Raised garden beds are popular for reasons ranging from aesthetics to usability. For example, they are usually built with beautiful cuts of wood that add richness to both porches and yards, and their design provides natural drainage. Furthermore, raised beds can placed on concrete or the ground depending on your preference, and they maintain soil moisture and temperature better than traditional gardens. They are perfect for seasoned gardeners and “green-thumbs” alike as long as they follow some basic raised bed guidelines. One in particular that can make or break a vegetable garden, is soil depth.
Soil depth is an important discussion, especially with regard to raised beds. Soil depth requirements depend on what you are growing – flowers may only need a few inches for their roots to spread, but vegetables will need more room to thrive. Before diving in and planting whatever vegetable comes to mind, a seasoned gardener assesses how much depth they will be able to work with. Depth is measured from the top of the soil down, which means gardeners can artificially create soil depth by increasing the garden bed’s height.
The following are some tips for providing the necessary soil depth for your plants and some vegetables that can be planted based on available soil depth.
Raised Beds Should Be 8 Inches or Taller
There isn’t a maximum soil depth limit for a garden, but there is a minimum depth requirement – at least for vegetable gardens. A good tip is to always err on the side of more soil, not less. For a vegetable garden to succeed, your raised bed should have at least 8 inches of soil depth, which means the bed itself needs to be 8 inches or taller. It is the minimum for nearly every vegetable and growing in less will produce stunted results.
The majority of a vegetable’s roots, often called a root ball, settle within the top 6-8 inches of a garden’s soil. Plants need available nutrient-rich soil surrounding them to sustain growth. If a plant’s roots meet resistance or blocks, growth will be stunted. That’s why it’s important to measure the amount of soil you have poured into your garden bed.
When you initially fill your raised garden with soil it may look like you have filled it to the brim, but new soil will compress. Well-designed raised beds are intentionally built 8 inches in height or taller for this root depth reason. To ensure maximum utilization, overfill your garden bed by about inch or two. Moisten the soil to allow it to settle and add more if the level falls below the top of the garden bed.
If your raised bed is seated on the earth, and not concrete, then you can gain soil depth through double-digging. Basically, it’s the preparation of soil underneath the raised bed so vegetables can continue growing past the raised bed limitations if necessary. Soil depth deeper than 8 inches is necessary for some larger, taproot type vegetables such as carrot, sugar beet, turnip, and radish varietals.
To double-dig, dig up to 2 shovel-blades deep into the ground where you plan to place your bed. Remove rocks and debris, and check for any roots that already occupy that space. Once it is cleared, pour the freshly aerated and sifted dirt back in, and set up your raised bed. Now your vegetables won’t meet any resistance as they grow from the raised bed soil into the ground.
If you don’t want something as labor intensive as double digging then you can try stacking. Raised garden bed engineers created stackable raised garden beds so you can have a deeper planting environment without double-digging.
It’s important to note that shallow rooted vegetables can still grow deeper – they just don’t need to. If you have a raised garden bed between 8 and 11 inches in height, you’ll be able to grow the majority of garden variety vegetables such as: spinach, broccoli, tomato, onion, cauliflower, pumpkin, and potatoes.
If you’re unsure of a plant's root depth needs, check what type of varietal the plant is and see what the characteristics are. For instance, Autumn King Carrots can grow up to 12 inches in length, while Chantenay carrots rarely exceed 6 inches. A quick search online of the varietal you’re considering should yield an easy answer.
Roots will spread where available, which is why some vegetable roots will spread outward in shallow environments. However, this comes at the cost of planting space because, at the end of the day, plants are fighting for nutrition. If you want to attempt something such as eggplants or peppers in shallower depths, be aware that they will spread their roots wider and compete with their neighbors.
Your best bet is to start off on the right foot. Ensure your chosen garden bed is 8 inches or taller (avoid anything that’s 6 inches or less), and fill it all the way to the top so your plants have plenty of room to grow. --
Authors: Wiley Geren III and Bryan Traficante. Bryan co-founded GardenInMinutes.com in 2013, a family-owned company passionate about crafting better ways to start a garden with their tool-free, cedar raised garden bed kits and the Garden Grid™ - the only planting guide and garden watering system, in one. Along with unique gardening solutions, Bryan shares time saving gardening insights on their blog, Facebook, and Instagram. You can read all of his MOTHER EARTH NEWS posts here.
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