Ask ten gardeners why they built raised beds and most will say something about not having to bend so far. That is a genuine benefit, but it is not the reason raised beds work. A raised bed is, first and foremost, a drainage device: lifting the root zone above the surrounding ground means water leaves faster, air gets in sooner, and the soil warms weeks earlier in spring. Everything else — the imported soil, the tidy edges, the easier reach — follows from that. Late August and September are the ideal months to build one, because a bed made now settles over winter and is ready to sow the moment spring arrives.
- Best width 1.2 m — reachable from both sides without stepping in
- Minimum depth 15 cm over soil, 30–45 cm over paving or concrete
- Fill mix roughly 60% topsoil, 30% compost, 10% grit
- Best time to build autumn — the fill sinks about a fifth in year one
What a raised bed actually changes
Three things happen the moment you lift a growing area above the surrounding ground. Drainage improves, because gravity now has somewhere to take the water. Soil temperature rises, because a raised mass of soil has more surface exposed to the sun and less contact with cold subsoil — typically worth two to three weeks in spring. And the soil stops being compacted, because nobody walks on it. That last one is quietly the biggest: compaction destroys the air spaces roots need, and a permanent bed you never tread on develops a crumb structure that open ground rarely matches.
Raised beds also let you sidestep the soil you have. Heavy clay, thin chalk, builder's rubble, a former lawn full of couch grass — a bed lets you grow in something better without the years of improvement that soil preparation would otherwise demand. That is why they suit new gardens so well.
Getting the dimensions right
Width matters more than anything else. Make the bed 1.2 metres across and you can reach the middle from either side; go wider and you will end up standing in it, which undoes half the point. Length is free — build to fit the space. Leave paths of at least 45 cm between beds, or 60 cm if you want a wheelbarrow through.
Depth depends entirely on what is underneath. Over cultivated soil, 15–20 cm of added depth is plenty; roots simply carry on down into the ground below. Over paving, concrete or heavy compacted hardcore, the bed is the entire root zone, so aim for 30 cm for salads and 45 cm for carrots, parsnips and anything that roots deeply. Building a 45 cm bed over open soil is not wrong, just expensive — you are buying a great deal of fill you did not need.
How to build and fill a raised bed
- Pick a spot with six hours of sun Beds are hard to move once filled. Watch the site for a day first, and run the long axis roughly north–south so taller crops do not shade the rest.
- Clear perennial weeds, then lay cardboard Dig out bindweed and couch grass properly — they will grow straight up through a new bed. Plain cardboard over the base smothers annual weeds and rots away within a season. Our weed control guide has more.
- Build the frame 1.2 m wide Untreated softwood lasts five to eight years, larch or oak far longer. Modern tanalised timber uses copper-based preservatives and is considered safe for food crops — the old creosote worry no longer applies. Add corner stakes on anything over 2 m long, or the sides will bow.
- Fill with a mix, not pure compost About 60% good topsoil, 30% garden or peat-free compost and 10% horticultural grit. Pure compost slumps, dries out at speed and is far too rich for root crops — it is a growing medium, not a soil.
- Overfill by a fifth and let it settle A new bed sinks roughly 20% in its first year as the organic matter breaks down. Filling proud in autumn means it is level by spring rather than a disappointing dip.
- Cover it or sow a green manure Bare soil over winter loses nutrients and grows weeds. Either mulch it or sow a cover crop — see green manures to sow in autumn, which is perfectly timed for a bed built now.
Raised bed or open ground?
Raised beds win for
Intensive, high-value cropping in a small space.
- Salads, spinach and cut-and-come-again leaves
- Carrots and parsnips — stone-free soil means straight roots
- Early sowings, thanks to warmer spring soil
- Wet, heavy or badly drained gardens
Open ground wins for
Space-hungry crops and anything drought-prone.
- Potatoes, pumpkins and squash — too sprawling for a bed
- Sweet corn, which needs a block to pollinate
- Deep-rooted crops on land that already drains well
- Any plot where watering is difficult in summer
That last point is the honest trade-off nobody mentions in the brochures: a raised bed drains better, which also means it dries out faster. In a hot spell, beds need watering when open ground does not. Mulch the surface and water deeply rather than often — the reasoning is in our guide to watering your garden.
What to grow, and how to keep the bed productive
Because you never tread on it, a raised bed can be planted far more densely than rows in open ground. Think in grids rather than lines, and follow one crop straight with another. Lettuce, spinach, radish, pak choi, mizuna and corn salad are the classic quick fillers; carrots, parsnips, leeks and chard hold the long positions. A row of peas or broad beans up a frame at the north end adds a whole extra layer — see supports and trellising. Keeping something in every square is easier with succession sowing, and companion planting is genuinely easier in a bed than a field.
Feeding is a no-dig job. Each autumn, spread 5 cm of compost over the surface and leave the worms to take it down; that both tops up the sinkage and maintains fertility. If you are not making enough of your own, our composting guide is the place to start, and our feeding guide covers what to add during the growing season.
Common mistakes
- Building it too wide — anything over 1.2 m means climbing in, compacting the soil and losing the main advantage.
- Filling with pure compost — it slumps by a third, dries out in days and gives forked, leafy root crops. Blend it with topsoil.
- Leaving perennial weeds underneath — bindweed and couch grass will find their way up through 30 cm of fresh soil with no difficulty at all.
- Forgetting that beds dry faster — the drainage that helps in March is a liability in July. Plan for mulching and, ideally, a hose within reach.
- Under-building the frame — wet soil is heavy. Long sides without a mid-point stake will bow outwards within two seasons.
- Sowing too shallow in fresh fill — new beds have a loose, fluffy surface that dries out fast. Firm it lightly and follow the depths in our guide to direct sowing outdoors.
Frequently asked questions
How deep should a raised bed be?
Over existing garden soil, 15–20 cm is enough, because roots continue down into the ground beneath. Over paving or concrete the bed is the whole root zone: allow 30 cm for salads and shallow crops, 45 cm for carrots, parsnips and anything deep-rooted.
What soil should I fill a raised bed with?
Around 60% good-quality topsoil, 30% compost and 10% horticultural grit. Topsoil supplies mineral structure and water retention, compost the fertility and organic matter, grit the drainage. Avoid filling with compost alone — it is too light, too rich and it collapses.
Is treated timber safe for growing vegetables?
Modern pressure-treated timber uses copper-based preservatives rather than the arsenic and creosote formulations of decades past, and is widely accepted as safe for food crops. If you would rather not, larch, sweet chestnut and oak all last well untreated, and a butyl liner on the inner face is an easy compromise.
When is the best time to build a raised bed?
Autumn. The fill has all winter to settle, the timber is easier to work while the ground is soft, and you can sow a green manure or mulch the surface straight away. Build now and the bed is level, settled and warm by the time the sowing season comes round again.
A raised bed is a season-long shortcut you build once: better drainage, warmer soil, no compaction and a growing medium you chose yourself. Build it this autumn, fill it proud, cover it for winter, and you will be sowing into warm, settled soil weeks before the rest of the garden wakes up. Start with the complete guide to growing vegetables from seed, then browse the vegetable seed range, the herb seeds, our organic vegetables and everything tagged for September sowing. And if you want a project for under glass while the beds settle, our new guide to growing jade vine from seed is the opposite extreme in every way. The rest of our growing guides will keep the beds busy.
