Uncovered soil compacts over winter, gets leached, and its surface crumbles to dust. A layer of organic material prevents all of that, and by spring it will have partly broken down — so mulching isn't only protection, it's slow-release feeding too.
What to mulch with
Leaves — free and the easiest to come by
The best ratio of benefit to effort, because in autumn you have them in abundance. But shred them, ideally by running the mower over them — whole leaves mat into an airless layer under which the soil goes sour. Oak and walnut leaves break down more slowly and acidify slightly, which only matters where you don't want it.
Straw — for vegetable beds
Airy, holds moisture well and doesn't compact easily. The drawback: it often carries cereal seed and attracts voles, which love overwintering under it. For beds close to the house, or anywhere you already have voles, choose something else.
Grass clippings — thin layers only
They break down fast and supply nitrogen, but in a thick layer they overheat and start to smell. Spread no more than 3–5 cm and let them wilt first.
Bark and wood chip — for perennials and shrubs, not vegetables
Long-lasting and good-looking, but as it decomposes it takes nitrogen out of the soil. Under perennials and shrubs that doesn't matter; on a vegetable bed it does — the crop would be competing with the mulch for nitrogen.
How thick the layer should be
| Material | Depth | Suits |
|---|---|---|
| Shredded leaves | 5–10 cm | vegetable beds, perennials |
| Straw | 10–15 cm | vegetable beds, strawberries |
| Grass clippings | 3–5 cm | vegetable beds |
| Compost | 3–5 cm | anything, doubles as a feed |
| Bark, wood chip | 5–7 cm | perennials, shrubs, paths |
The most common mistakes
- Mulch piled against the stem — leave a clear ring of 5–10 cm around the base of shrubs and perennials. Damp mulch against bark causes rot and invites voles straight to lunch.
- Mulching dry soil — mulch holds moisture in, it doesn't add any. If autumn has been dry, water the bed thoroughly before mulching.
- Mulching too early — mulch down in warm September weather and the soil stays warm for weeks, while rodents build nests under the layer. Wait until it cools.
- Fresh wood chip on vegetables — see above, nitrogen lock-up.
- A thick layer of fresh grass — it heats up and putrefies instead of decomposing.
What to do with mulch in spring
With leaves, grass and compost you usually need do nothing — most of it will have broken down by spring and the rest gets worked in when you prepare the bed. Straw and coarser materials should be raked aside in spring so the soil warms sooner; once you've sown and the plants are up, you can put it back.
One rule always holds: bare soil is always the worse option, whatever the mulch. Even an imperfectly mulched bed comes through to spring better than one that spent winter naked.
Frequently asked questions
When is the best time to mulch beds?
Once the soil has cooled in autumn but before it freezes — typically late October and November. Mulching in warm weather keeps heat in the soil and invites rodents to nest under the layer.
Can I use bark mulch on a vegetable bed?
It isn't advisable. Bark and wood chip draw nitrogen out of the surface layer as they decompose, and vegetables feel that keenly. Use leaves, straw, grass clippings or compost on vegetables; save bark for perennials, shrubs and paths.
Won't mulch attract voles?
Straw and thick layers of hay will — they're ideal vole quarters for winter. Shredded leaves and compost are considerably safer in that respect. Either way, leave a clear ring around the base of woody plants so bark-gnawing under cover doesn't go unnoticed.