How-to guide
Mulching: what to use, when and how deep
Mulching means covering the soil surface with a layer of material, usually 5 to 8 cm of something organic such as garden compost, leaf mould, bark or well-rotted manure. Do it on moist, weeded soil, ideally in late winter or early spring, or in autumn once rain has soaked the ground. A good mulch keeps moisture in, smothers weed seedlings, protects roots from frost and heat, and, if it is organic, feeds the soil as worms pull it down.
Few jobs give as much benefit for the effort. A single mulching session can halve the amount of weeding and watering you do in a season, and over a few years it changes the soil itself, opening up heavy clay and helping sandy soil hold water.
What mulch does
Holds in moisture. Bare soil loses water fast in sun and wind. A mulch acts like a lid, which is why it is one of the best defences against drought stress.
Suppresses weeds. Most weed seeds need light to germinate. A thick layer keeps them in the dark, and any that do come through are easy to pull from loose mulch.
Insulates the soil. It keeps roots cooler in summer and warmer in winter, which is useful for slightly tender plants such as dahlias and agapanthus left in the ground, and reduces frost damage to roots.
Improves the soil. Organic mulches are pulled down by worms and broken down by soil life, adding humus. You do not need to dig them in.
Choosing a mulch
Garden compost. The best all-rounder, and free if you make it. It feeds as well as mulches, so use it on vegetable beds, roses and hungry borders. Our guide to making compost explains how to produce your own.
Leaf mould. Made from autumn leaves stacked for a year or two. Low in nutrients but excellent for soil structure, and ideal around woodland plants such as hellebores, ferns and spring bulbs.
Well-rotted manure. Rich in nutrients, so best for vegetables, fruit and roses. It must be well rotted: dark, crumbly and with no strong smell. Fresh manure can scorch plants and is high in ammonia.
Composted bark and wood chip. Long-lasting, attractive and good for weed control around shrubs, trees and paths. Fresh wood chip from tree surgeons is often free. Lay it on top of the soil and leave it there; it only ties up nitrogen where it touches the surface, so avoid digging it in and keep it off seedbeds.
Straw. The traditional mulch under strawberries, where it keeps fruit clean and off the soil. Also useful around potatoes and squash.
Grass clippings. Spread thinly, no more than 2 to 3 cm at a time, or they turn into a slimy mat. Do not use clippings from a lawn treated with weedkiller.
Pine needles and composted conifer bark. A slightly acidic option for acid-loving plants such as camellias and rhododendrons.
Gravel, grit and slate. Inorganic mulches that do not feed the soil but look smart, stay put for years and keep the necks of drought-loving plants dry. Ideal for lavender, sedum, alpines and gravel gardens.
When to mulch
The golden rule is to mulch moist soil, not dry or frozen soil. A mulch keeps the soil in whatever state it is in when you apply it.
Late winter to mid-spring (February to April) is the main mulching season. The soil is still damp from winter rain and starting to warm, and weed seedlings have not yet got going. See our March jobs for what else to do at this time.
Autumn (October to November), once rain has soaked the soil, is the other good window. An autumn mulch protects roots over winter and gives worms months to work it in. Our October jobs page includes it.
Avoid mulching in the middle of a dry summer unless you have watered thoroughly first. Also avoid covering spring bulbs just as their shoots come through; mulch before they emerge or after the foliage dies down.
How to mulch
- Weed first. Remove perennial weeds with their roots. Mulch smothers seedlings but not established dandelions or bindweed.
- Water if dry. If the soil is dry a spade’s depth down, give it a thorough soaking and wait a day.
- Barrow it in. Tip small heaps around the bed, then spread with a fork or rake.
- Go deep enough. Aim for 5 to 8 cm. Less than 5 cm lets weeds through and breaks down within months.
- Keep it off stems. Leave a gap of a hand’s width around tree trunks, shrub stems and the crowns of perennials. Mulch piled against bark keeps it damp and invites rot.
- Top up yearly. As worms take it down, add a fresh layer each year.
How much to buy
A cubic metre of mulch covers about 20 square metres at 5 cm deep, or about 12 square metres at 8 cm. Builders’ merchants and landscape suppliers sell bulk bags that usually hold between 0.5 and 1 cubic metre; check the volume before you order. A typical 50 to 60-litre retail bag covers only about one square metre at 5 cm, which is why bulk delivery is far better value for a whole border.
Mulching vegetables and fruit
On the vegetable plot, spread compost or rotted manure on beds over winter and plant straight into it in spring. Around fruit bushes such as raspberries and blackcurrants, a deep mulch in late winter keeps the shallow roots cool and moist and the bed free of grass.
Do not mulch over the top of seed drills. Wait until seedlings are a few centimetres tall, then mulch between the rows.
Mulching pots
A layer of grit, gravel or decorative stone on top of a container cuts evaporation and keeps the compost from splashing. It also makes the surface less attractive for egg-laying vine weevil adults.
Common mistakes
Mulching dry soil. The mulch keeps rain out and the soil stays dry for weeks.
Volcano mulching. Piling mulch up against tree trunks causes rot and pest damage. Spread it flat, like a doughnut, not a cone.
Too thin a layer. A dusting of bark looks neat for a month and does little else.
Using fresh manure. It can burn roots and stems. Let it rot down for at least six months first.
Weed-control fabric under bark in borders. It stops the mulch feeding the soil and makes planting awkward. It is fine under paths and gravel but rarely worth it in planted beds.
Questions people ask
Does mulch attract slugs? Loose, damp mulches give slugs and snails somewhere to shelter. Keep mulch slightly back from vulnerable seedlings and hostas, and use grit or gravel around the most at-risk plants.
Should I remove old mulch before adding new? No. Leave it to rot into the soil and spread the new layer on top.
Can I use cardboard? Yes. Plain brown cardboard with tape removed, laid under a few centimetres of compost or wood chip, is an effective way to clear grass or weeds from a new bed over a few months.
Is bark mulch bad for plants? Not when used on the surface. It is only a problem if dug in or used on seedbeds.