How Deep Should Open-Bottom Raised Beds Be?

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Willie writes on small engine repair, landscaping, and lawn care. He owned a professional lawn care business in Alabama for almost 10 years.

Why the Depth of Your Open-Bottom Raised Garden Beds Matters

When you’re building open-bottom raised garden beds, you may wonder whether their depth even matters. The simple answer is yes.

If your garden beds are too shallow for the plants planted in them, their roots won’t have space to spread out properly, and this will adversely affect how your plants grow.

On the flip side, if your garden beds are too deep, there’s a risk that the weight of the soil will result in your garden beds collapsing in on themselves.

This is because, while the soil in your garden beds will likely be light and aerated when you first shovel it into the structure, it will settle and compact over time.

Compacted soil is denser than aerated soil (source), and the weight increases further when the soil is wet, either from rain or watering your plants. Furthermore, compacted soil drains water slower than aerated beds, so the weight does not dissipate quickly enough.

As a result, your raised garden beds may collapse in on themselves.

This is also why we recommend that you don’t make your garden beds too deep if the plants you’re growing in them don’t need it.

While herbs can grow just as effectively in 18-inch (45.72 cm) garden beds as in 6-inch garden beds (15.24 cm), the shallower garden beds will have a lower risk of collapse.

Additional Factors That Affect the Depth of Your Open-Bottom Raised Garden Beds

Aside from the needs of your plants, there are some other considerations that may affect how deep your garden beds will turn out to be:

The surface that the raised garden beds are placed on. If you are placing your open-bottom raised garden beds on hard surfaces, such as concrete, they should be deeper than they would be on softer surfaces (source).
Accessibility concerns. If your raised garden beds need to be wheelchair accessible, they’ll need to be taller (and therefore deeper) to make them accessible. On the other hand, if you hope to use your garden beds to teach your children about gardening, having them closer to the ground (and therefore shallower) can make them easier for kids to handle.
Location of the raised garden beds. If the beds are placed on the over soil, the roots of your plants will have additional space to grow. However, if they are placed over concrete and other hard surfaces, the roots will not be able to reach the soil underneath the hard surface and will be dependent on the soil in the bed. So, they will need to be deep enough that the roots can grow and spread out comfortably.

The Depth Open-Bottom Raised Garden Beds Should Be for Different Plants

Now that you understand why having the right depth is important, the next step is determining how deep your open-bottom raised garden beds should be.

Here are a few common plants grown in raised garden beds and how deep the beds for each should be:

Tomatoes: Tomatoes require garden beds that are at least 12 inches (30.48 cm) deep (source). However, some recommend that garden beds be as deep as 24-36 inches (60.96-91.46 cm).
Strawberries: Your raised garden beds will need to be 6-12 inches (15.24-30.48 cm) deep to grow strawberry plants. For safety, I recommend having garden beds that are at least 8 inches (20.32 cm) deep. While 6-inch (15.24 cm) beds can work, they must be impeccably maintained in shallower beds.
Herbs: Herbs need a minimum of 4-5 inches (10.16-12.7 cm) of soil (source). For safety, I recommend garden beds that are 6 inches deep (30.48 cm). This will give your herbs a little extra space and reduce the risk of having to transplant your grown herbs into a deeper bed.
Lettuce: Lettuce requires about 6 inches (30.48 cm) of depth to grow properly.
Carrots: Like tomatoes, carrots have long roots that can grow as long as 12 inches (30.48 cm). For this reason, your garden beds should be at least 12 inches (30.48 cm) deep – however, to ensure you have emergency space if their roots grow longer, we recommend choosing garden beds that are at least 18 inches (45.75 cm) deep.

Other Size Considerations When Building Open-Bottom Raised Garden Beds

Aside from choosing the proper depth, you must also choose the right width and length. This is because, when you plant your crops, there will need to be adequate space between each plant.

If there is inadequate space between each plant, they will compete with each other for resources, including sunlight and soil nutrients. There also won’t be enough space for their roots to spread properly. This will impact their growth and can result in some plants dying off.

Additionally, it can also increase the risk for fungal and pest infections.

As with depth, different plants require different spacing levels. Vegetables need anywhere from 1-3 inches (2.54-7.62 cm) of space between plants (for carrots) to 60-72 inches (152.4-182.88 cm) of space (for pumpkins). Additionally, there will also need to be adequate space between each row of vegetables.

So, before you build your garden beds, make sure you have considered the length and width, as well as the depth.

Final Thoughts

Open-bottom raised garden beds need to have adequate depth to allow your plants’ roots to grow properly. Most home gardens won’t require more than 8-12 inches (20.32-30.48 cm) of space.

However, the depth ultimately depends on the type of plants you grow, and some will require as much as 36 inches (91.46 cm) of space.

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