As you research project plans for building a raised bed, or you’re considering kits to buy and put together, you may be asking yourself: How deep should a raised garden bed be? There isn’t really one universal measurement when it comes to designs. But I’m going to share some tips on how to figure out the right dimensions for your garden.
One of the benefits of raised garden beds, if you’re building them yourself, is they can be absolutely any size. Whether you have a big backyard or a small patio, raised beds can be customized to fit the space. And they can help you overcome challenging conditions.
The height measurement of your raised garden bed, which determines the depth, is an important one. There are a couple of main issues to consider: the surface under which you’re going to place your raised bed and accessibility.
Why does raised bed depth matter?
You control the soil that’s put into a raised bed. Therefore, if you have concerns about the quality of the soil underneath where the garden will be placed, if it’s hard-packed, sandy, clay-based, or filled with tree roots, the height of your raised bed is important so that all your plants can grow within the parameters of the raised bed itself. Depth is also important if you’re placing your raised bed on patio stones or a driveway. The structure needs to be deep enough for the plants’ roots to grow downwards without hitting a “wall.” You’ll also want to make sure that your raised bed drains well.
If the soil underneath your raised bed is nice and loose and friable, the depth doesn’t matter as much because the plants can extend their roots past the raised bed structure into the subsoil underneath. Ideally you’d have at least another 18 inches (46 cm) of healthy soil underneath.
The case for building a shallow raised bed
Raised bed depth doesn’t matter as much if the soil underneath it is loose and healthy. Veggies can reach below the frame of the raised bed into the ground below, and grow healthy root systems beneath the garden. In this scenario, you’re really just framing out a garden for aesthetics. But having the edge keeps it tidy, and if you have more than one raised bed, you can create the rows between them, rather than between each type of vegetable plant itself as you would find in a traditional garden.
Another instance where you could have a shallow raised bed is if you’re only going to be growing greens. When I converted an old table into a lettuce garden for my first book, Raised Bed Revolution, the garden part only needed to be about four inches (10 cm) in height because lettuce can grow in a much shallower space. The same goes for my vertical raised bed. The “shelves” are only about seven inches (18 cm) deep. I grow lots of lettuces, baby kale, and herbs, like parsley and cilantro, with no issues. Do keep in mind a shallow raised bed will dry out much more quickly. You will need to keep on top of watering.
Raised beds at thigh or waist height are great options for those who have problems bending down or kneeling. Beyond the accessibility benefits, they are also very deep, meaning plants have lots of space to grow. However this also means they can be expensive to fill.