When you build a raised garden bed, one of the most important questions is how deep to make it. If it is too shallow, plant roots won’t have enough space to grow. If it is too deep, the whole thing can collapse due to the weight of wet soil.
So, how deep should you make raised garden beds? You should make raised garden beds at least 12 inches (30 centimeters) deep. Some plants with longer roots will require soil up to 36 inches (90 centimeters) deep. You may also want to leave additional space at the top of the raised bed for mulching later in the season.
Of course, there are other factors that will affect your decision on how deep to make your raised garden beds. For example, you might need a raised garden bed that is tall enough to allow wheelchair access.
Let’s take a closer look at how deep to make a raised garden bed, along with some other common questions about raised beds.
How Deep to Make Raised Garden Beds
A raised garden bed should be at least 12 inches (30 centimeters) deep. In fact, the Old Farmer’s Almanac suggests that 12 inches is the ideal depth for a raised garden bed.
A depth of 12 inches leaves plenty of room for soil so plant roots can grow freely. It also leaves space at the top of the raised bed for mulching later in the season.
The best depth for a raised garden bed also depends on the location. For example, the University of Georgia Extension suggests that a raised bed should be taller if it is built on a hard surface, such as concrete.
The University of Georgia also suggests the following raised garden bed depths for ergonomic and adaptive gardening:
- 24 inches (60 centimeters) to allow for wheelchair access
- 36 inches (90 centimeters) to avoid having to bend over too much
Finally, the type of crops you grow will play a role in how deep you make your raised garden beds. According to Earth Easy, some plants with the deepest roots need a raised garden bed depth of 36 inches or more.
Note: if you want to check the depth requirements for a specific plant, Earth Easy has a helpful table at the link above.
Let’s take a closer look at some of the factors that affect raised garden bed depth.
Location of Your Raised Garden Bed
The location of your raised garden bed matters when deciding on the depth.
As mentioned earlier, if you build over concrete, plant roots cannot grow below the bottom of the raised bed. In that case, the bed will need to be deeper to allow enough room for the roots to grow.
On the other hand, you can add depth underneath the raised bed if you build it over soil. To do this, till or dig up 6 inches of soil below ground level and amend it with compost. Then, the roots of plants in your raised bed can continue to grow right into the ground.
Just remember that you will need to remove any grass before you can work the soil underneath it. If you don’t want to work the soil, you can leave the grass in place and build over it, as long as you take the proper steps.
For example, you can use a layer of cardboard to smother grass in an area and then build a raised bed on top of it. You can learn more in this article, where I cover building raised garden beds on grass.
Ergonomic and Adaptive Gardening
If you are building a raised garden bed for ergonomic and adaptive gardening, you will want to make it deep. With a taller raised bed, you can sit on the edge to do your gardening work, instead of bending over.
A taller raised bed also allows easier wheelchair access. In addition to making your raised bed deeper, you can raise it up on legs (or “stilts”) to make it sit taller.
Remember that a raised bed should be at most 4 feet (1.2 meters) wide. This makes it easy to reach any spot in the raised bed from one side or the other.
Required Soil Depth for Plants
The type of crops you plant in your raised garden bed will also play a role in how deep it should be. Some shallow rooted crops may only need a soil depth of 12 to 18 inches (30 to 45 centimeters).
Crops with shallow roots include:
Certain root crops, such as carrots and turnips, grow a bit deeper and may need a soil depth of 18 to 24 inches (45 to 60 centimeters). You can learn more about the soil depth and space that carrots need in my article here.
Very deep-rooted crops may need a soil depth of 24 to 36 inches (60 to 90 centimeters).
Crops with deep roots include:
You may also want to leave extra space at the top of your raised garden bed. This makes room for adding mulch over the soil later in the season.
There are also some special considerations for depth when planting certain crops. For example, you may want to leave 6 to 12 inches above the soil surface when planting potatoes in a raised bed.
The reason is that you need to use hilling during the season when growing potatoes. Hilling means that you pile up soil around the base of the potato plant as it grows.
This protects the potato tubers (the part we harvest and eat!) from exposure to sunlight. If potato tubers are exposed to sunlight, they will turn green and produce solanine (a toxic substance you do not want to eat!)
Finally, take into account the need for drainage of water from your raised garden bed. If you are using a soil mix that retains water (heavy clay), then you might want to make your raised bed a little deeper.
This will encourage water to drain out of your raised bed, or at least drain to the lower portions of the bed (thanks to gravity!) You can learn more about how to improve drainage in raised beds in my article here.
What do I Put on the Bottom of a Raised Garden Bed?
You might want to allow the roots of plants to grow right into the soil below your raised garden bed. In that case, you can put nothing on the bottom of the raised bed.
However, if the soil in your raised bed is shallow, then grass or weeds can grow up from below. This is true even if you did not plant them – the seeds of weeds can be persistent!
As mentioned earlier, you can use a layer of cardboard to smother grass and then build a raised bed on top of it. You can also use a raised garden bed liner to keep moles and other pests away from your plants.
A raised garden bed liner could be made of plastic, landscape fabric, or other materials. You can learn more about raised garden bed liners in my article here.
What Soil to Put in Raised Beds?
You should put garden soil in your raised beds. Garden soil is top soil that has been improved with compost or aged manure, and possibly other fertilizers.
Do not use pure top soil, compost, or manure in your raised garden bed. Otherwise, you won’t get the best harvest possible.
Remember that manure that has not been aged properly can burn your plants (due to high nitrogen or salt levels). Don’t apply fresh or “hot” manure directly to your plants!
Also, keep in mind that mulch is meant to cover garden soil. Mulch is not meant for growing plants!
How Much Soil for Raised Beds?
You will need to know three dimensions to find out how much soil you need for your raised bed:
Measure each of these three dimensions in the same units. Then, multiply them together to get the volume of soil you need: (Volume) = (Length) x (Width) x (Depth)
For example, let’s say that you have a rectangular raised garden bed that is 6 feet long and 3 feet wide. Assume that you want a soil depth of 2 feet in the bed.
Then the volume of soil you need to fill the bed is 6x3x2 = 36 cubic feet.
If a bag of soil contains 2 cubic feet, then you would need 36 / 2 = 18 bags to fill your raised bed.
Remember that a taller raised garden bed will hold more soil. Deeper soil means more weight on the sides of the bed.
You might need to install cross supports for very deep or very long raised beds. This will help to prevent the sides from bowing out, especially when the soil gets wet and heavy after rain or watering.
What to Use to Build a Raised Garden Bed
You can use stone, plastic, or wood to build raised garden beds. There are lots of options within each of these categories, each with their own pros and cons.
Now you know how deep to make your raised garden bed. You also know what factors to consider when adjusting the height.
I hope you found this article helpful – if so, please share it with someone who will find the information useful.