Filling an allotment raised bed: choose the growing mix before the bag count

A bed can hold the right number of litres and still contain an unsuitable growing mix. Decide what you will grow and how the bed drains before ordering bags described simply as "compost".

Describe the bed and crop

Record its depth, whether it sits over soil or a hard surface, and what you plan to grow. Keep your site's rules and any structure or drainage questions with the plan. Don't assume a supplier can choose the right material from the bed's length alone.

The RHS says suitable bed size and depth depend on what you grow: 30 cm is enough for dwarf bulbs, salad leaves and strawberries, while fruit bushes, shrubs and small trees need 45 cm or more. Its general filling mix is mostly topsoil with smaller shares of organic matter and horticultural sharp sand. It isn't a recommendation to fill every bed with one soil improver.

Read material names carefully

Ask suppliers what is being sold: topsoil, garden compost, manure, a potting compost or a premixed bed-fill. Keep the description and intended use. Similar bag artwork doesn't mean two materials do the same job.

The RHS also notes that horticultural sharp sand is not the same as builders' sharp sand. Use the material the mix specifies, not something that shares part of the name.

Avoid turning one recipe into a rule

The RHS gives a general mix and adjustments for particular needs: more sand for plants that want sharp drainage, and a more free-draining mix for beds on hard surfaces. Check what your crops need and what lies beneath the bed before treating any example as universal.

If you're reusing existing soil or adding home-made organic matter, note its origin and condition, and ask for advice on anything uncertain. Don't bury unknown waste to cut down on bought fill.

Calculate only after the specification

Once you've agreed the usable depth and the mix, calculate each quantity in consistent units, using the supplier's sale basis. Keep bed volume, component proportions and bag rounding as separate steps. Our plot planner gives a bed volume to start from. A volume total says nothing about the weight a structure or surface can carry.

Allow for settling and final level as your supplier or source advises. Don't invent a universal extra percentage because a calculator has an allowance box.

Make delivery practical

Agree where materials can be placed and how they'll reach the plot without blocking paths or damaging shared access. Check the site's rules on deliveries and machinery before ordering a large load.

Use a brief order sheet: bed and crop, underlying surface, chosen mix, supplier's description, quantity calculation and delivery plan. Keep a note of the mix so a later top-up doesn't quietly change it.

The first decision is what the roots will grow in. Counting bags comes after that.

FAQs

Is "compost" a complete description of every raised-bed fill?

No. The RHS describes a general mix of topsoil, organic matter and horticultural sharp sand, with adjustments for particular plants. Specify the actual material.

Is horticultural sharp sand the same as builders' sharp sand?

No. The RHS says they are not the same.

Should I choose the fill before knowing what the bed will grow?

No. The RHS ties bed depth and mix choices to the plants. Decide the crop first.