GardenCalc
“Compost” is three different products sharing one word, and buying the wrong one is the commonest expensive mistake in the aisle. Garden compost conditions soil. Potting compost grows plants in containers. Bark and chippings are a mulch that sits on top and is not soil at all.
Biodegradable mulch only works at a depth most people never apply
This is where the money goes wrong. The RHS is specific: biodegradable mulches “need to be between at least 5cm (2in) and ideally 7.5cm (3in) thick”.1 A scattering looks tidy and does close to nothing — it will not hold moisture and it will not stop weeds. Too thin is worse than none, because you paid for it.
At the right depth it earns its keep: mulches “help soil retain moisture in summer, rain to penetrate the soil in winter, prevent weeds from growing and protect the roots of plants in winter”.1 Timing matters too — best applied “from mid- to late spring, when annual weeds have not yet germinated and herbaceous plants are dormant, and autumn”.1
People budget by area and forget depth. Going from a 2cm scatter to a working 7.5cm is nearly four times the material. Work out the volume before you buy, not in the car park.
Which one your job needs
Tired beds that grow weakly: garden compost dug in or spread as a surface layer. Containers and pots: potting compost, which is formulated for the job and not a soil substitute in the ground. Suppressing weeds and holding moisture: bark or chippings, at the depth above.
If you are filling something rather than feeding it, that is a soil question, not a compost one — which soil do you need covers that choice.
The three tools here, and when each one helps
- Compost calculator — how much compost a bed or border takes, in litres and bags.
- Mulch calculator — volume at a working depth, so you buy enough to actually do something.
- Bark chippings calculator — bark specifically, including paths and play areas.
Sources and checking notes
Every figure here was read off the source named, on 19 September 2026.
- Royal Horticultural Society — Mulch. rhs.org.uk/soil-composts-mulches/mulch. Checked 19 September 2026: biodegradable mulches “need to be between at least 5cm (2in) and ideally 7.5cm (3in) thick”; they “help soil retain moisture in summer, rain to penetrate the soil in winter, prevent weeds from growing and protect the roots of plants in winter”; best applied “from mid- to late spring, when annual weeds have not yet germinated and herbaceous plants are dormant, and autumn”.