How Deep Should a Raised Garden Bed Be? Size Guide by Plant Type

Raised bed depth is one of the first decisions you face when planning a build, and it affects everything from construction cost to plant selection to watering frequency. Go too shallow and deep-rooted crops struggle. Go too deep and you spend more on lumber and soil than necessary. Understanding exactly how much depth different plants need eliminates guesswork and helps you build beds that perfectly match your growing goals.

The answer depends on three factors: what you plan to grow, what surface the bed sits on, and how much you want to spend. A bed on native soil (where roots can penetrate below) needs less built depth than one on concrete where every inch of root space must be contained within the frame.

Key Takeaways

• 6 inches: Minimum for lettuce, herbs, and shallow-rooted annuals—but challenging to maintain adequate moisture
• 10-12 inches: The sweet spot for most vegetable gardens. Supports 90% of common garden crops with adequate root space
• 18-24 inches: Ideal for deep-rooted crops (carrots, parsnips, tomatoes) and beds on hard surfaces
• 24-36 inches: Provides accessibility for wheelchair users or gardeners with mobility limitations
• Beds on native soil effectively gain depth from root penetration below the frame

Depth Requirements by Plant Type

Shallow-Rooted Plants (6-8 inches minimum)

These plants have compact root systems that function in relatively shallow soil. A 6-inch deep bed supports them technically, though 8-10 inches provides better moisture stability and growing conditions.

• Lettuce and salad greens: Roots extend 4-6 inches. The most forgiving crop for shallow beds.
• Radishes: Standard varieties need 4-6 inches; larger types like daikon need 12+ inches.
• Herbs: Most culinary herbs (basil, cilantro, chives, thyme) function in 6-8 inches. Rosemary and dill prefer deeper soil for their taproots.
• Spinach: 6-inch roots. Excellent shallow-bed crop.
• Green onions: 4-6 inch roots. Perfect for even the shallowest beds.
• Strawberries: 6-8 inch root zone. Excellent in shallow beds and window boxes.
• Annual flowers: Most annuals (marigolds, petunias, zinnias) grow acceptably in 6-8 inches.

Medium-Rooted Plants (10-12 inches minimum)

The majority of popular garden vegetables fall into this category. A standard 12-inch raised bed (built from two 2×6 boards or one 2×12) handles all of these comfortably. This is why 12 inches is the most commonly recommended raised bed depth.

• Tomatoes: Root systems extend 12-24 inches, but 12 inches of raised bed soil over accessible native soil works well. In beds on concrete, 18+ inches is better. See our tomato container guide for depth optimization.
• Peppers: 10-14 inch root zone. 12-inch beds are fully adequate.
• Bush beans: 8-12 inch roots. Standard depth works perfectly.
• Cucumbers: 10-12 inch primary root zone with some deeper exploration.
• Kale and cabbage: 10-12 inch root systems. Standard beds suit all brassicas.
• Swiss chard: 10-12 inches. Reliable standard-depth performer.
• Peas: 8-12 inch roots. Standard beds work well with trellis support above.
• Garlic and onions: 8-10 inches for bulb development. Standard depth is ideal. Our garlic growing guide covers depth specifics.
• Beets: 10-12 inches for standard varieties. Cylindra types need slightly more.

Deep-Rooted Plants (18-24 inches preferred)

These crops develop extensive root systems that benefit from deeper soil. In beds on native soil, their roots penetrate below the frame, effectively getting the depth they need. In beds on hard surfaces, you must provide full depth within the frame.

• Carrots: Standard varieties (Nantes, Imperator) need 12-18 inches of loose, obstacle-free soil. Short varieties (Thumbelina, Paris Market) manage in 8 inches. Our carrot guide covers soil preparation for straight roots.
• Parsnips: 18-24 inches. One of the deepest-rooted common vegetables.
• Potatoes: 18-24 inches for adequate hilling and tuber development.
• Sweet potatoes: 18+ inches for proper tuber formation.
• Asparagus: 18-24 inches. Permanent planting—place in a dedicated deep bed.
• Rhubarb: 18-24 inches. Deep taproot needs room to establish.
• Indeterminate tomatoes: While manageable in 12 inches on soil, 18 inches produces notably better results when roots can’t access native ground below.

Special Depth Considerations

Beds on Native Soil vs. Hard Surfaces

When a raised bed sits directly on native soil (the most common setup), roots grow through the bed bottom into the ground below. Effectively, a 12-inch raised bed on soil provides 24+ inches of total root space. This is why standard 12-inch beds work well for even moderate-depth crops.

When building on concrete, asphalt, gravel, or other impermeable surfaces, the bed must contain the entire root zone. Add 6-8 inches of depth beyond what native-soil beds require. A 12-inch bed on soil becomes an 18-20 inch bed on concrete for equivalent performance.

Accessibility and Ergonomics

For wheelchair-accessible gardening, beds should be 24-30 inches tall with a maximum width of 2 feet (reachable from seated position). For gardeners with back or knee issues, beds of 24-36 inches reduce bending. Taller beds cost more for materials and soil but provide life-changing accessibility improvements. Factor in cost-saving filling methods when planning tall beds.

Drainage and Water Behavior at Different Depths

Shallow beds (6-8 inches) dry out faster because there’s less soil mass to retain moisture. You’ll water more frequently—potentially daily in summer. Deeper beds (12+ inches) hold significantly more moisture and maintain more consistent soil temperatures, reducing water stress on plants and reducing your watering frequency.

This moisture advantage is one reason why 12 inches is strongly recommended over 6-8 inches even for shallow-rooted crops. The extra soil depth creates a moisture buffer that makes the entire garden more resilient and less demanding.

Cost Implications of Bed Depth

Deeper beds cost more in both lumber and soil. Here’s a rough cost comparison for a 4×8-foot bed:

For tall beds (18+ inches), budget-friendly filling methods like hugelkultur (layered logs and organic matter in the bottom half) reduce soil costs substantially by filling the lower portion with free materials that decompose into fertile growing medium over time.

Recommended Depths by Garden Goal

Herb garden only: 8-10 inches is sufficient. Herbs don’t need deep soil and actually prefer slightly constrained root zones for concentrated flavor.

General vegetable garden: 12 inches. The universal recommendation because it works for 90% of vegetables with adequate performance.

Root crop focused: 18 inches. Provides the loose, deep soil that carrots, parsnips, and potatoes require for proper development.

Mixed garden on concrete: 18-24 inches. Compensates for lack of below-bed root access.

Accessible/raised for comfort: 24-36 inches. Prioritizes ergonomics alongside growing function.

Flower and ornamental garden: 10-12 inches. Most ornamental plants thrive at standard depth.

Frequently Asked Questions

Can I grow tomatoes in a 6-inch raised bed?

On native soil where roots can penetrate below, yes—with reduced performance. On a hard surface, 6 inches is too shallow for productive tomatoes. For best results, provide 12 inches minimum regardless of surface.

Is a 6-inch raised bed worth building?

For lettuce, herbs, and flowers on native soil, 6 inches works. But the minimal additional cost of building to 10-12 inches provides dramatically better moisture retention, wider crop options, and less demanding maintenance. The marginal investment in deeper beds pays for itself quickly.

Do I need to till the ground under a raised bed?

Not necessarily. Laying cardboard on the ground under the bed suppresses existing vegetation. Over time, worms and root activity naturally break up compacted soil beneath. If the ground is extremely compacted (construction hardpan), loosening the top 6-8 inches with a garden fork before placing the bed improves drainage and root penetration.

Can raised beds be too deep?

From a plant perspective, deeper is generally better—roots always appreciate more space. The practical limit is cost (material and soil) and the diminishing return beyond 24 inches for most crops. Extremely deep beds (36+ inches) waste soil in the bottom zone that most vegetable roots never reach. Fill the lower section with cheaper organic material using the hugelkultur method.

What is the cheapest way to build a deep raised bed?

Use the hugelkultur method: fill the bottom 50% with free materials (logs, branches, leaves, cardboard, grass clippings) and only fill the top 50% with quality garden soil mix. A 24-inch bed becomes affordable when only 12 inches needs purchased soil. See our budget filling guide for step-by-step instructions.