Key takeaways
- Elevated planters that list overall height but not chamber depth.
- “Deep” beds under 10 inches marketed for tomatoes or root vegetables.
- Liners or bottoms described as waterproof with no mention of drainage holes.
- Untreated fir or pine sold with no liner and a “long-lasting” claim.
- Painted rather than galvanized steel for a bed meant to hold wet soil for a decade.
- Beds sold for balconies at 16 inches or more deep without a weight warning.
- Kits that describe soil capacity in bags without stating cubic feet.
- Widths over 48 inches with two-sided access or over 30 inches with one-sided access.
What's inside
Soil depth is the raised-bed spec people get wrong most often, and in both directions. Some buy a 6- or 8-inch bed because it is cheaper and looks tidy, then watch tomatoes stall and lettuce wilt by 2 p.m. every July day because there is not enough soil to hold water or roots. Others buy a 30-inch elevated planter for carrots and discover that its actual soil chamber is only 8 inches deep, with legs and a false bottom making up the rest. A third group builds a 24-inch bed on good ground and spends $400 on bagged mix to fill a volume the plants never use. Depth has to match three things: what you are growing, what is underneath the bed, and how you want to work it. Once those are set, the right number is usually obvious and the wrong ones are expensive.
Decide These 3 Things First
1. What is under the bed. On open, workable soil, roots continue down past the bed walls, so 10 to 12 inches of added depth is plenty for most crops. On concrete, a deck, compacted clay, tree roots, or contaminated ground, the bed depth is the total root zone and needs to be 16 to 24 inches.
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2. What you will grow. Salad greens, herbs, radishes, and strawberries are happy in 6 to 8 inches of soil. Beans, peppers, bush tomatoes, and brassicas want 10 to 12. Carrots, parsnips, potatoes, indeterminate tomatoes, and squash prefer 16 to 18 or more. Match the bed to the deepest-rooted crop you actually plan to grow, not to the whole seed catalog.
3. How you want to work. Standing to garden without bending means a soil surface at 30 to 36 inches, which you can get either from a tall bed full of soil or from an elevated planter with legs and a shallower chamber. The two look similar and cost about the same, but one holds three times the soil.
Depth: The Numbers by Crop
Root depth drives everything. Lettuce, spinach, arugula, green onions, and most herbs root in the top 6 inches; a 6- to 8-inch bed or planter chamber is enough if you water daily in heat. Bush beans, peppers, determinate tomatoes, kale, cabbage, and broccoli send roots 10 to 14 inches down, so 12 inches is the working standard and the depth most 4 by 8 kits ship at. Carrots need loose soil to the full length of the root, so 12 inches for short varieties and 16 to 18 for standard; parsnips want 18 to 24. Potatoes need 12 inches minimum and produce better at 16 to 18 with room to hill. Indeterminate tomatoes, winter squash, and melons root 18 to 24 inches when allowed and reward depth with fewer waterings. Perennial asparagus and rhubarb need 18 inches or more and a permanent bed. A practical rule for a mixed vegetable bed on open ground is 12 inches; on a hard surface, 18 inches covers everything but parsnips.
Soil Volume and Cost
Depth multiplies cost quickly. A 4 by 8 foot bed at 12 inches holds 32 cubic feet (1.2 cubic yards); at 18 inches, 48 cubic feet (1.8 yards); at 24 inches, 64 cubic feet (2.4 yards). Bagged raised-bed mix runs $8 to $15 per 1.5 cubic foot bag and premium blends with castings and amendments can reach $20 to $32 per bag. That puts a 12-inch bed at $170 to $320 in bags or $50 to $110 in bulk delivered; an 18-inch bed at $260 to $480 bagged or $75 to $160 bulk; a 24-inch bed at $340 to $640 bagged. Above 12 inches, fill the bottom third with logs, branches, leaves, and straw (a technique borrowed from hugelkultur) or with cheap topsoil, and use the good mix only in the top 12 inches where roots feed. That cuts an 18-inch bed’s mix cost by roughly a third. A 46-inch by 24-inch wooden planter at 10 inches deep holds about 6 cubic feet, or 4 bags; a 72 by 24 inch bed at 16 inches holds 16 cubic feet, or about 11 bags.
Material and How It Affects Usable Depth
Walls determine how much of the listed height is actually soil. Wood beds (cedar, fir, redwood) list outside height, and boards of 1 to 1.5 inches sit on the ground, so a “12-inch” bed gives about 11.5 inches of soil. Galvanized steel kits list panel height; a 17-inch panel gives 16 to 17 inches of soil but heats the soil at the edges 5°F to 10°F above wood in full sun, which helps in spring and stresses roots in July. HIPS or composite wood-look beds at 72 inches long and 12 to 16 inches tall behave like wood thermally but never rot. Elevated planters on legs are the trap: a 72 by 24 by 30 inch unit typically has a 10- to 16-inch chamber, with the remaining height being legs and airspace. Look for the chamber depth in the spec sheet; 16 inches is the deep end and 8 to 10 inches is common. Fir planters need a liner or sealant to last more than 3 to 5 years; cedar and steel go 10 to 20.
Drainage, Bottoms, and Liners
Depth only works if water moves through it. Beds on open ground need no bottom; roots and worms pass into native soil and excess water drains away. Lay cardboard or landscape fabric first to smother grass, never plastic. Beds on concrete or decks need a bottom with drainage: hardware cloth or landscape fabric on slats, with 6 to 12 drain holes of 0.5 inch per 4 by 8 footprint, plus a 1- to 2-inch gravel or perlite layer if the bed is over 18 inches deep. Elevated planters ship with a plastic or fabric liner; check for pre-drilled drainage holes and add more if there are fewer than one per square foot. Weight matters on decks: wet soil weighs 75 to 100 pounds per cubic foot, so a 72 by 24 inch bed at 16 inches carries about 1,200 to 1,600 pounds, well above the 40 pounds per square foot most residential decks are rated for over that footprint. Keep deck beds to 8 to 10 inches and under 4 square feet, or put them on the ground.
Price Tiers
$20 to $80 for the bed: galvanized steel oval kits and small 46-inch fir planters at 10 to 12 inches. Soil for a 12-inch 3 by 6 bed adds $100 to $180 bagged. Best for greens, herbs, and a first tomato.
$80 to $200: 4 by 8 cedar kits at 12 inches, 72-inch composite beds at 12 to 16 inches, and elevated fir planters with 16-inch chambers. Soil adds $170 to $320 bagged or $50 to $110 bulk. Covers the full beginner vegetable mix.
$200 to $600: 17- to 32-inch galvanized or thick-cedar beds, modular metal systems, and elevated planters with 16-inch chambers and 400-pound-plus load ratings. Soil for an 18- to 24-inch 4 by 8 runs $260 to $640 bagged; use bulk and fill material to keep it under $200.
Warranty and Delivery
Steel kits typically carry 5- to 20-year rust-through warranties; check that the coating is galvanized or aluzinc rather than painted steel. Cedar kits often have no warranty beyond arrival condition; fir planters, 90 days to 1 year. Composite and HIPS beds run 2 to 5 years. A 4 by 8 cedar kit ships in one or two boxes of 40 to 80 pounds; a galvanized 17-inch kit is 25 to 40 pounds; an elevated 72-inch fir planter is 45 to 70 pounds. Assembly is 20 to 60 minutes with a screwdriver or the included bolts. Soil is the heavier delivery: 21 bags for a 12-inch 4 by 8 bed weigh about 900 to 1,000 pounds, which is why bulk delivery by the yard, at $40 to $90 per yard plus a $50 to $150 delivery fee, becomes cheaper and easier once you pass one bed.
Spec Checklist
| Spec | Budget ($20 to $80) | Mid ($80 to $200) | Premium ($200 to $600) |
|---|---|---|---|
| Soil depth (on ground) | 8 to 12 in | 12 in | 17 to 24 in |
| Soil depth (on hard surface) | Not recommended under 12 in | 12 to 16 in | 18 to 24 in |
| Elevated planter chamber depth | 8 to 10 in | 10 to 16 in | 16 in with reinforced base |
| Typical footprint | 2 x 4 to 3 x 6 ft | 4 x 8 or 2 x 6 ft | 4 x 8 or modular |
| Wall material | Galvanized steel, fir | Cedar, composite, galvanized | Thick cedar, aluzinc steel |
| Soil volume | 6 to 18 cu ft | 16 to 32 cu ft | 32 to 64 cu ft |
| Bagged soil cost | $40 to $180 | $170 to $320 | $260 to $640 |
| Bed lifespan | 3 to 10 years | 7 to 15 years | 15 to 20 years |
| Warranty | None to 1 year | 1 to 5 years | 5 to 20 years |
Red Flags
- Elevated planters that list overall height but not chamber depth.
- “Deep” beds under 10 inches marketed for tomatoes or root vegetables.
- Liners or bottoms described as waterproof with no mention of drainage holes.
- Untreated fir or pine sold with no liner and a “long-lasting” claim.
- Painted rather than galvanized steel for a bed meant to hold wet soil for a decade.
- Beds sold for balconies at 16 inches or more deep without a weight warning.
- Kits that describe soil capacity in bags without stating cubic feet.
- Widths over 48 inches with two-sided access or over 30 inches with one-sided access.
FAQ
Is 12 inches of soil deep enough for a raised bed?
Yes, for nearly all vegetables when the bed sits on open ground that roots can grow into. Greens, beans, peppers, brassicas, and most tomatoes thrive at 12 inches. Choose 16 to 18 inches for carrots, potatoes, and squash, or for any bed on concrete, a deck, or compacted clay.
How deep should an elevated planter’s soil chamber be?
Eight to 10 inches works for salad greens, herbs, and strawberries with daily watering in summer. For peppers, bush tomatoes, and beans, look for a 12- to 16-inch chamber; those are less common and usually cost $150 to $250. Check the spec sheet for chamber depth rather than overall height.
Can I fill a deep bed with something cheaper than soil mix?
Yes. In beds over 12 inches, fill the bottom third with logs, branches, leaves, straw, or inexpensive topsoil, then put 12 inches of quality raised-bed mix on top. Roots feed in the top foot, the organic fill breaks down over 2 to 4 years, and you save 30 to 40 percent on mix. Top up with 1 to 2 inches of compost each spring as the fill settles.
What depth should I plan for a new raised bed in 2026?
Twelve inches on open ground and 18 inches on any hard or poor surface. Those two numbers cover almost every home vegetable garden, keep soil costs manageable, and avoid both the shallow bed that dries out by noon and the oversized one that costs $500 to fill.
