A raised bed succeeds or fails in the first weekend. Get the depth, the soil volume and the water right at the start and the bed largely runs itself. Get them wrong and you spend the whole season fighting drought stress in a box of bagged topsoil that has compacted into a brick.
Late summer is a good time to build one. The soil is workable, bagged mix is easy to find, and a bed built now is ready for a fall planting of greens and garlic — with the whole structure settled and ready for spring.
How deep does the bed need to be?
| What you want to grow | Minimum depth | Comfortable depth | Notes |
|---|---|---|---|
| Lettuce, spinach, arugula, herbs | 6 in | 8 in | Shallow roots, dries out fast at 6 in |
| Bush beans, peppers, chard, onions | 8 in | 12 in | 12 in cuts watering frequency roughly in half |
| Tomatoes, squash, cucumbers | 12 in | 18 in | Needs a trellis or cage, plan for it now |
| Carrots, parsnips, daikon | 12 in | 18 in | Stones and compaction cause forked roots |
| Potatoes | 15 in | 24 in | Room to hill soil up as the plant grows |
| Blueberries, dwarf fruit | 18 in | 24 in | Permanent planting, build it once and build it well |
Width matters as much as depth. Keep any bed you can only reach from one side under 30 inches wide, and any bed you can walk around under 48 inches. Anything wider means stepping into it, and every step compacts the soil you just paid for.
The soil mix, with the math
Volume in cubic feet is length x width x depth, all in feet. A 4 ft x 8 ft bed at 12 in deep is 4 x 8 x 1 = 32 cubic feet, which is about 1.2 cubic yards. Bagged mix is usually sold in 1.5 or 2 cubic foot bags, so that bed needs roughly 16 to 21 bags. Almost everyone underestimates this by half.
A mix that holds water without going anaerobic:
- 50 percent quality topsoil or garden soil. Not bagged potting mix — it is too light and too expensive at this volume.
- 30 percent finished compost. This is the part that feeds the bed. Skimping here is the single most common reason a first-year bed underperforms.
- 20 percent coarse aeration. Pine bark fines, perlite or coarse sand. This is what stops the mix compacting into a brick by August.
- A balanced slow-release amendment worked into the top 6 inches at the rate on the label. See fertilizers and plant food.
Everything for the mix is in soil, mulch and growing media, and the beds themselves in raised garden beds. If a full bed is more than you want to commit to, grow bags give you the same depth at a fraction of the volume and can be moved.
First season, in order
- Site it for sun, then for the hose. Track the spot for one full day. Fruiting crops want 6 to 8 hours of direct sun; greens tolerate 4. Then check that a hose actually reaches — a bed you have to carry water to gets watered less.
- Level the ground and kill the grass. Lay cardboard over the footprint. It smothers the turf and breaks down within a season. Skip landscape fabric, which eventually clogs and blocks drainage.
- Assemble and square the frame. Measure both diagonals; when they match, the frame is square. On sloped ground, dig the high side in rather than shimming the low side up.
- Fill in layers and water each one. Add 6 inches, water it, add the next. Filling dry and watering at the end leaves dry pockets that stay dry all season.
- Install the drip line before you plant. A loop of 1/2 in tubing with 12 in emitter spacing takes ten minutes now and is a nuisance later. See drip irrigation.
- Set supports at planting time. Cages and trellises go in when plants are small — driving a stake past an established root system does real damage. See tomato cages and plant supports and trellises and arbors.
- Plant, then mulch 2 inches deep. Keep mulch an inch clear of stems. Mulch is what makes the difference between watering twice a week and watering daily.
- Expect settling. The mix will drop 1 to 2 inches in the first month. Top it up with compost rather than soil.
For a fall planting right now, greens, radishes, and overwintering garlic are the reliable choices — see fall vegetable garden and vegetable seeds.
Common questions
Do I need to line the bottom of the bed?
Only with hardware cloth if you have voles or gophers. Never line the bottom with plastic — the bed needs to drain into the ground below it.
Can I put a raised bed on a patio or deck?
Yes, but the weight is real: wet soil runs about 100 pounds per cubic foot, so a 4 x 8 bed at 12 in weighs roughly 3,000 pounds. On a deck, use shallow beds or planters and check the load rating first. Grow bags and large containers are the safer choice.
What wood is safe for a vegetable bed?
Cedar and untreated Douglas fir are the standard choices. Modern pressure-treated lumber uses copper-based preservatives rather than the arsenic compounds phased out years ago, but if you would rather avoid it entirely, line the inside face with heavy plastic sheeting while leaving the bottom open.
How often should a new bed be watered?
Daily for the first two weeks while roots establish, then move to deep soakings two or three times a week. Test with a screwdriver at 6 inches rather than guessing.
Do I need to replace the soil each year?
No. Top-dress with 1 to 2 inches of compost each spring and fall. A well-built bed improves for years; the main thing to avoid is walking on it.
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