Build beds that outlive the hype

Raised beds solve the three biggest city-growing problems at once: bad ground, contaminated ground, and ground that isn't yours to dig. You control the soil from day one, drainage is built in, and nobody's knees touch the pavement. They also make a space look intentional, which matters when neighbours and landlords are watching.

Wood choice is where most builds go wrong. Use untreated timber - larch, oak, or cedar last 10-15 years untreated; pine gives you 5-8, which is still plenty. Skip pressure-treated lumber for food beds, especially anything old enough to contain CCA (chromated copper arsenate), and skip railway sleepers - they're soaked in creosote. If free pallets are your budget, look for the HT stamp (heat treated), not MB (methyl bromide).

Size for human arms, not aesthetics. Maximum 120 cm wide so you can reach the middle from either side without stepping in - stepping in compacts the soil and undoes your work. Length is whatever the space allows. Depth: 20 cm minimum for salads and herbs, 30 cm for most vegetables, 40+ cm if you're growing carrots and parsnips or building on concrete.

Don't buy soil by the bag - fill the bulk for free. The bottom half of a deep bed can be logs, branches, twigs, and coarse woody stuff (this is the hugelkultur trick - the wood holds moisture and feeds the bed as it rots for years). On top of that, layer like lasagna: cardboard, then alternating greens and browns - grass clippings, leaves, half-done compost, manure if you can get it.

Only the top 15-20 cm needs to be the good stuff: a mix of roughly equal parts topsoil and compost. That's where the seeds and seedlings live for the first season. Everything underneath is a slow-release pantry that improves every year as it breaks down.

Expect the whole bed to sink 10-20% in the first months as the layers settle and the wood compresses. That's normal, not failure. Top it up with compost each season and the level holds. By year two, the worms will have moved in and the lasagna layers will be one dark, living mass.

On concrete or a rooftop, line the base with a permeable barrier (landscape fabric, not plastic sheet) so soil stays in and water gets out. And before you build heavy beds on any roof - read the city-rules guide first. Wet soil weighs roughly a tonne per cubic metre, and roofs have opinions about that.