How to choose a raised garden bed: size, depth and material

A raised bed solves several problems at once: poor native soil, drainage, weeds creeping in from the lawn, and a sore back. Choosing one is mostly about three numbers (width, depth and soil volume) and one decision (material).

Width: can you reach the middle?

You should be able to reach the center of the bed without stepping into it, because stepping on garden soil compacts it.

• Bed accessible from both sides: up to about 4 feet wide.
• Bed against a fence or wall: about 2 to 3 feet wide.

Length is flexible. Leave paths wide enough for you, and a wheelbarrow or garden cart if you use one.

Depth: what are you growing?

Depth depends on your plants and on whether the bed sits on open ground.

Depth	Works for
6–8 inches	Lettuce, greens, many herbs, when the bed sits on loosened soil that roots can grow into
About 12 inches	A versatile depth for most vegetables, including tomatoes and peppers on open ground
18–24 inches	Carrots, parsnips and other deep roots; beds over poor ground; beds you want to reach without bending

Open-bottom beds placed on the ground let roots go deeper into the soil below, so they can be shallower. Closed-bottom planters on a patio, deck or legs hold all the soil the plant will ever have, so they need enough depth on their own and they dry out faster.

Elevated planters on legs are great for herbs and for gardeners who can't kneel, but they're usually shallow and need frequent watering.

Soil: the hidden cost

Filling a bed takes more soil than most people expect. Calculate the volume:

Length × width × depth (all in feet) = cubic feet.

A 4 × 8 foot bed that's 1 foot deep holds 32 cubic feet, about 1.2 cubic yards. Bagged soil often comes in 1 to 2 cubic foot bags, so that's a lot of bags. For large or multiple beds, bulk delivery by the cubic yard is usually cheaper.

Fill with a mix made for raised beds or containers rather than plain topsoil, which can compact and drain poorly.

Materials compared

Material	Pros	Cons
Galvanized or coated steel	Long-lasting, modern look, often modular	Can heat up in strong sun; sharp edges on cheap kits; thin panels can bow
Cedar or redwood	Naturally rot-resistant, classic look, stays cool	Costs more; still weathers over years
Pine or fir	Inexpensive and easy to build	Rots sooner when untreated
Composite or recycled plastic	Doesn't rot, low maintenance	Can be expensive; check UV resistance
Fabric grow beds	Cheap, light, good drainage and aeration	Wear out in sun after a few seasons; dry quickly

About pressure-treated wood: lumber treated with chromated copper arsenate (CCA) was phased out of most residential uses in the U.S. at the end of 2003. Today's treated lumber typically uses copper-based preservatives instead. Many gardeners use it; others prefer untreated naturally rot-resistant wood or line the inside of the bed. If you buy treated lumber, choose wood rated for ground contact.

Build quality to check in a kit

• Corner brackets and hardware: sturdy corners keep the sides from bowing once the bed is full of heavy, wet soil.
• Panel thickness for metal beds, and board thickness for wood. Thin material bulges.
• Safety edges on metal beds, such as rolled or capped tops.
• Bracing for long beds; long sides need a cross brace in the middle.

Questions people ask

Should I put anything at the bottom of a raised bed?

For beds on the ground, many gardeners lay cardboard to smother grass and weeds, which breaks down over time. Hardware cloth at the bottom can keep out burrowing animals like gophers and voles. Avoid filling the bottom with gravel; it doesn't improve drainage the way people expect.

Are galvanized steel raised beds safe for vegetables?

Galvanized steel is widely used for vegetable beds. If you have concerns, look for beds with a food-safe coating or line the inside.

How deep should a raised bed be for tomatoes?

About 12 inches on open ground is generally enough, because roots can continue into the soil below. In a closed-bottom planter, go deeper.