There are different kinds of raised garden beds, and people choose raised-bed gardening over in-ground gardening for different reasons. But no matter the reason, the depth of your raised garden bed has to be right in order for your growing season to be successful. To determine how deep a raised bed should be, you first have to be clear about two distinct considerations: how much "root room" do the plants need, and how high should the raised bed be so that working in it is easy for you?
So the issue of correct depth must be looked at from two perspectives: the plants' needs and your own needs. Here's how to figure out how deep your raised garden bed should be.
What Is a Raised Garden Bed?
A raised garden bed is a garden bed that's been raised to a level above that of the surrounding ground. Typically, this raising is achieved in one of three ways:
- Double-digging a patch in your garden, mounding up and amending the soil afterward, and holding it in place with a low frame installed on the ground
- Installing a higher frame on the ground, then filling it with amended soil
- Moving your bed off the ground altogether with a raised planter box
Raised Garden Bed Depth
We'll tackle the issue first from the perspective of your plants' needs. Simply put, most plants need a certain amount of soil under them for their root systems to thrive. As much as we focus on the above-ground growth on a plant, what's going on down below (the root system) is just as important (maybe more so). Hamper a plant's root system by depriving it of the soil depth it needs, and you're decreasing the likelihood that it will perform well. Thus the importance of getting the depth right.
What the minimally-required depth is depends on the plant. But, on average, a raised garden bed should accommodate about 20 inches of soil for the roots of flowers and vegetables to grow in.
However, if you have performed double-digging first on the patch of ground where you're building a raised bed, then you have already met this requirement: By definition, when you "double-dig," you're turning the soil over to a depth of 24 inches. But the extra depth provided by the raised bed is not wasted: You'll be placing soil amended with compost in it, which helps plants grow better. All you need for a raised-bed wall are two 2x6 boards, stacked one upon the other, running horizontally.
If you're not double-digging, you must make the raised bed higher to meet the 20-inch requirement.
What to Grow in a Raised Garden Bed
While it's possible to grow larger plants in raised beds, people typically grow flowers and vegetables in them. Even these smaller plants vary in terms of depth needed. They fall into three categories:
- Shallow-rooted plants need 12 to 18 inches of soil; an example is lettuce. Here, you could get away with a shallower bed.
- Medium-rooted plants need 18 to 24 inches of soil; an example is peppers.
- Deep-rooted plants need 24 to 36 inches of soil; an example is tomatoes.
Raised Garden Beds vs. In-Ground Gardening
Most gardeners are familiar with the pros and cons of in-ground gardening. For example:
- Pro: Instead of having to begin by buying or building a raised bed, you can get right to the gardening.
- Con: But if the spot where you've chosen to garden is rocky, you must remove the rocks first before you can grow plants easily.
Likewise, gardening in raised beds has pros and cons:
- Pro: Ground that doesn't drain well isn't problematic for raised-bed gardening, since you will be creating a new bed, on top of that ground, with its own ideal mix of soil and amendments.
- Con: Cost. Depending on the size of the bed and the materials used (lumber, masonry, etc.), the cost may be prohibitive for people on a tight budget.
But one of the biggest benefits of raised-bed gardening is that it allows you to tailor the garden environment to your own needs.
We've already looked at the issue of raised-bed depth from the perspective of your plants' needs, but what about your own needs? The latter will also help determine the dimensions of your raised garden bed. There is one dimension you don't have to worry about: length. Your raised bed will extend whatever length you require/can afford. But don't make your raised bed wider than about 4 feet: You must ensure that you can reach any part of the bed from one side or the other.
To answer the question of what the correct depth is, from the perspective of your own needs, ask yourself: At what height will it be easiest for me to maintain plants in this raised bed? For many people, the answer will be, "I don't have a preference." If that's your answer, then the depth will be determined solely by your plants' needs.
But other people do have a strong preference, usually because they don't want to perform garden chores while hunched or bent over. And if you build the raised bed high enough, you won't have to hunch over. Some raised beds are even designed so that you sit at the edge of them while you perform your gardening chores.
Can a Raised Garden Bed Be Too Tall?
As long as it's easy for you to work in it, it is not possible for a raised garden bed to be too tall, with one proviso: a taller bed is often a deeper bed, and the deeper the bed, the more soil it will hold, and the more pressure there will be on the sides of the bed. To counteract this pressure, reinforce the bed with cross supports or 4x4 posts driven into the ground; you can also drill drainage holes into the bed sides (near the bottom).