Raised Beds - Soil Depth Requirements

A frequent question we receive in our online store is "How tall a raised bed do I need to grow vegetables?"

The ideal height for a raised bed is a matter of preference for gardeners. Considerations include the cost of raised beds, the condition of the soil beneath the bed, soil depth requirements for the intended crop, and especially important to mature gardeners, how much bending over you want to do.

Container gardening also requires gardeners to understand the rooting requirements of different crops. Since gardening containers and planters have bottoms, the soil depth is limited.

These considerations are discussed in the article below, with charts showing the rooting depths needed for different vegetables and the sizes of plants at maturity, which helps to determine the location and spacing of seeds and transplants.

• 'double dig' the ground beneath the raised bed
Raised garden beds are open on the bottom which enables plant roots to access soil nutrients below ground level. When first setting up a raised bed, gardeners should ‘double-dig’ the soil which underlies the raised bed. This only needs to be done once, and is much easier if done before the raised bed is assembled in place.

Double-digging refers to two shovel blade lengths, or approximately 24” in depth. This is done to remove rocks and any debris which would obstruct root growth, and it gives the gardener a chance to see if there are any other roots encroaching into the soil space. Nearby trees, for example, may be sending roots as far as 50’ laterally underground in search of available nutrients. Double-digging provides a reservoir of nutrients and water which can be accessed by the plant’s deeper roots.

Double-digging also lets the gardener see the condition of underlying soil so it can be determined which soil amendments to add. For example, if the soil appears clay-like then it will need to be lightened with peat to provide aeration and improve drainage.

• add amendments to improve the subsoil
Once the ground plot has been dug and cleared of rocks, add peat moss as needed to lighten the soil. Since peat is acidic, add lime to balance the soil ph. Rock phosphate can also be sprinkled in at this time. Now the raised bed can be assembled and topped up with soil. Once the bed is filled to within a few inches of the top, add fertilizer and compost. These final amendments should be added within a week or two of planting, since you don't want early spring rains to wash these valuable amendments too deeply into the soil.

To learn more about building rich soil, see 6 Tips for Building Soil in Raised Garden Beds and Planters.

 

The soil in these raised beds is being prepared for planting. Compost and fertilizer will be added to top up the beds.

If you look carefully, you can see the aluminum cross-braces on the lower bed. These will be covered with more soil.

Note the beds are built with a taper to the sides. This is because the garden is on sloping ground. Read our article How to build raised beds on sloping ground.