Raised bed gardening: the complete guide

Quick facts about raised beds

Contents

• Quick facts about raised beds
• Why garden in raised beds?
• Bed size, shape and how many beds you need
• Is pressure-treated lumber safe for raised beds?
• Where should you put a raised bed?
• The base: grass, fabric and hardware cloth
• How to fill a raised bed from the bottom up
• How much soil does a raised bed need?
• Bed depth and working height
• Watering: why raised beds dry out faster
• Feeding and topping up the soil every year
• Row cover, hoops and season extension
• Zone table: the raised bed season in zones 3–6
• Winter care from October to March
• What can you grow in a raised bed?
• Settling, drying out, slugs and voles
• Common mistakes
• Frequently asked questions

Why garden in raised beds?

You choose the soil. Heavy clay, stony glacial till, thin soil over bedrock, compacted builder's fill — none of it matters once the bed is filled. You start from zero instead of from minus.

The soil warms and drains earlier. Clemson and Utah State extension both note that raised beds warm faster in spring. In zones 3 to 5 that is typically one to two weeks of workable soil ahead of the open ground.

You never step on the growing soil. At 120 cm (4 ft) wide you can reach the middle from either side, so the soil stays uncompacted and needs no digging.

Everything is easier to manage. A defined rectangle is easier to plan, weed, water and cover, and it makes crop rotation straightforward. The drawbacks are real too: the soil dries out faster, you buy soil in the first year, root depth is capped by the frame, and untreated wood in soil contact does not last forever.

Bed size, shape and how many beds you need

Width is the only dimension with a hard rule: no wider than 120 cm (4 ft) if you can walk around the bed, no wider than 75 cm (30 in) if one side is against a wall. The RHS puts the limit at 1.5 m (5 ft), but 120 cm is where an average adult reaches the centre line without putting a knee in the soil. Length is free — 1.2 × 2.4 m (4 × 8 ft) is the standard workhorse at 2.9 m² (32 sq ft), popular because that is how lumber comes, so nothing is cut or wasted. A 1.2 × 1.2 m (4 × 4 ft) bed is a good first one.

How many beds?

• One bed. Salad, herbs, radishes and a couple of tomato plants.
• Two or three. Salad, herbs, beans and some roots all season. The usual starting point.
• Four to six. It shows in the grocery bill, with room for a four-block rotation.
• Eight or more. A kitchen garden in its own right; one to two hours a week in peak season.

Two well-filled beds beat six half-filled with cheap soil. Plan the paths before you build: 45 cm (18 in) minimum, 90 cm (3 ft) if a wheelbarrow has to get through, which it will every spring. Surface them with wood chip or gravel over fabric.

Is pressure-treated lumber safe for raised beds?

The most asked question in raised bed gardening, and the honest answer has two halves. The old worry was arsenic: chromated copper arsenate, CCA, was withdrawn from residential lumber in the United States in 2003, so anything sold for garden use since is copper-based — ACQ, copper azole, micronized copper. Clemson extension advises lumber labelled UC4A or higher, and warns against railroad ties, which are creosote-treated and should never go near food crops.

The remaining question is copper. Some leaching always occurs, which is why several European gardening bodies advise against treated wood in food beds. On the other side, research summarised by Swedish wood protection authorities, referencing the Swedish University of Agricultural Sciences, found no effect on plants or root development, and noted that garden soil often already contains more copper than leaches from the timber over its life. Utah State extension suggests a plastic barrier between treated lumber and soil. The risk is small but not zero, and easy to avoid.

Four ways to sidestep the question

1. Use naturally durable wood. Cedar, cypress, black locust and dense larch last 10–15 years untreated — more up front, less over a decade.
2. Line the inside. Heavy polythene stapled to the inner faces separates soil from timber. Leave the bottom open so water drains.
3. Char the inner faces. Scorching the soil-facing side with a torch until it is black slows rot fungi without adding anything to the soil.
4. Go non-wood. Galvanised steel beds last 20 years or more and concrete blocks indefinitely.

Lumber dimensions

For a wooden frame, 2×10 or 2×12 boards are the practical choice: a 2×10 gives a 235 mm (9.25 in) deep bed and a 2×12 gives 286 mm (11.25 in), neither needing stacking. Use 100 mm (4 in) deck screws into a 4×4 corner post rather than screwing board ends together, which pulls apart within two seasons. Untreated pine, spruce or fir lasts three to seven years in soil contact, and moisture decides almost everything — setting the frame on paving slabs, so the lowest board is never permanently wet, is the cheapest life extension there is.

Where should you put a raised bed?

Sun. Six hours of direct sun is the floor. Fruiting crops — tomatoes, cucumbers, squash, peppers, beans — want eight or more; leafy greens and herbs manage on five. Check the site rather than guessing: look at it at 9am, 1pm and 5pm on a clear day. A tree that is bare in April casts heavy shade by July.

Orientation. Run the long axis north to south, as the RHS recommends: the sun then tracks across the bed and every plant gets an equal share, while an east–west bed puts the north row in the shade of the south row as soon as plants get tall. If the site forces east–west, put the tallest things along the north edge.

Water and proximity. Run a hose to the site before you fill the bed. A raised bed needs water two or three times a week in midsummer, and if that means carrying two cans across the garden it will not happen — the most common reason raised bed gardens are abandoned after one season. A bed you walk past every day also gets tended; slugs and drought are caught in time only if you go by.

The base: grass, fabric and hardware cloth

On lawn

You do not need to strip the turf. Mow it short, set the frame in place and line the bottom with ten layers of flattened cardboard, overlapping seams by 20 cm (8 in). Water it so it stays put. The grass dies from lack of light and becomes worm food, and the cardboard breaks down over a season — exactly what you want, because roots can then reach the native soil.

On perennial weeds or hard surfaces

Cardboard will not stop couch grass, bindweed, horsetail or ground elder. Use woven landscape fabric, or dig the roots out thoroughly first.

On concrete, asphalt or a deck, two conditions apply. Drainage: lay 5 cm (2 in) of coarse gravel with filter fabric over it. Depth: roots cannot travel downward, so you need at least 30 cm (12 in), and 45–60 cm (18–24 in) for tomatoes, peppers and squash, matching the University of Maryland recommendation for hard surfaces. Check the load — a filled 4 × 8 ft bed weighs 360–550 kg (800–1,200 lb) and belongs over a joist.

Hardware cloth against voles and gophers

Burrowing rodents do the damage that hurts most, because you see nothing until the carrots are hollow and a row of lettuce has vanished overnight. If you have voles or gophers, galvanised hardware cloth in the base is the only reliable answer.

Lay 13 mm (1/2 in) mesh across the whole footprint before filling, or 6 mm (1/4 in) against mice. Run it up the inside of the frame at least 5 cm (2 in) and staple it, or animals get in at the edge. Plastic netting is chewed through and 25 mm chicken wire lets a vole pass. The mesh cannot be added later without emptying the bed, so if unsure, fit it anyway.

How to fill a raised bed from the bottom up

Filling the whole frame with bagged mix works, but it is expensive and not the best soil you can build. Layering is better, on the hügelkultur principle: slow-rotting bulk at the bottom that holds water like a sponge, growing soil on top.

1. Set the frame and check it is level. A bed on a slope waters unevenly, and you notice only when one end stays dry.
2. Fit hardware cloth if you have rodents. 13 mm (1/2 in) mesh, stapled 5 cm up the inside. This step cannot be taken later.
3. Line the base with cardboard. Ten layers, 20 cm (8 in) overlaps, tape removed, watered through.
4. Add coarse material. Pruned branches up to wrist thickness, twigs and old stems, filling a quarter to a third of the depth. Do not compress it.
5. Fill the gaps with leaves and rough compost. Autumn leaves are free and excellent. Water each layer as you go.
6. Add a manure layer. Five to ten centimetres (2–4 in) of well-rotted manure. Fresh poultry manure will burn roots.
7. Top with growing soil, 3–5 cm (1–2 in) above the rim. The top 20–25 cm (8–10 in) is what roots live in: two parts topsoil to one part finished compost. It will settle.
8. Soak it thoroughly. 150–250 litres (40–65 gallons) in a new 4 × 8 ft bed. Dry bagged mix is water-repellent and has to be wetted through once.
9. Let the bed settle one to two weeks before planting, as the RHS advises. Decomposition starts and you can see where to top up.

Getting the mix right

The variable that matters most is organic matter. University of Maryland extension puts it at 25–50 percent by volume, Clemson lower at 10–20 percent. Both bracket the same idea: too little gives a dense mix that sheds water, too much a bed that collapses as it decomposes and dries out fast. Filling a bed with pure compost is a mistake, however good the compost.

Check the pH if you buy soil from an unknown source; most vegetables want 6.2–6.8. Blueberries and other acid-lovers need their own bed with ericaceous mix.

How much soil does a raised bed need?

Length times width times depth — arithmetic almost nobody does before ordering. Here it is, done for you.

Two numbers are worth memorising. A 4 × 8 ft bed at 25 cm (10 in) takes almost exactly one cubic yard, and a cubic yard is 18 bags of 1.5 cu ft — which is why bagged soil stops making sense above one bed, bulk delivery being roughly a third of the price.

Layering changes the sum entirely. Fill the bottom half of a 45 cm (18 in) bed with branches, leaves and rough compost and the bought soil drops from 1,360 litres to around 700. That is the strongest argument for building deep: root depth and a comfortable working height for what a shallow bed costs.

Bed depth and working height

Twenty-five to thirty centimetres (10–12 in) is the sweet spot; the RHS puts 30 cm (12 in) as the minimum for salad leaves and strawberries and 45 cm (18 in) for most other things. The frame is not the whole root zone, though: loosen the soil beneath with a fork before filling and roots carry on down, so a 25 cm bed behaves like a 40 cm one. On concrete they cannot, which is why hard surfaces need deeper frames.

Beds above about 45 cm (18 in) need bracing, because wet soil bows the long sides within a season. Use a corner post at every corner, and on beds longer than 2.4 m (8 ft) a cross-tie at half height. Do it before filling.

Watering: why raised beds dry out faster

If you take one thing from this guide, take this: a raised bed dries out faster than you expect, and that is the most common cause of a disappointing harvest. The soil is raised, so water drains actively out. The sides are exposed to air and sun, so evaporation happens from four faces, not just the top. And roots are partly cut off from the water table. Utah State extension states it plainly: raised beds must be irrigated, because they cannot live on rainfall.

The target is 25 mm (1 in) a week, the same as open ground. Over a 4 × 8 ft bed that is 2.9 square metres times 0.025 metres, about 75 litres (20 US gallons) a week. In a hot, windy week it can double, and a deep bed of full-grown tomatoes takes 30 litres (8 gallons) a day.

Water deeply and rarely rather than shallowly and often; a deep soaking pulls roots downward, while a daily splash keeps them in the top few centimetres where soil dries first. Newly sown seed is the exception. Water at the base, and check with a finger — the surface can look bone dry while it is damp 5 cm (2 in) down.

The two things that make the biggest difference are drip line and mulch. A soaker hose on a timer cuts consumption sharply and makes a July holiday possible; 15 m (50 ft) covers two beds. Mulch is free and nearly as effective: 3–5 cm (1–2 in) of clippings or straw halves evaporation and suppresses weeds.

Feeding and topping up the soil every year

A raised bed is a closed system: what the harvest removes has to go back, or yields fall year on year. Before the season starts, spread 5–10 cm (2–4 in) of well-rotted compost or manure across the surface and work it lightly into the top few centimetres. On a 4 × 8 ft bed that is 150–300 litres (5–10 cu ft), replacing both nutrients and the volume lost to settling. Add granular organic fertiliser at the same time, and do not dig the bed over.

Heavy feeders — tomatoes, cucumbers, squash, brassicas — need more than the base dressing carries; give liquid feed every two weeks from midsummer. Leafy crops want nitrogen, fruiting crops potassium once flowering starts. Beans and peas need none, since they fix their own.

Topping up

Raised bed soil sinks. It decomposes, it compacts and some leaves with the harvest. Expect 5–10 cm (2–4 in) of settling in the first year, then 2–5 cm (1–2 in) a year after.

Here is the conversion that makes it manageable: on a 4 × 8 ft bed, every centimetre of settling is about 29 litres, so an inch is roughly 75 litres or 2.7 cubic feet — two bags. In practice you add 75–150 litres (3–5 cu ft) per bed per year, as compost rather than plain potting mix.

The free alternative is to top up with organic material in autumn. Leaves, clippings and half-finished compost go in during October and November, heaped above the rim, and have broken down by May. Do not replace the soil as routine — a bed that gets compost every year improves with age.

Row cover, hoops and season extension

Floating row cover is the most versatile material. It passes light, air and water while raising the temperature underneath by 2–4 °C (4–7 °F) and holding off frost to about −2 to −4 °C (25–28 °F). Lay it loosely and weight the edges; plants lift it as they grow. Its second function is often more valuable: it excludes carrot rust fly, cabbage white and flea beetles, as in the guide to growing carrots. For summer heat use finer mesh: 1.3 mm stops cabbage white, 0.8 mm flea beetles and aphids too.

Hoops of plastic or galvanised wire push into the soil along the long sides at 50–60 cm (20–24 in) intervals; five per 2.4 m (8 ft) bed makes a stable frame. Over them goes row cover in spring, insect mesh in summer and clear plastic in autumn. A polycarbonate lid works as a cold frame and gives 4–6 °C (7–11 °F) on a sunny spring day. Ventilate it: a closed lid in April sun hits 30 °C (86 °F) within an hour and cooks the seedlings.

Added up, the effect is larger than people expect: the bed itself gives about a week in spring, row cover one to two more, a lid or plastic hoops two to three more. Autumn works the same way in reverse — four to six extra growing weeks, which in zone 3 or 4 decides whether the cucumbers ripen.

Zone table: the raised bed season in zones 3–6

These are guide dates. Raised bed soil thaws and dries roughly two weeks ahead of open ground on the same site, which the table accounts for. Local conditions outweigh the zone — a bed against a south wall behaves a zone warmer, one in a frost pocket a zone colder.

Two things follow. The spread between zone 6 and zone 3 is about six weeks at each end, so zone 3 has roughly 90 to 120 frost-free days against 180 or more in zone 6. And the colder the zone, the more a raised bed is worth: in zone 6 the head start is convenient, in zone 3 it decides whether there is a crop.

Winter care from October to March

A raised bed looks after itself over winter, but a couple of hours in October buys a noticeably better April.

Never leave the soil bare. Rain caps the surface, nutrients leach and weed seeds get an early start. Leaves are easiest: 10 cm (4 in) across the bed with a few branches on top so it does not blow away. A cover crop is the alternative — sow winter rye or phacelia in late summer and let the frost-killed residue lie as mulch.

Clear what has to go. Foliage from crops with disease problems belongs in municipal waste or a hot heap, not back in the bed; healthy residues can stay. Note what grew where while you remember — rotation is easiest to plan in November: move each block on one place. The guide to companion planting covers combining within a single bed.

Allow for deeper freezing. Raised bed soil freezes harder and faster than ground soil, because cold comes at it from four sides. For annuals that is irrelevant, but perennials — strawberries, chives, rhubarb — meet harsher conditions than in the ground. Mulch those beds with an extra 15 cm (6 in) of leaves or evergreen boughs in zones 3 and 4. Growing rhubarb in a raised bed also needs 45 cm (18 in) of depth.

What can you grow in a raised bed?

Almost anything, with depth as the only real constraint. A 15 cm (6 in) bed handles lettuce, spinach, radish, herbs and strawberries. At 25–30 cm (10–12 in) you open up carrots, beets, onions, beans, brassicas and tomatoes. Deeper is needed mainly for potatoes and parsnips. Avoid sprawling pumpkins and sweetcorn.

The full list, with spacings, plants per bed and planting plans, is in the guide to the best raised bed crops. For the easiest possible start, read the guide to growing lettuce first.

Settling, drying out, slugs and voles

The bed dries out even though you water. Either the water runs down the inside of the frame without wetting the middle, which happens once the soil has shrunk away from the boards — water in several passes until it swells back. Or the mix is too high in peat, which turns water-repellent when dry; blend in mineral soil next spring.

Slugs. The frame edge is ideal slug habitat — damp, dark, sheltered — and they gather in the gap between soil and timber. Patrol in the evening after rain, keep the grass short and do not run mulch up against the wood. Wide self-adhesive copper tape along the top edge works as a barrier.

Voles and mice. They come from below, and hardware cloth in the base is the only reliable answer. If the bed is already filled without it, the options are traps in the paths, lining it next spring, or growing only above-ground crops there.

Hedge and tree roots. A bed near a hedge is threaded with roots within two or three years, and it shows as plants wilting despite watering. Keep at least 2 m (6 ft) from a hedge and 4 m (13 ft) from a tree.

Common mistakes

• Siting the bed where it looks best instead of where the sun is. Under six hours of sun you get thin plants and small harvests. Check the shadows at 9am, 1pm and 5pm before you fill.
• Building wider than 120 cm (4 ft). You end up stepping into the bed, compacting the soil you paid to build.
• Skipping hardware cloth in the base. Ten minutes before filling, essentially impossible afterwards. Use 13 mm (1/2 in) mesh, stapled 5 cm up the inside.
• Filling the whole bed with bagged mix. Expensive, and worse soil than a layered fill. Branches and leaves in the bottom give better structure at a third of the cost.
• Watering shallowly and often. Light sprinklings keep roots in the top few centimetres, where soil dries first. Water deeply — 60–75 litres (16–20 gallons) at a time.
• Not topping up every spring. Beds settle 5–10 cm the first year and 2–5 cm after. Without 75–150 litres of compost per bed per year, yields decline.
• Leaving the soil bare over winter. Nutrients leach and weeds get a head start. Ten centimetres of leaves in October solves it.

Frequently asked questions

How much soil does a 4 × 8 ft raised bed need?
At 25 cm (10 in) deep it takes almost exactly one cubic yard: 755 litres or 27 cubic feet, which is 18 bags of 1.5 cu ft. At 30 cm (12 in) it rises to 905 litres (32 cu ft). Fill the bottom third with branches and leaves and the bought soil drops by a third.

Is pressure-treated lumber safe for a vegetable raised bed?
Arsenic-based CCA was withdrawn from residential lumber in 2003 and modern treatments are copper-based. Research referenced by Swedish wood protection authorities found no effect on plants, though some leaching always occurs, so the risk is small but not zero. To avoid it entirely, use cedar or larch, line the inside with plastic, or choose a steel bed.

What should you put at the bottom of a raised bed?
On lawn, ten layers of cardboard with 20 cm overlaps — the grass dies and worms move up into your new soil. Over that, coarse material: branches, twigs, leaves and rough compost filling a quarter to a third of the depth. Add 13 mm hardware cloth under the cardboard if you have voles. Skip gravel; it does not improve drainage in a raised bed.

How deep should a raised bed be?
Fifteen centimetres (6 in) is enough for salad, herbs and strawberries. Most vegetables want 25–30 cm (10–12 in), and tomatoes, squash and deep-rooted crops do better at 45 cm (18 in). If the bed sits on loosened open soil roots continue downward and a shallower frame works; on concrete, go to 45 cm.

How often do raised beds need watering?
Budget about 75 litres (20 gallons) per 4 × 8 ft bed per week in normal summer weather, which is two deep waterings. In a heat wave that doubles, and a deep bed of mature tomatoes takes 30 litres a day. Water deeply and rarely, and test with a finger 5 cm down.

Do you have to replace raised bed soil every year?
No. Raised bed soil improves with age if you add 75–150 litres (3–5 cu ft) of compost each spring. Replace it only after a soil-borne disease such as clubroot, if fertiliser salts have built up, or if the bed is threaded with hedge roots. Otherwise build on what is there.

Can you put a raised bed on concrete or a deck?
Yes, with two conditions. Drainage: lay 5 cm (2 in) of coarse gravel with filter fabric over it, or the base turns sour. Depth: roots cannot go downward, so you need at least 30 cm (12 in), and 45–60 cm for tomatoes. A filled 4 × 8 ft bed weighs 360–550 kg (800–1,200 lb), so check the load.

How long do wooden raised beds last?
Untreated pine, spruce or fir gives three to seven years in soil contact; cedar, cypress and dense larch 10–15; galvanised steel 20 or more. Moisture decides almost everything, so setting the frame on paving slabs with an air gap under the lowest board extends its life significantly.

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