Why mulch at all?

Mulching is one of those jobs that feels almost too simple to matter. You spread a layer of material over the soil, stand back, and… that's it. Yet it is the single most useful thing you can do for a border, a shrub bed or a fruit bush, and it pays you back all season.

A good mulch does three jobs at once. It blocks light from reaching weed seeds, so annual weeds struggle to get going and the ones that do germinate are weak and easy to pull. It slows evaporation from the soil surface, which means far less time standing about with a watering can in July. And it keeps the soil beneath it cooler in summer and warmer in winter, buffering roots against the extremes we seem to get more of every year.

There's a fourth benefit that's less visible but arguably the most important: mulch feeds the soil. Spread it on top rather than digging it in, and the worms and soil fungi do the incorporation for you, pulling organic matter down and building the crumb structure that makes soil hold water and drain freely at the same time.

What homemade compost brings that a bag of bark chips doesn't

Bark chips are perfectly decent mulch. But they're mostly lignin and take years to break down, so they sit there suppressing weeds and doing very little else. Your own compost is a different proposition entirely.

Home compost is biologically alive. A single handful contains more micro-organisms than there are people on the planet, and those organisms are what release nutrients from the soil's mineral particles and from last year's fallen leaves. It also carries nutrients itself — nitrogen, phosphorus, potassium and a long list of trace elements — in slow-release form that won't burn roots or leach away with the first heavy shower.

• Weed suppression: a 5cm layer cuts light to weed seeds far more effectively than a thin scattering of chips.
• Water retention: compost holds many times its own weight in water, releasing it slowly to plant roots.
• Soil life: worms are drawn to it in numbers, and their tunnelling aerates the ground beneath.
• Disease resistance: a lively soil community competes with the organisms that cause root rots and damping off.
• It's free. Made from kitchen scraps, grass clippings and prunings you'd otherwise bin, it costs you nothing but patience.

Getting your compost mulch-ready

You can't spread compost the day you tip it out of the bin. Fresh material can be still heating, and immature compost can actually rob nitrogen from the soil as it finishes breaking down. Leave it to mature for a few months after the last addition, turning it occasionally if you have the energy.

Ready compost smells sweet and earthy, like a woodland floor after rain. It shouldn't smell sour or sharp, and you shouldn't be able to recognise the banana peel you put in last week. For mulching, sieve it to about 10mm — the coarse bits that don't pass through can go back in the bin to start the next batch. A rough, crumbly texture is ideal; fine, dusty compost tends to cake and shed water rather than absorb it.

If your compost is on the coarse side, that's no bad thing for mulching. Chunky material lets air and rain through and lasts longer on the surface. Use the finer stuff for potting mixes and seed sowing instead.

How to spread it — and the one rule you must not break

Timing matters more than most gardeners realise. The sweet spot is early spring, once the soil has warmed and you've had a good weed, or early autumn while the ground is still warm and damp. Avoid spreading onto frozen ground or onto soil that's bone dry — you'll simply insulate the dryness in.

Water the border thoroughly the day before if conditions are dry. Then weed first, by hand, getting the roots out. Mulching over established perennial weeds such as bindweed or ground elder is a mistake; you've just given them a lovely blanket.

Depth depends on what's growing:

• Shrubs, roses and fruit bushes: 5–8cm, spread out to the drip line or beyond.
• Herbaceous perennials and soft fruit: 3–5cm, enough to cover the soil without smothering low crowns.
• Vegetable beds and annuals: 2–3cm, worked between rows rather than right up to stems.

And the rule: keep the mulch clear of stems, trunks and crowns. Leave a gap of 5–10cm around every woody stem. Piling compost against bark holds moisture there, softens the tissue and invites rot, and it gives slugs and mice a covered route straight to your plant. Think of it as a ring doughnut, not a cone. Spread it evenly, break up any lumps with the back of a fork, and don't firm it down.

Which plants thank you most

Everything in a border benefits, but some plants repay mulching spectacularly. Roses put on stronger growth and suffer less from blackspot when they're not stressed by drought. Clematis, with their cool, shaded roots, love a generous collar of compost. Fruit bushes — blackcurrants, gooseberries, raspberries — crop noticeably better on mulched ground, and you'll pick far fewer weeds out of the canes.

Around hungry feeders such as dahlias, courgettes and rhubarb, a compost mulch does double duty, suppressing weeds while trickling nutrients down to shallow roots. Trees and shrubs planted in the last two years benefit most of all, as their roots are still establishing and hate competition.

Small mistakes worth avoiding

Don't mulch right up against the base of a wall or fence and expect the plants there to thrive — it stays dry and can harbour pests. Don't use compost that still contains perennial weed roots or seeding annuals; your bin rarely gets hot enough to kill them, and you'll simply be planting the problem. And don't top up endlessly year after year without checking the depth — more than 10cm can seal the soil off from air and rain.

Finally, don't mulch everything on the same day in a fit of enthusiasm. Do one bed well, then stop. Your soil will notice the difference within a season, and next year's compost will be ready before you know it.