How to Make a Raised Bed for Gardening

A raised bed is easy to knock together and easy to get wrong. Two numbers decide whether it works: 1.2m wide, so you can reach the middle from either side without treading on the soil, and deep enough for whatever you intend to grow in it.

Late winter into early spring is the right moment to build one. The garden is quiet enough to measure and mark out, and close enough to the season that you will actually plant it.

• Sizes that work
• Choosing the timber
• Building it
• Where to put it
• What to fill it with
• Beyond vegetables
• Frequently asked questions

Sizes that work

Width. 1.2m is the standard for a reason: it puts the centre within arm's reach from both sides for most people. If the bed sits against a wall or fence and you can only reach it from one side, halve it to 60cm.

Depth. This is the decision people get wrong, usually by building too shallow.

• 15cm to 20cm: salads, herbs and annual flowers, and only over decent soil the roots can grow down into
• 30cm: the general-purpose depth. Most vegetables, and the sensible default
• 45cm to 60cm: root crops, permanent planting, or beds over concrete and hardstanding where there is nothing underneath to root into
• 75cm or so: standing height, for anyone who would rather not bend at all

Length. Whatever fits. Beyond about 2.4m, put a brace across the middle, because wet soil pushes hard against long sides and they will bow outwards within a season or two.

Choosing the timber

Naturally durable timber lasts longest without treatment: oak, larch and sweet chestnut all cope with ground contact for years. Scaffold boards are the popular choice because they are cheap, thick and already weathered, and at roughly 225mm deep two boards stacked give you a 45cm bed.

Treated softwood is fine for edibles by modern standards, since the copper-based preservatives used today are not the creosote of forty years ago. If it worries you, line the inside faces with a permeable membrane so the soil is not in direct contact.

The one thing to avoid outright is old railway sleepers. The genuinely old ones are saturated with creosote, which leaches, smells in hot weather and is not something to grow food in. New "sleeper-style" timbers are a different product and are fine.

Building it

1. Mark out the footprint with string and pegs, and check the diagonals match so it is square.
2. On grass, lay cardboard across the base to smother what is underneath. Do not line the bottom with plastic, because you want worms moving up into the bed.
3. Drive a 50mm square post into each corner, and at the midpoint of any side over 2.4m. Leave them proud until the boards are on, then cut flush.
4. Screw the boards to the inside faces of the posts, using exterior-grade screws. Nails work loose as the timber moves.
5. Check the top edges are level before you fill, because you will not be able to adjust it afterwards.

An afternoon's work for a single bed, and the levelling is the only fiddly part.

If you would rather watch it done, David has filmed the whole build: how to build timber raised beds.

Where to put it

Raised beds warm up earlier in spring and drain more freely than open ground, which is most of their appeal. The flip side is that they dry out faster, and that shapes where you put them.

• Sun: most vegetables want full sun. Shade-tolerant planting is perfectly possible, but decide which you are doing before you build.
• Shelter: an exposed site costs you moisture and stresses the plants. A hedge or fence upwind is worth more than it looks.
• Water: put the bed within reach of a tap or a butt. You will be watering more than you would in open ground, and a long walk with a can is how beds get neglected in July.

Raised beds also let you sidestep bad ground entirely. Compacted soil, contamination or persistent wet all become irrelevant when you lift the root zone above them.

What to fill it with

Not compost alone, which is the most common and most expensive mistake. Pure compost slumps, dries out hard, and runs out of structure by the second season.

A blend that holds up:

• Topsoil for mineral content and long-term structure, making up the bulk of it
• Composted organic matter for fertility and moisture retention, at roughly a third to 40 per cent
• Grit or sharp sand where you specifically need drainage, such as for alpines or Mediterranean herbs

Buy peat-free. Expect the level to drop by ten to fifteen per cent over the first year as it settles, so fill slightly proud, and top up each spring with homemade compost or leaf mould rather than digging.

For smaller beds and containers, coir blocks are a tidy way to buy growing medium: a 650g block expands to around 10 litres with water, so there is no part-used bag going hard in the shed. Those are in Garden Supplies.

Beyond vegetables

Raised beds get filed under vegetable growing, and that undersells them badly.

Because you control what goes in, each bed becomes its own micro-habitat. An ericaceous mix lets you grow blueberries and rhododendrons in a garden with alkaline soil. A gritty, free-draining blend suits alpines and Mediterranean herbs that would rot in heavy clay. A richer, moisture-retentive mix serves leafy crops and moisture-loving perennials. You stop fighting your garden's soil and start choosing it.

They also work as structure. Beds define paths, divide a space, frame a view and add height to a flat garden, and raised planting brings small, detailed plants up to eye level where you can actually see them. Repeat the same shape or material across a garden and it reads as deliberate.

The accessibility case is the one most worth making. A bed built to a comfortable working height, with a firm edge to steady yourself on and a clear path around it, keeps people gardening for years longer. Build higher for standing and lower for a seated gardener.

There is far more on all of this in David's own The Raised Bed Book, which covers siting, materials, depths, peat-free mixes and planting in detail. We stock it signed, with the option of a personal dedication, alongside the rest of our gardening books.

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Frequently asked questions

How wide should a raised bed be?

1.2m if you can reach it from both sides, which puts the centre within arm's reach for most people and means you never have to stand on the soil. Treading on a bed compacts it, which undoes the main advantage of building one. If the bed backs onto a wall or fence and is only accessible from one side, make it 60cm instead. Length is far less critical, but anything over about 2.4m wants a brace across the middle to stop the sides bowing outwards under the weight of wet soil.

How deep does a raised bed need to be?

30cm is the sensible general-purpose depth and suits most vegetables. You can go down to 15cm or 20cm for salads, herbs and annual flowers, but only over reasonable soil that roots can grow down into. Over concrete or hardstanding, or for root crops and permanent planting, go to 45cm or 60cm, since whatever is in the bed is all the plants get. For gardening without bending at all, build to around 75cm, which is a comfortable standing working height.

What timber should you use for a raised bed?

Naturally durable species last longest untreated: oak, larch and sweet chestnut all cope with years of ground contact. Scaffold boards are the popular practical choice, being thick, cheap and already weathered, and at around 225mm deep two stacked boards give a 45cm bed. Modern treated softwood is acceptable for growing food, since today's copper-based preservatives are not the creosote of decades past. Avoid genuinely old railway sleepers, which are saturated with creosote that leaches and smells in hot weather.

Do you need to line a raised bed?

Line the sides, not the base. A permeable membrane against the inside faces is worth adding if you are using treated timber and would rather the soil was not in direct contact with it. The base should stay open so worms and roots can move between the bed and the ground beneath, which is what keeps the soil alive. If you are building over grass, lay cardboard across the bottom to smother it. It breaks down within a season and lets everything through.

What should you fill a raised bed with?

Not compost alone, which is both expensive and a poor long-term filling, because it slumps, dries hard and loses structure by the second season. Use topsoil as the bulk of the mix for mineral content and structure, with composted organic matter at roughly a third to 40 per cent for fertility and moisture retention, and grit or sharp sand added only where you need extra drainage. Buy peat-free, fill slightly proud since it will settle by ten to fifteen per cent in the first year, and top up each spring rather than digging.