Which mulch should I use — bark, wood chip, gravel or compost?
What each mulch material is actually good at, where each one is the wrong choice, and the depth that decides whether any of them works.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 3 min
Short answer
Compost and rotted manure feed the soil but break down in a year. Bark and wood chip last two to three years and suppress weeds better but add nothing. Gravel and slate are permanent and suit dry, Mediterranean planting. Whichever you choose, 50 to 75mm deep or it does nothing.
Mulch does three separate jobs — suppressing weeds, holding moisture and feeding the soil — and no single material does all three well. Choosing badly usually means choosing a material that is good at a job you did not need doing.
Step by step
- Decide which job matters most.Feeding a hungry border, keeping weeds down on a shrub bed, holding moisture in a dry garden, or looking a particular way. The answer changes the material.
- Choose compost or rotted manure to feed.Both improve soil structure and fertility as worms take them down. They break down within a season, suppress weeds only moderately, and need reapplying annually.
- Choose composted bark for shrub borders.Lasts two to three years, suppresses weeds well, looks tidy. It adds very little nutrition, so feed separately. Fine grades break down faster than chunky ones.
- Use wood chip on paths and around trees.Cheap or free from a tree surgeon, long-lasting, excellent for paths and around established woody plants. Keep it off vegetable beds and away from soft young plants.
- Never dig fresh wood chip in.Decomposing wood locks up nitrogen. On the surface that only affects the top few millimetres and is harmless; mixed into the soil it starves plants for a season.
- Choose gravel or slate for permanence.Ideal over a Mediterranean or gravel-garden planting, on well-drained soil. It keeps stems dry, reflects heat and never needs replacing. It is also permanent — very hard to remove if you change your mind.
- Use straw only for specific crops.Under strawberries and around cucurbits it keeps fruit off wet soil. It blows about, harbours slugs and often contains weed seed, so it is a crop tool rather than a border mulch.
- Apply at 50 to 75mm.Thinner than 50mm and weed seeds still germinate through it. This is the single most common reason mulching disappoints.
- Mulch onto wet, weeded soil.Mulch slows water going in as well as out, so mulching dry soil keeps it dry. Weed thoroughly first, because perennial weeds grow straight through any of these materials.
- Keep it clear of stems.Leave about 100mm of bare soil around trunks and crowns. Mulch piled against bark holds it wet and rots it.
Tips
- 50 to 75mm or do not bother — a thin scatter is decoration, not mulch.
- Mulch after heavy rain, never onto dry ground.
- Free wood chip from a tree surgeon is excellent on paths and around trees and wrong on a vegetable bed.
Common mistakes
- Laying membrane under a bark mulch in a planted border — It stops the mulch reaching the soil, weeds root into the bark on top of it, and after a few years you have a bed you cannot plant into and a membrane surfacing everywhere.
- Mulching over dry soil in a drought — It seals the dryness in. Water heavily first, or wait for a wet spell, and then mulch.
If it doesn't work
Weeds through the mulch within weeks
Cause: Layer too thin, or perennial roots left in — Fix: Top up to at least 50mm, and dig out perennial weed roots before mulching.
Bed drier after mulching
Cause: Mulched onto dry soil — Fix: Rake the mulch back, water deeply, and replace it.
Plants pale and stunted after wood chip
Cause: Chip mixed into the soil rather than laid on top — Fix: Rake it back onto the surface and apply a nitrogen feed.
Stems rotting at the base
Cause: Mulch piled against the trunk or crown — Fix: Pull it back to leave a clear collar of bare soil.
Questions people ask
How deep should mulch be?
50 to 75mm. Anything less does not block enough light to stop weed seeds, and does not slow evaporation appreciably.
Does bark mulch rob nitrogen from the soil?
Only where it is mixed in. As a surface layer, the nitrogen effect is confined to the top few millimetres where nothing is rooting, so it is not a practical problem.
Can I mulch with grass clippings?
In thin layers around established plants, yes, and they are free. Thick layers heat up, go slimy and smell. Never use clippings from a lawn treated with weedkiller.