Why Mulch Earns Its Keep

A generous layer of mulch is one of the kindest things you can do for a flower bed. It suppresses weeds, slows evaporation from the soil surface, keeps the roots cool in a hot spell and, as it breaks down, feeds the soil life that feeds your plants. It also makes a border look finished, which matters more than we like to admit.

But mulch is not one substance. Bark, compost and leaf mould behave quite differently in the ground, and choosing between them comes down to two things: what your soil is like now, and what you are asking that border to grow. Get the match right and you will spend less time watering and weeding. Get it wrong and you can smother shallow roots, lock up nitrogen or create a slug haven.

Bark Mulch: Structure and Staying Power

Bark is the neat, long-lasting option. Chipped bark and bark nuggets break down slowly, so a 5–7cm layer will still be visible a year later. That slow breakdown is exactly why it is good for permanent plantings: shrubs, roses, hydrangeas and anything in a mixed border where you want a tidy, weed-free surface.

Two cautions. Fresh, unrotted woodchip and bark can temporarily rob the soil surface of nitrogen as microbes work to break it down, so keep it clear of annuals and young bedding plants, or add a handful of general-purpose fertiliser before you spread. And because bark sits loose and airy, water can struggle to reach the soil if the layer is too thick — 7cm is plenty, and less on heavy clay.

• Best for: shrub borders, roses, acid-lovers such as rhododendrons, camellias and pieris
• Lasts: 12–24 months
• Watch out for: slugs sheltering underneath in wet summers

Compost and Well-Rotted Manure: Feeding the Soil

If your soil is thin, sandy or has been worked hard for years, compost is the mulch that gives the most back. It is dark, crumbly and full of the organic matter that holds water and nutrients. Spread it 5cm deep in spring and the worms will pull it down within a season, improving structure as they go.

Well-rotted manure works in the same way but is richer, so use it more sparingly — 3–5cm on hungry plants such as roses, clematis and dahlias. Only use manure that is genuinely well rotted, at least six months old, or it can burn young growth and, if it came from a stable using aminopyralid-treated hay, damage broad-leaved plants for years. Home-made compost is ideal provided it is mature and you have not added perennial weed roots.

Compost mulches are less effective at suppressing weeds than bark because weed seeds germinate happily in that rich, damp surface. A thin layer of bark on top of the compost gives you the best of both.

Leaf Mould: The Patient Gardener's Gold

Leaf mould is the gentlest mulch of the three. Made from rotted leaves — oak, beech and hornbeam take two years; sycamore and horse chestnut break down in one — it is low in nutrients but superb at improving soil structure and encouraging the fungal life that woodland plants adore.

Use it around woodland perennials, ferns, hellebores, epimediums and spring bulbs. It is also the best mulch for acid-loving plants if your leaves came from acid trees. Because it is fine and crumbly, it knits into the soil surface rather than sitting on top, so it suppresses weeds less well than bark. A 3–5cm layer is ample, and leaf mould is the one mulch you can safely heap a little closer to stems and crowns.

Matching Mulch to Soil and Plants

Start with the soil in front of you.

• Heavy clay: you want organic matter to open it up. Compost and leaf mould first, with bark on top for tidiness. Avoid thick bark layers, which keep clay cold and wet.
• Sandy or free-draining soil: compost and manure, applied every spring. Moisture retention is your priority.
• Chalky or alkaline soil: stick to compost and leaf mould. Bark is slightly acidifying, which is a small help but no substitute for choosing lime-tolerant plants.
• Acid-loving borders: bark and leaf mould from oak or beech, kept topped up.
• Alpine and gravel gardens: none of the above. Sharp grit keeps foliage dry and crowns firm.

Think about the plants, too. Herbaceous borders want something fine enough for shoots to push through in spring — compost or leaf mould. Shrub and rose borders want bark. Beds full of self-seeders such as aquilegia and verbena do better with a thinner mulch, or you will smother next year's seedlings along with the weeds.

How Thick, When, and a Few Practicalities

Spring is the main mulching moment in the UK: the soil has warmed, it is still damp, and weeds are just getting going. Wait until the ground is workable and any bulbs have finished. A second, thinner layer in autumn helps on light soils and around newly planted shrubs, but do not mulch frozen or waterlogged ground.

• Depth: 5–7cm for bark, 3–5cm for compost and leaf mould. More is not better.
• Keep clear: leave a 5–10cm collar of bare soil around trunks, stems and crowns. Piling mulch against bark invites rot and mice.
• Water first: mulch locks in moisture, it does not create it. Soak dry ground before spreading.
• Weed first: mulch suppresses weeds, it does not kill established ones. Perennial roots will simply push through.

Whichever you choose, buy or make more than you think you need. A border mulches down faster than anyone expects, and a bag that looked generous in the shop covers a surprisingly small patch of ground.