How deep should raised beds be for optimal growth?
Written by
Michael Sullivan
Reviewed by
Prof. Charles Hartman, Ph.D.The depth of any raised bed should be optimal for the types of plants it contains for maximum growth. Not enough depth stunts root growth, and too much depth causes soil to be wasted. Different plants require various depths for best growth. For instance, greens with shallow roots will thrive in beds eight to twelve inches deep, while carrots require a minimum depth of eighteen inches for their long taproots to grow straight. Having a depth of bed suited to the crop will prevent a common failure.
To develop healthily and avoid forking, root crops like carrots and parsnips require at least 18" (45 cm) deep soil for proper growth. My first crop of carrots was planted in 12" beds and produced twisted roots. Deeper beds are necessary for unrestricted downward growth into the deeper soil. Provide additional depth for compost layering techniques, which improve soil fertility over time.
Soil Layering
- Place logs or branches in bottom third for decomposition
- Add 6 inches of leaves or straw as middle layer
- Top with quality soil-compost mix for root zone
Elevation Solutions
- Build beds 6+ inches above ground for drainage
- Use cinder blocks or risers for extra height
- Create terraced designs on sloped terrain
Root Management
- Install root barriers for invasive plants
- Use fabric liners to contain vigorous roots
- Prune taproots of perennials in shallow beds
Tomatoes and peppers do best in beds which are 12-18 inches (30-45cm) in depth, giving ample root room for their extensive root systems. I grow my indeterminate types of tomatoes in beds that are 18 inches deep, and I find that the deep roots in beds like these produce about 40% more fruit than shallow, container-grown plants. They require deep roots in loose, well-drained soil, and to accomplish this, proper bed construction is necessary to prevent compaction.
Raise your beds a minimum of six inches above ground level for a significant drainage advantage. Without this airspace, waterlogging follows in the case of heavy rains. My garden, which had clay soil, greatly benefited from the introduction of gravel bases under the elevated beds. A suitable height of beds, combined with adequate soil depth, yields good results.
To grow greens and herbs, shallow beds measuring 8 to 10 inches are ideal. Lettuce, radishes, and basil become firmly rooted in soil at this depth. I succession-plant transplant quickly growing crops in shallow beds to provide an ample supply for the home. Only see that it gets enough moisture during dry weather. The soil being only shallow, a more rapid drying is experienced.
Read the full article: 10 Key Benefits of Raised Beds
