Mulching is the job people skip because nothing visibly happens, and then wonder in July why the borders bake dry and the weeding never ends. Nothing about it looks like much on the day, and it saves more work than anything else you do in autumn.
What it's actually doing
Four things at once, which is why it's worth the effort:
Holds moisture in. A covered soil surface loses far less water to evaporation, so borders ride out a dry spell that would otherwise have you watering every other evening.
Stops weed seeds getting going. Many annual weed seeds need light to germinate, and the ones that don't still can't push up through three inches of mulch before they run out of steam. Either way, cover the soil and you stop a large share of them.
Evens out the temperature. The soil under a mulch warms and cools more slowly, which suits roots and soil life both.
Feeds the soil as it rots. Organic mulch is pulled down by worms over a season or two, and it's the main way a no-dig bed gets fed at all.
Worth being clear about what a feeding mulch does not do. At two or three inches it will not kill established perennial weeds — bindweed, ground elder and couch come straight back up through it.
Smothering those out is a different job with different numbers: cardboard first, then something like eight inches of bark or chip, left down for two or three growing seasons. That does work, and it's the main non-chemical option there is. A thin weed-control fabric won't do it; bindweed roots go down several feet and it simply travels and comes up at the edge.
When to put it down
The timing rule matters more than the material, and it's the bit most often got wrong.
Mulch onto soil that is moist — not dry, not frozen, and not squelchy. Autumn once the rain has properly returned is the classic window, and mid-spring is the other one.
Too dry is the common mistake. A mulch holds in whatever moisture is already there, so put it on dry ground and all you've done is preserve the dryness. A shower then soaks into the mulch and evaporates back off it before it reaches the soil at all; it takes a real downpour to get through. If the ground is dry and you're set on doing it, soak it properly first.
Too wet is the likelier mistake here, on the clay. Mulching sodden ground caps the wet in and keeps it cold and airless. Moist means damp when you push a finger in, not standing water. Frozen ground is the same problem again — you'll insulate the cold in and the soil will be slower to wake up.
How deep
Two to three inches — 5 to 7.5cm — over the open soil. That's the number worth remembering, and it's more than most people put down.
Thinner than that and weed seeds still find light and the moisture benefit largely goes. Much thicker and on heavy ground you can start to starve the surface of air, particularly with fine material that mats. (The exception is the deliberate smothering above, where depth is the whole point and you're not trying to grow anything in it yet.)
The other half of the rule matters just as much:
Keep it clear of stems and trunks. Leave a mulch-free circle about 10cm (4in) across around any woody stem, and wider round the trunk of an established tree. Mulch heaped against bark holds damp on it, and that is how you get rot at the collar — the mounded "volcano" around a young tree is genuinely damaging, not just untidy. Don't bury the crowns of perennials either.
Weed first, properly. Every perennial root out, not just the tops off. Two inches of mulch over ground elder buys you a month.
Edge first too. It's far easier to cut a clean edge before the mulch goes down than to tidy one afterwards.
What to use
For beds and borders, well-rotted organic matter:
Garden compost — free, and the best of the lot if you've got enough of it. Almost nobody has enough of it.
Leaf mould — a year or two of leaves in a heap or a bag. Low in nutrients, superb for soil structure, and effectively free in autumn, which is the one time you have a great deal of raw material lying about.
Composted bark — the tidy option, lasts well, holds its look. You buy it, which is the drawback.
Well-rotted manure — excellent on hungry ground, with one warning below that is worth more than the rest of this article.
The manure warning
Rotted enough is not the only question to ask about manure.
If the hay or silage the animals ate came off a field sprayed with aminopyralid, the residue passes straight through the animal and stays active in the muck for years. Stacking does not break it down. Hot composting does not break it down. It has no smell, so the usual "if it still smells of the farmyard, stack it another year" test tells you nothing about it at all.
What it does is deform everything you then grow in that bed — cupped, fern-like, twisted growth on tomatoes, beans, potatoes, dahlias and sweet peas — and the ground can be out of use for a long time.
So: ask the supplier what has been sprayed on the fields, and if you don't get a straight answer, test before you spread. Mix some of the manure half and half with soil in a pot, sow a few broad beans or put in a tomato seedling, and give it three weeks. Distorted growth means don't put it anywhere.
Wood chip, gravel, slate
Fresh wood chip goes around established plants, not onto ground you're about to plant. Not for the nitrogen reason people repeat — spread on the surface it doesn't rob the root zone. It's that fresh chip, conifer especially, contains compounds that check young plants. Stack it three or four months and that passes off. Straight off the chipper it's a path material, and fine around established shrubs and trees. Keep it off vegetables and soft fruit, which root too shallow for it.
Gravel and slate are a different tool again. They cut evaporation from the surface and slow weeds down, but they feed nothing, and once dust and seed settle between the stones you'll be weeding them anyway. They suit a dry, gritty planting scheme rather than a hungry border.
The autumn version specifically
Autumn is the easy window because the material is right there. Leaves come off the lawn anyway — and leaving them on it costs you a lawn in March — so the same barrow that clears the grass can start next year's leaf mould.
A border mulched in autumn comes out of winter with the worms having done half the work, and wants noticeably less watering the following summer. The payoff turns up about eight months after the effort, which is why the job gets skipped.
Bottom line
Weed it, edge it, wait for the soil to be moist, then two to three inches of well-rotted organic matter, kept off the stems. That's the whole job.
If your borders are past the point where a mulch on its own will fix them, clearing them back first is the honest starting point.
Happy to take it on if you'd rather not — I do border work and autumn clearance across Ledbury, Bromyard and out to Ross-on-Wye. Give me a call.