Mulching is the closest thing gardening has to a free lunch: one spring afternoon spreading organic matter saves a summer of watering and weeding, and quietly improves your soil every year you do it. Yet it is often done too thinly, at the wrong time, or piled against stems where it does harm. Here is what mulch actually does, which material to choose, and how to get it right.
What a mulch actually does
A mulch is simply a layer of material spread over the soil surface. Its effects are physical and cumulative:
- Holds moisture. A 5 cm layer dramatically slows evaporation from the soil surface, so beds stay damp for weeks longer after rain — the single biggest benefit in an average British summer.
- Suppresses weeds. Most annual weed seeds need light to germinate; a decent mulch blocks it. Weeds that do appear root into the loose mulch and pull out easily. (No mulch stops established bindweed or ground elder — be honest with yourself about that.)
- Feeds soil life. Organic mulches are gradually dragged down by earthworms and broken down by fungi and bacteria, improving structure on both clay and sand. You rarely need to dig mulch in; the worms do it.
- Buffers temperature. Mulch insulates roots against winter frost and summer baking, useful for borderline-hardy plants such as agapanthus or Melianthus (typically H3–H4).
- Protects the surface. It stops heavy rain capping bare soil and reduces mud splash onto leaves and fences.
Choosing a material
Garden compost
The best all-rounder if you make enough of it: moderately nutritious, excellent for soil structure, and free. Fine texture means it breaks down within a season, so it suits vegetable beds and hungry borders that get topped up annually. Home compost may carry some weed seeds; municipal green-waste compost is hotter-composted and usually cleaner.
Well-rotted manure
The richest common mulch — good around roses, fruit and vegetables. It must be well rotted (dark, crumbly, not smelling of ammonia); fresh manure scorches roots and stems. Stack fresh manure for six months to a year before use, and buy from a source that can vouch it is free of aminopyralid herbicide residues, which can distort tomatoes, beans and potatoes.
Bark and woodchip
Low in nutrients but long-lasting — two to three years from a single application — and the tidiest-looking option for shrub borders, paths and around trees. A persistent myth says fresh woodchip "steals nitrogen" from the soil. Nitrogen lock-up only happens where wood is mixed into the soil; laid on the surface, fresh chip is fine around established trees and shrubs, because the effect is confined to the thin contact layer. Avoid digging it in, and avoid deep fresh chip around shallow-rooted annuals and vegetables.
Leafmould
Rotted autumn leaves, one to two years old. Almost nutrient-free but superb as a soil conditioner and a gentle mulch for woodland plants, hellebores, ferns and shade borders — it mimics exactly what they get in nature. Free if you gather leaves each autumn into a wire cage or punctured bin bags.
Gravel and grit
The right choice for alpines, Mediterranean herbs, and any plant that hates winter wet around its neck. A 2–3 cm collar of gravel or horticultural grit keeps the crown dry, reflects light, deters slugs a little, and never rots down. Use it over free-draining soil for lavender, sedums, sempervivums and dianthus rather than organic mulches, which hold exactly the moisture these plants resent.
What to avoid
Uncomposted grass clippings in thick layers (they slump into an airless mat — use thin layers or compost them first), peat (environmentally indefensible and poor as mulch anyway), and rubber or dyed decorative chips near food crops.
How deep, and how much to buy
The commonest failure is meanness. A dusting of 1–2 cm looks tidy for a fortnight and does almost nothing: light gets through, weeds germinate, and moisture escapes. Aim for 5–7 cm of organic mulch (a little more for coarse bark, a little less for fine leafmould on delicate plantings).
The arithmetic is simple: one square metre covered 5 cm deep needs 50 litres of material. So:
| Bed size | Depth | Material needed |
|---|---|---|
| 1 m² | 5 cm | 50 litres (one large bag) |
| 10 m² | 5 cm | 500 litres (half a bulk bag) |
| 20 m² | 7 cm | 1,400 litres (1.5 bulk bags) |
A standard builder's-style bulk bag holds roughly 750–1,000 litres depending on supplier. For anything beyond a couple of beds, a loose or bulk-bag delivery is far cheaper per litre than 50-litre bags from the garden centre — and honest suppliers state the volume, so do the sums before ordering.
When to mulch
The golden rule: mulch locks in whatever conditions exist underneath it. Spread it on moist, warming soil and it locks in moisture; spread it on dry or frozen soil and it locks that in instead.
- Mid-March to April is the prime window for most gardens: the soil is thoroughly wet from winter, starting to warm, and weeds have not yet got going. Mulch after a good rain, never during a dry spell — or water deeply first.
- October to November is a good second window, particularly on heavy clay (protecting the surface from winter rain) and around borderline-hardy plants, where a dry mulch of bark or bracken over the crown adds real frost protection.
- Any time you plant a tree or shrub, finish the job with a mulched circle at least 60 cm across — it is the cheapest way to improve establishment, mainly by stopping grass competing for water.
How to do it properly
- Weed first. Remove perennial weed roots; a mulch buried over live bindweed is a growbag for bindweed.
- Water or wait for rain, so the soil beneath is properly moist.
- Spread 5–7 cm evenly with a fork or by the bucketful, breaking up lumps.
- Keep it clear of stems and trunks. Leave a 5–10 cm gap of bare soil around the base of every plant. Mulch heaped against bark holds moisture on it and causes rot — the "mulch volcano" around a tree is the classic error, and it can kill over a few years. A doughnut, never a volcano.
- Top up annually or when the layer thins below about 3 cm.
Common mistakes
- Too thin. Under 3 cm, most of the benefits vanish. Do half the garden properly rather than all of it thinly.
- Mulching dry soil. A spring mulch after a dry March seals drought in for months. Water first.
- Piling against stems. Causes rot on trees, roses and perennial crowns alike.
- Smothering emerging perennials and bulbs. In early spring, work around clumps rather than burying the new shoots; delicate crowns can be mulched more lightly.
- Digging fresh woodchip in. Surface, yes; incorporated, no — that is where nitrogen lock-up genuinely occurs.
- Using rich manure on plants that hate it. Mediterranean and alpine plants want grit, not muck.
- Expecting weed miracles. Mulch suppresses germinating annuals; it does not defeat established perennial weeds without membrane, persistence or excavation.
Mulch once a year, generously and at the right moment, and within three seasons you will notice the difference underfoot: darker, crumblier soil, fewer weeds, and beds that shrug off a dry fortnight that once had you out with the hose.
Sources and further reading
- Royal Horticultural Society — guidance on mulch materials, application depths and soil health.
- Garden Organic — composting, leafmould and organic soil management techniques.
- Woodland Trust — how leaf litter and decaying wood support soil ecosystems, the model garden mulching imitates.