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Mulch beds in late winter or spring onto moist, weed-free soil, keeping it clear of plant stems and tree trunks. Bark suppresses weeds and lasts 2-3 years; compost feeds the soil and vanishes into it in one, which is the point. Gravel mulches want a membrane beneath.
What The Mulch Calculator Works Out
- Volume needed: bed area multiplied by depth. 20 m2 at the standard 75mm takes 1.5 m3. Depth is the specification that does the work: 50mm for an annual top-up, 75mm for a proper new layer, 100mm where weed suppression is the mission; thin scatterings underperform everywhere.
- As bags: the volume in 70 litre bags or 0.73 m3 bulk bags, both rounded up. Twenty-odd bags or two bulk sacks for the example, and the bulk sack wins on price for anything past about half a cubic metre.
- Weight for delivery: density decides the delivery and the barrowing. Bark runs light at about 250 kg per m3, composts around 600 kg, gravel 1.6 tonnes, which is why the same barrow load feels so different.
- Custom depth and allowance (advanced): any depth in millimetres, plus a percentage for settling and the inevitable spillage. Bark and chip settle 10% to 20% in the first months; add 10% if you want the finished depth to match the number.
- Bag size (advanced): bark comes in 50 to 100 litre bags and compost in 40 to 70; choose the size you will buy so the count and cost are right.
- Bagged versus bulk cost (advanced): whole bags at your price against bulk bags delivered. Typical UK prices are GBP 6 to GBP 10 for a 70 litre bag of bark and GBP 90 to GBP 140 for a bulk bag, so the 20 m2 bed costs about GBP 180 in bags or GBP 220 for three bulk bags with plenty spare; the depth table shows which route is cheaper at each depth.
- Barrow loads (advanced): the volume in 80 litre wheelbarrow loads and the weight of each, which turns a kerbside bulk bag into a morning's work.
- Weed membrane (advanced): gravel mulches want a woven membrane beneath, so the bed area is costed at your price per m2 (about GBP 1 per m2 for standard grade). Organic mulches go straight onto moist, weeded soil, kept clear of stems and trunks; the "mulch volcano" piled against bark rots it.
Further reading: RHS: mulches and mulching.