Planting in raised garden beds gives people flexibility when gardening. When considering dimensions for a raised bed, planting depth is essential. Determining how deep a raised garden bed should be depends on what you're growing and your preferences. Before you build a raised garden bed, consider the "root room" the plants need and how high you want to raise it so that working in it is easy for you.
To determine the correct depth, consider the issue from two perspectives: the plants' and yours. 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, 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.
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Raised Garden Bed Depth
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 you focus on the above-ground growth of 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, it is essential to get the depth right.
The minimum required depth 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.
Double-Digging
However, if you have performed double-digging first on the patch of ground where you're building a raised bed, you have already met this requirement: When you "double-dig," you're turning the soil over to a depth of 24 inches. But the extra depth the raised bed provides is not wasted: You'll place soil amended with compost, 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 raise the raised bed to meet the 20-inch requirement.
What to Grow in a Raised Garden Bed
While growing larger plants in raised beds is possible, people typically grow flowers and vegetables. Even these smaller plants vary in terms of the depth needed. They fall into three categories:
Shallow Bed (12 to 18 Inches)
- Lettuce, spinach, cabbage, Brussels sprouts
- Herbs
- Radishes
- Garlic, onions, chives
- Peppers
- Strawberries
Medium-Depth Bed (18 to 24 Inches)
- Beans, peas
- Cantaloupe
- Squash, zucchini, cucumbers
- Eggplant
- Beets, carrots, turnips
- Potatoes
- Tomatoes
- Corn
- Kale
Deep Bed (24 to 36 inches)
- Artichoke, asparagus
- Rhubarb
- Watermelon
- Pumpkins
- Sweet potatoes
- Parsnips
Other Dimension Considerations
How Long Should a Raised Bed Be?
One dimension you don't have to worry about is length. Your raised bed will extend whatever length you have space for or can afford. A good rule of thumb is not to make your raised bed wider than about 4 feet. You must ensure you can reach any part of the bed from one side or the other.
Can a Raised Garden Bed Be Too Tall?
At what height will it be easiest to maintain plants in this raised bed? Some people don't have a preference. However, others might not want to perform garden chores while hunched or bent over. If you build the raised bed high enough, you won't have to hunch over. Some raised beds are even designed so that you can sit at the edge while performing your gardening chores.
As long as it's easy for you to work in it, a raised garden bed can't 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).
Raised Garden Beds vs. In-Ground Gardening
In-Ground Gardening
- Pro: You can get right to gardening without having to begin by buying or building a raised bed.
- 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.
Raised Bed Gardening
- Pro: Tailor the garden environment to your own needs. For example, ground that doesn't drain well isn't problematic for raised-bed gardening since you will create a new bed on top of that ground with its 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.
What Do You Put in the Bottom of a Raised Garden Bed?
Soil is the most obvious answer for what you use to fill a raised garden bed, but soil is expensive to fill the raised bed entirely. Other viable alternatives for lining the bottom of a garden bed include landscape cloth, gravel, sticks, leaves, compost, wood chips, and some use a plastic liner. Although, a plastic liner is not a good option.
A plastic liner may keep your soil contained within the bed, but a big downside is plastic impedes natural drainage. Your raised bed will become like a swimming pool for your plants, potentially drowning the roots and killing any good growth potential.
Line the bottom with landscape cloth, a porous membrane that allows for drainage while keeping weeds at bay. Note what it's made of since some are made with plastic materials, which can be costly and not entirely eco-friendly.
Burlap is a good natural alternative that contains the soil while allowing drainage. Another cheap, earth-friendly option is cardboard. Cardboard is porous and biodegradable. Ensure you use unmarked cardboard (free of dyes) and remove any plastic tape before using it.
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