Quick facts
- Suitable for
- Bare soil in borders, around trees, shrubs, roses and fruit, on vegetable beds after clearing, and on the surface of long-term containers.
- Timing
- Mid-autumn to early spring, when the soil is moist and not frozen. Late October and early November are ideal in most of Ireland; March works too.
- Difficulty
- Easy. The only real skill is judging depth and keeping it off stems.
- You will need
- A barrow, a fork or shovel, and roughly one 40-litre bag per square metre at 5cm depth.
What a mulch actually does
A mulch is a layer laid on top of the soil rather than dug into it. Describe what it does rather than what it is and the value becomes obvious.
It stops light reaching weed seeds, so annual weeds never germinate. It slows evaporation, so the soil under it stays damp for weeks longer into a dry June. It buffers temperature, keeping roots cooler in summer and warmer in a cold snap. It takes the impact of heavy rain, so the surface does not cap and seal. And a biodegradable mulch is dragged down by worms over winter, giving structure and fertility as a by-product.
One layer, four jobs, no digging.
Autumn beats spring because of moisture. A mulch does not add water, it holds what is already there. Lay it in late October on soil still warm and full of rain and you seal that in. Lay it on dry soil in May and you seal the dryness in instead.
Choosing a mulch
Biodegradable mulches
These break down and improve the soil as they go. Use them anywhere you want the ground to get better.
- Garden compost. The best all-rounder for borders and vegetable beds. Free, if you make it. Weed seeds can survive a cool heap, so keep the poorest compost for the back of shrub borders.
- Well-rotted manure. Excellent on hungry ground, roses and fruit. It must be genuinely well rotted — at least a year old, dark, crumbly and odourless. Fresh manure scorches roots and can carry pasture herbicide residues that distort tomato and bean foliage for seasons.
- Leafmould. Two years of stacked leaves in a wire cage gives a low-nutrient, beautifully textured mulch. A conditioner rather than a feed, and ideal around woodland plants, ferns and hellebores.
- Bark and wood chip. Long-lasting and tidy, so it suits paths, shrub borders and the ground around trees. Being high in carbon, it ties up nitrogen where it meets the soil, so keep it on the surface around established woody plants and out of vegetable beds.
Non-biodegradable mulches
These do not feed the soil, but they hold moisture, suppress weeds and change the growing conditions at the surface.
- Gravel and grit. A 3–5cm layer around lavender, Stipa tenuissima, Sedum and alpines keeps the crown dry through an Irish winter. Given our rainfall, that matters more here than the drought protection it is famous for elsewhere.
- Slate or stone chippings. The same function with a different look, and useful in contemporary planting.
We gravel-mulch anything Mediterranean on our own projects as a matter of course, because the plant almost never dies of cold — it dies of a wet collar.
How to lay it
- Weed first, thoroughly, including perennial roots. A mulch stops weeds germinating; it will not kill established bindweed or scutch grass.
- Check the soil is moist. If it is not, wait for rain.
- Spread evenly to 5–7.5cm. Thinner than 5cm and light gets through and weeds come anyway. Much thicker on heavy clay and you risk holding too much water at the surface.
- Leave a clear collar of 5–10cm around shrub stems, tree trunks and the crowns of perennials. Mulch heaped against bark keeps it permanently damp and invites rot at exactly the point the plant cannot afford it.
- Do not dig it in. Leave it and let the worms do the work.
When not to mulch
- Waterlogged ground. Mulch keeps water in. On a site that is already sitting wet, it makes the problem worse. Sort the drainage first.
- Frozen ground. Mulching over frozen soil insulates the cold in and delays spring warming.
- Dry soil. You are locking in a drought.
- Right up to a house wall. Keep mulch below the damp-proof course.
- Over emerging bulbs. Mulch before they push through, not after, or mulch lightly and let them come up through it.
Common mistakes
- The "mulch volcano" — a cone of bark piled against a tree trunk. It is the commonest killer of young trees we see, and entirely avoidable.
- Mulching a weedy bed and expecting the weeds to give up.
- Using fresh wood chip or unrotted manure straight onto a growing bed.
- Laying 2cm and calling it done. Below 5cm you get the tidiness and none of the benefit.
- Mulching in high summer on soil that has already dried out.
If you are unsure which mulch suits your borders, bring a photograph in and one of our team will talk it through with you.
