Why Mulch Earns Its Keep

If there is one job that pays you back twice over in the garden, it is mulching. Spread a layer of organic material over the soil surface and you are doing three things at once: starving weeds of the light they need to germinate, slowing evaporation so your watering goes further, and feeding the soil life that turns that material into next year's good earth.

The weed suppression alone is worth the effort. Most annual weed seeds need light to sprout, and a decent blanket of mulch simply denies them that. You will still get the odd determined thistle or bindweed pushing through, but the endless chickweed and bittercress that colonise bare soil will thin out dramatically. Meanwhile, the soil beneath stays cooler in a heatwave and warmer in a cold snap, which suits the roots of most plants far better than the bake-and-freeze cycle of bare ground.

There is a longer game too. As bark, leafmould and garden compost break down, worms pull the material down into the soil, improving structure and water-holding capacity. Do it year after year and heavy clay gradually becomes crumbly and workable. That is not marketing talk — it is just what happens when you keep feeding the ground from above.

Choosing the Right Mulch for the Job

Almost any organic material will do, but each has its strengths. Think about what you are mulching around and what you want the bed to look like.

• Garden compost: the all-rounder. Fine-textured, rich, and ideal for vegetable beds and borders. Apply 2–3cm; it breaks down quickly, so expect to top up twice a year.
• Well-rotted manure: superb for hungry crops such as courgettes, beans and roses. Make sure it is genuinely rotted — at least a year old — or it can scorch young stems.
• Bark chippings: the tidy choice for shrub borders and around trees. Coarse grades last two to three years and look smart, though they are best kept away from vegetable beds and acid-hating plants.
• Leafmould: free if you have trees. Brilliant for woodland plants, ferns and anything that likes a cool root run. Sieve it for containers.
• Straw: the classic around strawberries and tomatoes, keeping fruit clean and slugs slightly baffled. It is light, so damp it and weight it down.
• Gravel or slate: not organic, but excellent for alpines, Mediterranean herbs and pots, where damp foliage is the enemy.

Whatever you choose, avoid anything fresh, green or smelly. Unrotted material robs the soil of nitrogen as it decomposes and can attract pests.

How to Apply It Properly

The technique matters as much as the material. First, weed thoroughly and water the ground if it is dry — mulching over bone-dry soil traps that dryness in, and it is surprisingly hard to rewet afterwards. Wait until the soil has warmed in spring, usually from mid-April in most of the UK, rather than laying it in February when it will simply keep the cold in.

Then spread a layer of 5–8cm for coarse materials such as bark or straw, and 2–3cm for fine compost or leafmould. Thinner than that and weeds will punch straight through; thicker and you risk smothering shallow-rooted plants and keeping rain out.

The most important rule is the one people break most often: keep mulch clear of stems, trunks and crowns. Leave a gap of 5–10cm around the base of shrubs and trees, and keep it well away from the centre of perennials such as delphiniums, lupins and strawberries. Piled-up mulch against stems holds moisture where it does the most damage, softening bark and encouraging rot. Think of a doughnut with the plant sitting in the hole, not a volcano with the trunk buried in the middle.

Mulching Different Parts of the Garden

In the vegetable plot, mulch after seedlings are established and the soil is properly warm. A straw or compost layer under courgettes, squash and tomatoes cuts watering dramatically in a dry August. Around leafy crops, a thinner compost layer is plenty. On the flower borders, bark or leafmould applied in spring saves hours of hoeing and sets off the planting beautifully.

For containers and hanging baskets, a 1–2cm topping of gravel or compost slows evaporation and stops compost splashing onto leaves during watering, which helps keep diseases such as blackspot and blight at bay. And around fruit bushes and trees, a wide ring of mulch out to the drip line will reward you with better crops, provided you keep it off the trunk.

Mistakes to Avoid

• Mulching dry ground. Water first, then mulch. Once it is on, rain struggles to penetrate a thick layer of bark.
• Using fresh grass clippings in a thick layer. They mat down, smell, and go slimy. Mix them with brown material or compost them first.
• Mulching damp, cold soil in early spring. You will simply hold the cold and slow growth.
• Ignoring slugs. Mulch gives them somewhere to hide. Water in the morning rather than the evening and patrol after rain.
• Forgetting to top up. Organic mulches disappear — that is the point — so check levels each spring and autumn and top up as needed.

Get into the habit and you will spend far less time weeding and far more time enjoying the garden. An hour with a barrow and a fork in spring buys you a whole summer of easier watering and cleaner beds — the best bargain in gardening.