shallow-rooted crops grow, because they can take nitrogen from the surface as they break down.

When to apply

Late winter or early spring is the best time for most beds. The soil is moist from winter rain, weeds have not germinated, and the mulch will still be there when the summer dry spell arrives.

Autumn is the other good window. It protects the soil from winter rain and lets worms pull the material down. Do not mulch frozen ground or soil that is already sodden.

If you are mulching in summer, water the bed first and apply the mulch over damp soil. Mulching a dry bed seals the dryness in.

How to apply it

1. Clear perennial weeds. Annual weeds can be smothered, but bindweed and couch grass will push through.
2. Water the soil if it is dry.
3. Spread the mulch in a layer 5 to 7.5 cm deep, working backwards so you do not tread it down.
4. Keep a 10 cm gap around the stems of shrubs, trees and woody herbs. Piling mulch against a stem keeps it damp and can rot it.
5. Leave a little space around seedlings so they are not buried.

What can go wrong

Mulching dry soil. The layer keeps the water out as effectively as it keeps it in. Water first.

Mulching against stems. Damp mulch against a stem is an invitation to rot.

Fresh manure. It can burn young roots and bring weed seeds. Compost it first.

Slugs. A thick organic mulch gives slugs somewhere to hide. Keep it slightly away from tender seedlings, and use whatever slug control you normally use.

Expecting it to replace watering. Mulch slows evaporation; it does not create water. In a long dry spell, you will still need to soak the bed.

The payoff

A mulched bed can go several days longer between waterings than a bare one. Over a summer, that is the difference between watering once a week and watering every other day. It also means less weeding, which is its own reward.

For the water side, see when to water vegetables and water butts and ollas. For the soil side, the raised bed soil calculator works out how much compost a bed needs.