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Last Updated on September 18, 2026

How Deep Should a Raised Bed Be? Root Depth by Crop

On open, loosened ground, build the bed at least 12 inches deep for most vegetables, since roots keep growing into the soil below the frame. On a closed bottom or a hard surface like a patio, decking, or landscape fabric over compacted ground, go deeper: at least 8 inches for shallow crops like lettuce and beans, and 12 to 24 inches for tomatoes, peppers, squash and other crops with deeper roots, because the bed is the entire root zone with nothing to grow into underneath.

Why the number changes with what is under the bed

A raised bed on tilled or loosened native soil is still an in-ground garden. University of Maryland Extension puts it directly: when a bed sits on top of the ground, roots grow through its soil into the soil below, so the bed is an in-ground garden even though it is framed and elevated. Utah State University Extension goes further: beds shallower than 12 inches should have no bottom, so roots can reach the soil below, and recommends tilling that soil 6 to 12 inches deep first. University of Missouri Extension agrees that most plants need at least a 6 to 12 inch rooting zone, some of it borrowed from below the existing grade.

A raised bed on a hard, impermeable surface, such as a patio, decking, gravel, or landscape fabric over compacted ground, cannot borrow depth from underneath. University of Maryland Extension says these beds are fine at 8 inches deep for leafy greens, beans and cucumbers, but need 12 to 24 inches for peppers, tomatoes and squash, and warns they dry out faster since roots cannot reach water below the box. That is also why Purdue University Cooperative Extension Service, writing about containers and raised beds on hard surfaces exposed to wind and sun, recommends a minimum depth of 10 to 12 inches, noting that most vegetables root 2 to 3 feet deep in good soil but can be grown shallower at some cost to size and yield.

That is the whole disagreement in the popular lists you have probably already seen: a page built around beds on open ground lands near 6, 12 and 18 inches by crop, and a page built around beds with a closed bottom lands near 12 to 18, 18 to 24 and 24 to 36 inches. Neither is wrong. They are answering two different setups.

Root depth by crop

The table below uses the effective root zone depth published by University of Nebraska Extension (NebGuide G2189), which sorts vegetable crops into shallow, moderate and deep rooted for irrigation planning. Shallow crops are comfortable in the practical open ground minimum, medium crops want the top of that range or a bit more, and deep crops want the deeper end of the hard surface range even on open ground, if you want the roots fully inside the added soil rather than reaching into whatever is underneath. Crop names link to that crop’s square foot spacing page.

Root depth group	Crop	Effective root depth
Shallow	Chives	6 in
	Green onions	6 in
	Lettuce	6 in
	Radish	6 in
	Spinach	6 in
	Strawberry	6 in
Medium	Onions, dry	12 in
	Celery	12 in
	Shallots	12 in
	Swiss chard	12 in
	Beets	18 in
	Broccoli	18 in
	Cabbage	18 in
	Carrots	18 in
	Cauliflower	18 in
	Collards	18 in
	Cucumbers	18 in
	Eggplant	18 in
	Kale	18 in
	Kohlrabi	18 in
	Okra	18 in
	Peas	18 in
	Peppers	18 in
	Potatoes	18 in
	Snap beans	18 in
	Turnips	18 in
Deep	Butternut squash	24 in
	Cantaloupe	24 in
	Corn	24 in
	Pumpkin	24 in
	Squash	24 in
	Tomato	24 in
	Watermelon	24 in
	Zucchini	24 in

Effective root zone depth figures are from University of Nebraska Extension NebGuide G2189, Water Wise Vegetable and Fruit Production, citing the USDA NRCS New Jersey Irrigation Guide. “Onions, dry” covers storage onions and shallots at 12 inches; green onions grown as scallions are shallower at 6 inches. Squash covers both summer squash, like zucchini, and winter squash, like butternut, at 24 inches. Some crops with a per-square-foot page on this site are left out because no fetched extension source gives a root depth number for them.

Depth follows what you plan to grow, so it helps to decide that first. Plan My Garden is free to start: draw the bed, pick your crops, and it places each plant at square foot spacing.

What extra depth costs

Soil volume climbs fast with depth, since a raised bed is a box and volume is length times width times height. A 4×8 foot bed, the most common size, holds 32 cubic feet of soil at 12 inches deep. At 18 inches it holds 48 cubic feet, half again as much. At 24 inches, double the original depth, it holds 64 cubic feet, twice the soil to buy and haul. The raised bed soil calculator runs that math for any bed size and depth and converts it to bags or cubic yards.

Lumber cost rises with depth too, in steps rather than smoothly. A cedar 2×6 gives about 5.5 inches of wall; two stacked gives roughly 11 inches, close to the 12 inch open ground minimum. Reaching 18 or 24 inches means stacking three or four boards, or moving to a 2×10 or 2×12, plus taller corner posts so the wall does not bow under the added soil. Choosing between cedar, pine and metal covers how each material holds up at that height, and the raised bed lumber calculator works out board count and cost for a given size.

Beds on legs, over hardscape, and drainage

An elevated or tabletop bed is built like a bed on a hard surface: it has a closed bottom, so the soil inside is the entire root zone. Iowa State University Extension describes elevated and tabletop gardens built 30 to 33 inches tall, with 6 to 12 inches of soil plus 1 to 2 inches of mulch. It recommends drilling half inch drainage holes every 6 inches across the base and lining the bottom with landscaping fabric before adding soil, so water drains through but the soil stays in.

The same logic applies to a bed on a patio, deck or driveway. University of Maryland Extension notes these beds drain well onto whatever is underneath, but dry out faster than an in-ground bed since there is no soil moisture below for roots to draw on, and reflected heat from the surface adds stress. Check a hard surface or elevated bed for dryness more often, and water before the top few inches go dry.

Let the planner place them

Plan My Garden is free to start: enter your zip code, draw your beds, pick tomatoes, lettuce and everything else you want to grow, and it places each plant at square-foot spacing and builds a planting schedule from your frost dates.

Quick questions

How deep does a raised bed need to be for tomatoes?

At least 18 to 24 inches. University of Nebraska Extension lists tomatoes at a 24 inch effective root depth, and University of Maryland Extension puts tomatoes in the 12 to 24 inch range on a hard surface.

Is 12 inches deep enough for a raised bed?

Yes, for most vegetables, as long as the bed sits on loosened native soil so roots continue below the frame. University of Missouri Extension and Utah State University Extension both put the minimum rooting zone at 6 to 12 inches.

Do raised beds need a bottom?

Not when they sit on open ground. Utah State University Extension recommends building beds shallower than 12 inches with no bottom, so roots reach the tilled soil underneath. A bed with a solid bottom needs to be deep enough on its own.

How deep should a raised bed be for root vegetables like carrots?

University of Nebraska Extension lists carrots at an 18 inch effective root depth, the same as beets and turnips, so a bed in the 12 to 18 inch range gives the roots room to reach full length.

What is the minimum raised bed depth on a patio or deck?

University of Maryland Extension puts it at 8 inches for shallow crops like leafy greens, beans and cucumbers, and 12 to 24 inches for tomatoes, peppers and squash, since a hard surface has no soil below for roots to reach.