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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.
How deep should a raised garden bed be?
When you look at your standard raised bed size, which is three to four feet wide, by six to eight feet long, the structure is usually at least 10 to 12 inches (25 to 30 cm) high. That makes it easy when shopping for lumber, as two stacked 2×6 boards are about 11 inches in height. And, it’s possible to get boards that are a foot high.
There are charts online that explain the root depth certain veggies need to grow. Tomatoes, for example, which benefit from being planted deeply, require about 24 to 36 inches (60 to 90 cm) of depth for their roots to grow. However, if your raised bed is about a foot high, the growth of the plant may simply be slightly stunted. I have successfully grown tomatoes in a one-foot-high raised bed where there was landscape fabric placed along the bottom (something I would not recommend doing for various reasons). The point I’m making in this case is the roots couldn’t extend below the base of the raised bed because of that barrier. And I’ve grown enormous tomato plants in that garden. One thing you can do with a shallower raised bed is look for patio varieties. The plants remain small and compact, and don’t require as much space to grow.
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.
Why have a higher raised bed?
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.
There are some wonderful no-build options, like galvanized raised beds, which have become popular. Actual stock tanks only require removing a plug for drainage, while modern kits, like Birdies Raised Beds, have the stock tank look of the frame, but are bottomless (and much lighter).
Raised beds don’t have to be built from the ground up. Another option is to consider elevated raised beds. These raised beds sit on legs. While they are less deep than a stock tank, for example, they are deep enough to grow an abundance of vegetables. Jessica has shared some growing tips in her article about elevated raised beds.
My friends at Bufco (The Backyard Urban Farm Co) have created a wheelchair accessible raised bed that allows for a wheelchair to be rolled right up to the garden, rather than having to pull in sideways. Though only five and a half inches (14 cm) deep, it provides ample space to grow a variety of beans, herbs, radishes, and even some tomato varieties.
A couple of tricks for filling a tall raised bed
One of the tricks I’ve learned over the years for filling a deep raised bed is to place large, empty plant containers upside down inside a galvanized raised bed, followed by a layer of boards. Then, line the top half of the stock tank with landscape fabric. This means only filling about half the raised bed with soil. I refer to this tip as false bottom fakery.
Another budget-saving tip for a tall raised bed is to fill the bottom third with yard waste—sticks and twigs, composted leaves, etc. You want to make sure the soil that’s around the plants and the roots is composed of soil and fully decomposed material, like compost. The bottom layer is strictly to fill up some of the space.
Find other raised garden bed articles and inspiration
- The best soil for a raised garden bed
- Raised garden bed materials
- Set up a self watering raised bed
- Raised bed designs for gardening
- The best vegetables to grow in raised beds
- 4×8 raised bed vegetable garden layouts
