If your outdoor space is mostly paving, a balcony or a modest courtyard, growing your own vegetables can feel like a stretch. It isn't. A single shallow raised bed, no bigger than a doormat and a half, will keep you in cut-and-come-again salad from spring right through to autumn, with a few herbs on the side. This one is built from untreated timber in an afternoon, needs no digging, and can be moved or taken apart if you change your mind about where it lives.

Why keep the bed shallow

Salads and herbs are shallow-rooted. Lettuce, rocket, spinach, chard, radish and most culinary herbs do their best work in the top 15cm of soil, so a deep, heavy raised bed is simply wasted effort, compost and money. Aim for a finished depth of 15–20cm, which is enough for roots to spread and for you to keep on top of watering without needing a wheelbarrow of topsoil.

The size that works hardest in a small garden is roughly 1.2m long by 0.6m wide. That is long enough to sow a couple of short rows, narrow enough to reach the middle from either side without treading on the soil, and light enough that two people can shuffle it across the patio when the mood takes you.

Choosing the right untreated timber

Untreated timber is the sensible choice for a bed growing food. It will weather to a soft silver-grey within a season or two, and although it won't last as long as treated boards, it avoids any doubt about what is leaching into your soil.

• Untreated gravel boards, usually about 150mm by 22mm and sold in 1.8m lengths, are ideal: cheap, straight and just the right depth for a salad bed.
• Larch, Douglas fir and oak are naturally durable and last considerably longer than softwood. Oak is the hardest of the three and the most expensive.
• Avoid old railway sleepers and any reclaimed timber of unknown origin. Creosote and other old preservatives have no place near edibles.
• Check pallet wood carefully. If it is stamped MB, it has been treated with methyl bromide — leave it. Stamped HT means heat treated, which is fine.

Two 1.8m boards will do the whole job. Cut each one into a 1.2m length and a 0.6m length, and you have all four sides with nothing wasted.

Tools and fixings

• Two 1.8m untreated gravel boards, 150mm by 22mm
• Twelve 60mm stainless steel or galvanised screws
• A saw, a drill with a 3mm bit, a screwdriver bit, a tape measure and a try square
• A short length of permeable membrane, if you have some

Stainless steel costs a little more but will outlast the timber, which is exactly what you want from a fixing.

Assembling the bed

Butt the joints rather than mitring them. It is quicker, stronger and far more forgiving of a slightly ragged saw cut. Lay the two long sides out, stand a short side between them, and drill pilot holes through the long board into the end grain of the short one — two screws per corner, one near the top edge and one near the bottom. End grain splits easily, so the pilot holes are not optional.

Before you tighten everything up, measure the two diagonals. If they match, the frame is square; if they don't, rack it gently until they do, then finish driving the screws. Sit the finished frame on level paving. If your patio falls slightly, slip a thin batten or a couple of plastic packers under the low corners so it doesn't rock and twist.

Line the base with a strip of permeable membrane if you have it. Soil won't wash out through the gaps, the paving underneath stays cleaner, and water still drains away freely.

Filling it well

Your finished frame holds roughly 100 litres, which is a manageable amount to buy or mix. Use two parts peat-free multipurpose compost to one part garden soil or loam-based compost, then stir through two generous handfuls of horticultural grit to keep the mix open. If you have well-rotted manure or a slow-release fertiliser to hand, fork a little through the bottom half before you top up.

Fill to about 2–3cm below the rim, so water doesn't run straight off the surface, then firm the mix gently with your hands — not your boots — and water it well. Leave it to settle for a few days before sowing, topping up any dips that appear.

What to sow, and when

Start with a short row of each of the following and you will have a genuinely useful bed rather than a decorative one:

• Loose-leaf lettuce — pick outer leaves and the plant keeps producing for weeks.
• Rocket and spinach — fast, forgiving, and happy in light shade.
• Chard — cut young leaves for salad or leave them to grow for cooking.
• Radish and spring onion — quick crops that fill the gaps between slower rows.
• Herbs — basil, parsley, coriander and chives all thrive here. Keep mint in a pot sunk into the mix, or it will take over.

Sow little and often, every two or three weeks, rather than one large sowing. You will get a steady supply instead of a glut followed by nothing.

One last thing: a raised bed dries out far faster than open ground, especially on a sunny patio. Check the top centimetre of compost daily through summer and water in the cool of the evening. Get that habit right and this small, simple box will earn its place many times over.