Depth is the one specification you cannot change after you buy. You can improve the soil, move the bed, change what you plant and top it up every spring, but an 8 inch bed is an 8 inch bed forever. That makes it the decision worth getting right, and the answer depends on something most guides never mention: whether the bed has a bottom.
The short answer
A raised bed sitting on open soil needs 6 to 8 inches for salad and herbs, 10 to 12 inches for most vegetables, and 18 inches or more for asparagus, rhubarb and berries. A bed with a solid floor, on a patio or deck, needs the full mature root depth of the crop because roots cannot continue into the ground below.
That distinction changes the numbers by a factor of two, and it explains why gardeners argue about raised bed depth so much. They are answering different questions. This guide gives you the depth by crop, the reason the open bottom matters, and the honest case for going shallower than the internet tells you to.
What you'll learn:
- Minimum and comfortable depth for every common crop
- Why an open bottom halves the depth you need
- When deeper genuinely pays, and when it just costs more
- How to work with a bed that is already too shallow
How Deep Should a Raised Bed Be, by Crop?
These figures assume an open-bottomed bed on unsealed ground. The minimum column is what the crop will tolerate; the comfortable column is where it stops being a constraint on yield.
| What you want to grow | Minimum depth | Comfortable |
| Salad, spinach, herbs, radish | 6 in (15 cm) | 8 in (20 cm) |
| Beans, peppers, cucumbers | 10 in (25 cm) | 12 in (30 cm) |
| Carrots, beets, potatoes | 12 in (30 cm) | 15 in (38 cm) |
| Tomatoes, squash, brassicas | 12 in (30 cm) | 18 in (46 cm) |
| Asparagus, rhubarb, berry bushes | 18 in (46 cm) | 24 in (61 cm) |
Compiled from US cooperative extension guidance including University of Maryland Extension and NC State Extension.
If most of that list is annual vegetables, 12 inches is the number to buy. It covers everything except the perennials, and it is the point where cost per extra inch starts to outrun the benefit.
Why Does an Open Bottom Change the Answer?
Because roots do not stop at the timber. A bed with no floor, sitting on soil you have loosened rather than paved, gives the plant the bed depth plus whatever it can reach below.
The University of Maryland notes that raised beds commonly run from 2 to 12 inches tall, which is fine for salad and limiting for much else if you treat the bed as the whole rooting zone. It is not the whole rooting zone in an open bed. NC State Extension describes asparagus crowns being progressively covered until they sit around 8 inches deep, and the plant then roots far below that, well past the bottom of any normal bed.
Why This Works
The bed is not a container, it is a soil improvement. You are raising the surface and improving the top layer while leaving the connection to the wider soil system intact. Mycorrhizal networks extend past the frame, worms migrate in and out, and drainage follows the ground rather than pooling on a false floor. That connection is why an open-bottomed bed needs less input every year than a sealed one, and it is the reason the depth question has two different answers.
What If the Bed Has a Solid Floor?
Then depth is the whole story and you should buy deeper than feels necessary. A bed on a patio, balcony, roof or decking is a large container. Roots reach the bottom and stop.
Two things follow. First, use the comfortable column in the table above rather than the minimum, because there is no ground below to buffer a bad week. Second, go deeper than 12 inches if you can, because a taller column of soil also drains better: the saturated zone at the base sits at a roughly fixed height regardless of bed depth, so the deeper the bed, the more freely draining soil sits above it. We cover that in detail in our guide to raised bed drainage.
Common Mistake to Avoid
Do not line a ground-contact bed with plastic sheeting to protect the timber. It converts an open bed into a sealed one, which means the depth figures in the first column no longer apply to you and every crop deeper-rooted than lettuce is now constrained. If the material worries you, change the material rather than wrapping it.
Is Deeper Always Better?
No, and the cost curve is steeper than most people expect. Depth is the main driver of fill volume, and fill is usually the largest single expense in building a raised bed.
A 4 by 8 foot bed at 6 inches deep needs 16 cubic feet. The same bed at 18 inches needs 48. You have tripled the soil bill to grow crops that in an open-bottomed bed would have been perfectly happy at 12 inches. That money is almost always better spent on better soil, or on a second bed.
There are three cases where the extra depth genuinely pays: you are growing perennials, the bed has a sealed floor, or the ground beneath is so compacted or contaminated that you want the plants to stay above it. Outside those, buy 12 inches and put the saving into the mix. Our guide to the best soil for raised garden beds covers what to spend it on, and if you are filling a deep bed, the layered filling method cuts the purchased volume by roughly two thirds.
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Send Me the ChartWhat Can You Do With a Bed That Is Too Shallow?
Quite a lot, as long as it is open at the bottom. A shallow bed is a constraint on crop choice rather than a write-off.
Loosen the ground beneath
Fork or broadfork the soil under the bed without turning it over. This is the single cheapest way to add rooting depth, and it costs nothing but an afternoon.
Match the crop to the depth
Salad, spinach, radish, bush beans and most herbs are perfectly productive in 6 to 8 inches. Save the carrots and the perennials for a deeper bed.
Add a second tier
Most modular metal and timber beds accept an extra course. Adding 6 inches to an existing frame is far cheaper than replacing the bed, and the fill for one extra course is modest.
Top up every spring
Organic matter breaks down and the level drops. An inch or two of compost each spring maintains depth and feeds the soil biology at the same time.
If you are still choosing, depth is the criterion our raised bed buyer's guide ranks above everything else, precisely because it is the one you cannot revisit.
Frequently Asked Questions
How deep should a raised bed be for vegetables?
Twelve inches suits almost every annual vegetable, including tomatoes, squash, carrots, beets and potatoes. Salad, spinach, radish and herbs manage in 6 to 8 inches. Only asparagus, rhubarb and berry bushes need 18 inches or more. These figures assume the bed has an open bottom and sits on soil you have loosened, so roots can continue below the frame. If the bed has a solid floor, use the higher figure in each range because the bed is the entire rooting zone.
Is 6 inches deep enough for a raised bed?
For salad, spinach, radish, most herbs and bush beans, yes, provided the bed is open at the bottom and the ground below is not compacted. Six inches is not enough for carrots, potatoes, tomatoes or anything perennial. If you have a 6 inch bed already, fork the ground beneath it rather than replacing it, and choose shallow-rooted crops. Adding a second course to the frame is usually cheaper than a new bed.
How deep should a raised bed be on concrete or a patio?
Deeper than you think, and 12 inches is a sensible minimum. A bed on a hard surface is a container: roots reach the bottom and stop, so the bed must supply the full mature rooting depth of the crop. Depth also improves drainage in a sealed bed, because the saturated zone at the base sits at a fixed height whatever the bed depth, so a taller bed leaves more freely draining soil above it. Eighteen inches is better if the structure will take the weight.
Do deeper raised beds grow better vegetables?
Only up to a point, and only for some crops. Once a crop has the rooting depth it needs, more depth adds cost rather than yield. A 4 by 8 foot bed needs 16 cubic feet of fill at 6 inches and 48 cubic feet at 18, so depth is the main driver of what a bed costs to fill. Beyond 12 inches in an open-bottomed bed, the money is usually better spent on a better soil mix or a second bed than on extra height.
How deep do carrots need in a raised bed?
Twelve inches is the working minimum and 15 is comfortable. Carrots fork and stunt when they meet a hard layer, so what matters as much as depth is that the soil is loose and free of stones for the full length of the root. A shorter cultivar such as Chantenay or a round type will crop well in 8 inches if that is what you have. Avoid adding fresh manure before a carrot crop, which also causes forking.
Does raised bed depth affect how often I need to water?
Yes, and deeper beds are more forgiving. A greater volume of soil holds more water and buffers the swing between watering, so shallow beds dry out fastest and need attention most often in hot weather. This is most pronounced in sealed beds, where there is no moisture exchange with the ground. Mulching the surface makes a bigger difference than an extra couple of inches of depth, and costs far less.
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