Raised Garden Bed Guide: Size, Soil Mix and Materials That Last

A raised garden bed works when it is 4 ft (1.2 m) wide, 8 to 12 ft (2.4 to 3.7 m) long, and 12 to 18 in (30 to 46 cm) deep, filled with a mix of 60 percent topsoil, 30 percent compost and 10 percent aeration material — get those three numbers right and the bed drains well, warms early, and grows vegetables that outperform an in-ground row. Price the fill with the Topsoil Calculator, price the base layer with the Gravel Calculator, compare upkeep costs in the garden maintenance cost guide, and pair the bed with storage using the garden shed buying guide so tools and soil stay close to where you garden.

Size and placement: the dimensions that make a bed easy to garden

Build raised beds 4 ft (1.2 m) wide, because 4 ft is the widest bed a person can reach across from either side without stepping into it — the center of the bed is 2 ft (61 cm) from each edge, which is a comfortable arm's reach for planting, weeding and harvesting. Beds against a fence or wall should be 2 to 3 ft (61 to 91 cm) wide, since they can only be reached from one side. Length is flexible: 8 ft (2.4 m) is the standard because it matches common lumber lengths and gives about 32 sq ft (3 sq m) of growing space, while 12 ft (3.7 m) suits a long row of one crop. Paths between beds need 2 to 3 ft (61 to 91 cm) for a wheelbarrow — 2 ft is enough for a person, 3 ft is enough for a wheelbarrow and a kneeling gardener side by side — and the whole bed area needs 6 to 8 hours of direct sun a day for vegetables, with morning sun preferred because it dries dew early and reduces fungal disease.

Depth decides what the bed can grow. A 6 in (15 cm) deep bed suits lettuce, herbs and radishes, which root in the top few inches. A 12 in (30 cm) deep bed suits most vegetables — tomatoes, peppers, beans, carrots and squash — because their roots use the full 12 in and reach into the native soil below if the bottom is open. An 18 in (46 cm) deep bed suits root crops like parsnips and potatoes, gardeners who cannot bend low, and sites with poor native soil or contaminated soil, because 18 in of good mix isolates the roots from what is underneath. Deeper than 18 in, the soil volume and cost climb fast — a 4 ft by 8 ft bed at 18 in deep holds 48 cu ft (1.36 cu m) of soil, versus 32 cu ft (0.91 cu m) at 12 in — and the extra depth buys little for most crops. Put a bed where water is close: a hose bib within 25 ft (7.6 m) or a rain barrel beside the bed, because a bed that needs a 100 ft hose run gets watered less often, and a raised bed dries out faster than in-ground soil in summer heat.

Orient long beds north to south so both sides get even sun through the day, and place tall crops (tomatoes, corn, pole beans) on the north end so they do not shade shorter crops. Leave the bed bottom open to the native soil unless you have a reason not to: an open bottom lets worms in, lets excess water drain, and lets deep roots reach down. Line the bottom with hardware cloth (1/2 in, or 13 mm, galvanized mesh, $20 to $40 per bed) only if gophers, voles or moles are a real problem, and lay cardboard or newspaper over grass before filling — it smothers the grass in 4 to 6 weeks and rots away, so there is no need to dig out the sod first. Never use landscape fabric under a raised bed: it blocks worms, traps water, and tears into shreds that contaminate the soil within two seasons.

Materials: wood, metal, stone and what lasts longest

Cedar is the best wood for a raised bed, because it lasts 10 to 15 years untreated, resists rot and insects naturally, and does not leach chemicals into the soil. A cedar 4 ft by 8 ft by 12 in bed costs $120 to $250 in lumber; the same bed in pressure-treated pine costs $60 to $120 and lasts 8 to 12 years, and modern pressure-treated lumber (ACQ or MCA, used since 2004) is considered safe for vegetable gardens by the US EPA, because the old arsenic-based CCA treatment was phased out. Avoid railroad ties and old creosote-treated timbers entirely — creosote is toxic and leaches into soil — and avoid untreated pine, which rots in 3 to 5 years and means rebuilding the bed just as the soil is at its best. Use 2 in (5 cm) thick boards, not 1 in, because 1 in boards bow outward under the weight of wet soil within a season; a 4 ft by 8 ft bed filled with wet soil pushes outward with several hundred pounds of force, so corner posts of 4x4 lumber and a cross-brace on beds longer than 8 ft are not optional extras.

Metal beds are the fastest-growing alternative, and for good reason: galvanized steel beds last 20 years or more, cost $80 to $250 for a 4 ft by 8 ft by 12 in bed, and assemble in 30 minutes with no tools beyond a screwdriver. Their drawback is heat: a steel bed in full sun runs 5 to 10°F (3 to 6°C) hotter at the edges than a wood bed, which stresses roots in hot climates and dries the edge soil faster, so metal beds suit cooler climates and spring and fall crops better than desert summers. Corten steel (weathering steel) is the premium metal option — it develops a rust-colored patina, costs $300 to $800 per bed, and lasts decades — but it stains patios and should not sit on a deck. Stone, brick and concrete block beds last effectively forever and look permanent, but they cost $300 to $1,000 per bed, need a level footing, and their thermal mass keeps soil cool in spring, which delays planting by a week or two in cold climates. For most gardeners, cedar or galvanized steel is the right answer: cedar if the garden is a landscape feature, steel if speed and longevity matter most.

Build the bed to last by getting the joints right. Screw boards together with 3 in (76 mm) exterior deck screws, not nails, because screws hold against the outward soil pressure and can be tightened later; use two screws per joint, pre-drill hardwood like cedar to prevent splitting, and cap the top edge with a 2x6 board laid flat, which gives a comfortable seat and a shelf for tools and a coffee cup. Set the bed level with a spirit level, not by eye, because a bed that is 2 in out of level across 8 ft shows as a visible lean once it is filled and waters unevenly — the low end stays wet and the high end dries out. Anchor a tall bed (over 18 in) with stakes driven 12 in (30 cm) into the ground at each corner, and if the bed sits on a slope, terrace it: cut the uphill side into the slope and build a small retaining edge on the downhill side, rather than letting the bed sit tilted.

Soil mix: the recipe that grows vegetables, not weeds

Fill a raised bed with 60 percent topsoil, 30 percent compost and 10 percent aeration material, because that mix holds water without waterlogging, feeds plants for a season, and stays loose enough for roots to grow. Buy topsoil by the cubic yard, not by the bag: a cubic yard (0.76 cu m) costs $25 to $50 delivered in bulk versus $3 to $5 per 1 cu ft bag, which works out to $81 to $135 per cubic yard — bulk is a third of the price. Screened topsoil is worth the small premium over unscreened, because unscreened topsoil arrives with clods, stones and weed seeds that take a season to deal with. Compost should be finished, dark, crumbly and earthy-smelling; if it smells sour or like ammonia, it is not finished and will rob nitrogen from the soil as it breaks down. Mix in perlite, coarse vermiculite or rice hulls at 10 percent for aeration — perlite is permanent and never breaks down, vermiculite holds more water and suits dry climates, and rice hulls are cheap and break down over two seasons, which is fine because you will top up the bed every year anyway.

Do not fill a raised bed with bagged garden soil alone, because bagged garden soil is mostly peat and bark, dries out fast, shrinks as it decomposes, and costs three times as much as bulk topsoil for a worse result. Do not use pure compost either: compost holds too much water, compacts, and is too rich in salts for seedlings. The right fill for a new bed is bulk topsoil plus compost plus aeration, mixed on a tarp or in a wheelbarrow before it goes in, watered thoroughly as it is filled so it settles evenly, and topped up after the first watering because a fresh fill settles 2 to 3 in (5 to 8 cm) in the first week. Expect to top up every spring with 2 to 3 in of compost — a 4 ft by 8 ft bed needs about 8 cu ft (0.23 cu m) of compost a year for that top-up — because soil levels drop as organic matter decomposes and plants use nutrients. Test the soil every 2 to 3 years with a $20 to $30 lab test (your state cooperative extension service runs them) rather than guessing at fertilizer: the test reports pH, nitrogen, phosphorus, potassium and organic matter, and tells you exactly what to add, which is usually lime to raise pH in acidic soils or sulfur to lower it in alkaline soils.

Water a raised bed deeply and less often, because shallow daily watering grows shallow roots that wilt by noon. A new bed needs 1 to 1.5 in (2.5 to 3.8 cm) of water a week, delivered in two or three deep waterings rather than seven light ones, and a soaker hose or drip line ($25 to $50 per bed) beats a sprinkler because it waters the soil, not the leaves, which cuts fungal disease and saves 30 to 50 percent of the water. Mulch the surface with 2 to 3 in (5 to 8 cm) of straw, shredded leaves or wood chips after seedlings are established: mulch cuts watering by a third, suppresses weeds, keeps soil temperature even, and breaks down into the compost the bed needs next year. Keep the mulch 2 in away from plant stems, because mulch piled against a stem holds moisture against it and invites rot and slugs.

Planting, yields and the first-year plan

Plant a first-year bed with the crops that forgive beginners and pay back fast: tomatoes, zucchini, beans, lettuce, herbs and peppers. A single 4 ft by 8 ft bed holds 4 tomato plants (spaced 2 ft, or 61 cm, apart), 8 pepper plants, 16 bean plants, or 32 lettuce heads in succession — and those numbers are why spacing matters more than the number of seeds in the packet. Follow the square-foot method for small beds: divide the bed into 1 ft (30 cm) squares and plant 1 tomato, 4 lettuces, 9 beans or 16 radishes per square, which turns a small bed into a productive grid instead of a crowded mess. Rotate crops each year — never plant tomatoes, peppers or potatoes in the same bed two years running, because soil-borne diseases build up — and plant a cover crop of crimson clover or winter rye in the fall, which adds nitrogen and organic matter over winter and is dug in as green manure in spring. A well-run 4 ft by 8 ft bed yields 50 to 100 lb (23 to 45 kg) of vegetables in a season, which at grocery prices of $2 to $4 per lb is $100 to $400 of produce from a bed that cost $200 to $400 to build and fill — the bed pays for itself in one to two seasons, and then it is pure profit.

Start seeds indoors 6 to 8 weeks before the last frost for tomatoes, peppers and eggplant, because those crops need a long season and a transplant, while beans, squash, carrots and lettuce go straight into the bed after the last frost. Harden transplants off for a week before planting out — a few hours outside on day one, a full day by day seven — because a tomato moved straight from a windowsill to full sun and wind wilts within hours. Feed the bed, not the plant: side-dress tomatoes and peppers with a handful of compost or a balanced organic fertilizer (10-10-10) at flowering, and skip high-nitrogen fertilizer, which grows leaves at the expense of fruit. Watch for the three problems that hit raised beds first: slugs, which hide under boards and mulch and are caught with a shallow dish of beer sunk to soil level; aphids, which are blasted off with a strong jet of water before they colonize; and blossom-end rot on tomatoes, which is a calcium uptake problem caused by uneven watering, not a calcium shortage — consistent deep watering fixes it, and crushed eggshells do not, because they break down too slowly to help this season.

Costs, maintenance and mistakes to avoid

Budget $200 to $400 for a complete 4 ft by 8 ft by 12 in cedar bed filled with bulk soil, broken down as $120 to $250 for cedar lumber, $60 to $120 for soil and compost delivered, and $25 to $50 for a soaker hose and mulch. A galvanized steel bed runs $150 to $350 all-in, and a premium corten steel bed runs $400 to $900 all-in. Annual running costs are small: $20 to $40 for compost top-up, $10 to $20 for seeds, and $10 to $20 for mulch. Maintain the bed with a 30-minute spring routine: check the boards for rot and loose screws, top up with 2 to 3 in of compost, fork the top 6 in lightly to break compaction, test the soil every two to three years, and replace the mulch. A cedar bed rebuilt after 12 years costs only the lumber, because the soil inside is by then the best soil in the garden — which is the real return on a raised bed: not the first season's tomatoes, but the living soil that gets better every year you tend it.

Avoid the mistakes that waste a first season: a bed wider than 4 ft that forces you to step into it and compact the soil, a bed in shade that grows leggy plants and no fruit, pure bagged soil that dries out and shrinks, no mulch so the bed needs watering every day, and planting tomatoes in the same bed two years running. Run this final check before planting: the bed is level, the soil is 2 in below the top edge so water does not run off, the paths are wide enough for a wheelbarrow, the hose reaches, and the sun hits the bed for at least 6 hours. Then plant, water deeply, mulch, and let the bed do what it does — which, given good soil and enough sun, is grow more food than you expected from 32 sq ft of ground.