Mulch Calculator

How deep should mulch be?

Two to four inches is the range recommended by most university extension services, and three inches is the safest default for vegetable beds, flower beds and around shrubs. Less than two inches does little to suppress weeds or hold moisture, and it breaks down so fast that you will be re-applying within weeks. More than four inches can hold too much water against stems and shallow roots, and in wet climates it can go slimy underneath. Fine mulches such as compost or leaf mould settle and compact, so they can go on a little thicker; coarse bark holds its structure and does its job at two to three inches. Around trees, spread mulch out to the drip line if you can, but never pile it against the trunk — the “mulch volcano” that you see around street trees traps moisture against bark and invites rot and pests. Leave a clear gap of a few inches around every stem and trunk.

Which mulch should you use?

There is no single best mulch; the right one depends on the job. Bark and wood chips last longest, look tidy and suit paths and shrub beds, but they break down slowly and can temporarily use up nitrogen at the soil surface, so they are better around established plants than around seedlings. Straw is cheap, light and easy to spread, and it is the classic mulch for vegetable beds — it keeps soil cool, suppresses weeds and breaks down into organic matter within a season, but it can contain weed seed if it is actually hay rather than straw, so buy from a trusted source. Compost and leaf mould are the best mulches for feeding the soil, because they improve structure as they rot, but they disappear fast and need re-applying. Grass clippings are free and rich in nitrogen, yet they mat into a slimy layer if spread thick — use thin layers and let them dry first. Shredded leaves are excellent and free every autumn. Whatever you choose, avoid dyed mulches for food beds and avoid fresh wood chips piled deep around young plants.

When to mulch — and when not to

Mulch in spring once the soil has warmed up and seedlings are established; mulching cold spring soil too early keeps it cold and slows growth. In autumn, a fresh layer protects soil over winter, suppresses weeds and feeds soil life as it breaks down. Do not mulch bone-dry soil in a drought — water the bed first, because mulch slows evaporation but does not add water, and a dry layer can even shed rain away from the roots. Do not mulch a bed you plan to direct-sow in the next few weeks, since seeds need warm, bare soil to germinate; sow first, then mulch around the seedlings once they are a few inches tall. And remember that mulch is not a substitute for watering: it reduces how often you need to water, it does not remove the need. Use the watering calculator to work out how much water the bed still needs.

Buying mulch: bags or bulk?

Bags are convenient for small areas and for carrying home in a car, and the bag size is printed on the label — usually 2 or 3 cubic feet. For anything over about a cubic yard, bulk delivery is much cheaper per cubic foot; one cubic yard at three inches deep covers roughly 108 square feet, which is a useful rule of thumb when you are standing in the yard deciding how much to order. If you are filling a new bed first, work out the soil with the raised-bed soil calculator and the soil volume calculator, then mulch on top. For planting dates and what to grow under the mulch, see the planting calendar and the full list of garden tools.