Same hole, very different delivery
Fill one raised bed — 4 metres by 1.2, 100 millimetres deep — and the volume is the same whatever you put in it. The weight is not, and the difference is much larger than people expect.
With the standard settlement and waste allowances, that bed takes about 0.61 cubic metres of material. Filled with topsoil that is roughly 0.79 tonnes. Filled with compost, about 0.36. Filled with bark mulch, about 0.29. Filled with decorative stone, about 0.97.
So bark and stone, occupying identical space, differ by a factor of more than three in weight. That matters for the vehicle, for whether a bulk bag can be craned into position, for how many barrow trips someone is doing, and for whether the bed structure or the roof below it can take the load. It does not, notably, change the price much — landscaping materials are usually sold by volume, so you pay for the space and the weight comes along for the ride.
Choose by function first. Then check the weight before you book the delivery.
Organic mulches
These break down over time and feed the soil while they do it. That is the point of them, and it is also why they need replacing.
- Bark and wood chip: the standard decorative mulch. Long-lasting compared with finer organics, good at suppressing weeds, and very light. Coarse bark lasts longer than fine; fine looks tidier.
- Compost: partly a mulch and partly a soil improver. Richer than bark, breaks down faster, and better mixed into the top of a bed than left thick on the surface.
- Leaf mould and well-rotted manure: soil conditioners used as a mulch layer. Excellent for structure, but not a long-lived surface covering.
- Straw: common on vegetable beds. Cheap, light, effective, and disappears within a season.
The trade with all of them is the same. They improve the soil, and they consume themselves doing it. Budget for topping up annually or every couple of years rather than treating it as a one-off.
Two cautions worth knowing. Fresh, undecomposed wood chip in contact with soil can temporarily tie up nitrogen at the interface as it breaks down, which is why it belongs on the surface rather than dug in. And piling any organic mulch against a stem or a trunk holds moisture where it does harm — leave a clear gap around plants.
Inorganic mulches
These do not break down, so they do not feed anything and they do not need replacing.
- Decorative stone, gravel and slate: permanent, heavy, and effective at suppressing weeds while looking deliberate. Best over a permeable membrane so the stone does not gradually sink into the soil.
- Rubber and glass chippings: hard-wearing and used mostly for paths and play areas rather than planting beds.
The advantage is longevity. The disadvantages are weight, cost, and that they are genuinely difficult to remove once laid. Stone also stores heat, which suits a Mediterranean planting scheme and does not suit a shade bed.
How deep
Mulch depth is a different question from bed depth, because you are covering a surface rather than filling a root zone.
- Bark or wood chip: around 50 to 75 mm. Thinner and weeds come through; much thicker and water struggles to reach the soil.
- Compost as a surface mulch: 25 to 50 mm.
- Decorative stone: around 40 to 50 mm, which is enough for coverage without becoming a wading experience.
- Around new planting: keep the mulch clear of stems and crowns regardless of depth.
Depth is the number that decides your order, and mulch is one of the few materials where laying it too thick is actively harmful rather than merely wasteful.
Estimating it
The arithmetic is the same as any other spread material — area times depth, plus allowances — but two things are specific to mulch.
Settlement is larger for organic mulches than for anything else on the list, because they consolidate and then continue to decompose. The standard 15 percent allowance is a reasonable starting point for the initial settling, but the ongoing loss is not an estimating problem, it is a maintenance schedule.
Decorative stone barely settles at all. There is nothing soft in it and nothing to rot, so if you are laying stone you can reasonably drop the settlement allowance well below the default. The waste allowance still applies — stone spills, and it rolls further than soil does.
Choosing, in one paragraph
If the soil needs improving, use an organic mulch and accept that you will be topping it up. If the bed is planted and you want it to look after itself with minimal weeding, bark is the usual answer. If you want a permanent, low-maintenance surface and the structure can take the weight, use stone. And if you are working above ground level — a roof terrace, a balcony, a bed on a structure — the weight figures at the top of this guide are not a detail, they are the constraint the whole design has to satisfy.