It can be challenging to decide how deep your raised garden beds should be, as many factors go into determining the best size. You should consider the plants you want to grow, how far you want to bend down to tend to your garden, and how much work you want to do when installing the bed.
A raised garden bed should be at least four inches deep. While in-ground garden beds can be relatively short, an above-ground raised garden bed must be deep enough to sustain your plants’ roots. Usually, above-ground beds should be at least one foot deep. However, we recommend a height of at least 36 inches.
So, let’s get into the details and figure out exactly how many inches tall your raised garden bed should be. Then, I’ll tell you more about how to choose your raised garden bed depth depending on your plants, your mobility, and the style of bed you choose.
- What Determines How Deep a Raised Garden Bed Should Be?
- How Wide Should a Raised Garden Be?
- Why Raised Garden Beds?
- Frequently Asked Questions (FAQs)
- Final Thoughts: What Is the Ideal Raised Bed Size?
What Determines How Deep a Raised Garden Bed Should Be?
The answer to every good question is: it depends. So, why would this question be any different? While the best garden bed depth will depend on a few different factors, it’s pretty easy to figure out the ideal size with just a bit of research – that’s why we’re here to give you the details.
A raised garden bed should be a different depth depending on the raised garden bed style, what you’re planting in it, and your ability to bend over.
So, let’s explore these factors next and determine the best depth for your garden.
In-Ground vs Above-Ground Raised Garden Beds
We’ve all seen raised garden beds, but did you know that there are multiple types of raised gardens, each with its own unique features?
There are generally two different raised garden bed types:
- In-ground raised garden beds: Raised gardens where the plant roots grow into the soil beneath
- Above-ground garden beds: Raised gardens where the plant roots don’t grow into the soil below
Each of these garden bed types have different minimum heights.
How Deep Should In-Ground Raised Garden Beds Be?
In an in-ground raised garden bed, your plant’s roots can grow into the soil beneath the raised garden bed. These beds do not have liners or bottoms because they are simply bottomless boxes that you stake on top of the soil.
In-ground raised garden beds should be at least four inches deep, but the deeper they are, the better. These garden beds don’t have a bottom, allowing your plants to grow their roots into your native soil as they mature. For that reason, they don’t have to be very deep.
In-ground raised garden beds are easy to install and work well for perennial plants. That’s because, since the bed has no bottom, your plants can continue to develop roots that go deep into your native soil, giving them access to nutrients below ground level.
Additionally, an in-ground raised bed helps plants get established in the nice, fertile soil that you’ve added. Once the plants get growing and mature, their roots will grow down into the natural soil beneath the raised bed.
This type of raised garden works really well because your plants get a good start to life as they root into the nice, fertile potting soil or humus on the top layers of the bed. However, by using an in-ground garden bed, you’re also improving the soil in that particular spot as the soil and organic matter you add to the raised bed will filter down into the natural soil beneath.
If you ever move the raised bed, you’ll have a beautiful, improved in-ground garden underneath, which you can choose to plant in or leave as-is, of course.
Here’s an example of what an in-ground raised garden bed looks like:
How Deep Should Above-Ground Raised Garden Beds Be?
In an above-ground raised garden bed, your plants’ roots do not grow into the soil beneath the raised garden bed. Instead, the bed either has a bottom, a liner, or legs, preventing your plants from expanding too much.
This type of raised garden is enclosed, or you’ve added a barrier between the raised garden and the natural soil.
An above-ground raised garden bed should be at least one foot deep, but that is just the minimum depth. Above-ground beds cut off your plants from the native soil, preventing the roots from growing beyond the beds. So, your plants may outgrow an above-ground bed if you do not provide them with enough vertical space.
Above-ground beds usually have bed drain holes at the bottom, but they don’t always have them. If you end up choosing an above-ground bed, you should check to be sure that it has some sort of irrigating drain before you plant anything in it. Drainage holes are a must if you want to keep the bed in good shape and keep your plant roots healthy.
If you can avoid using an above-ground garden bed, we recommend you do so. Creating a stand-alone ecosystem makes it harder to get things right in your garden. Plus, it might cause problems with drainage.
Remember that, in this type of bed, what you add is all your garden plants have. They can’t get their own nutrients or water. It’s up to you to provide them with everything.
Here’s an example of an above-ground bed:
Tall raised garden beds on legs are perfect for people who don't want to or can't bend over to tend their gardens. Beds like this one also increase drainage and keep the bed off the ground, making the wood last longer.
While it’s easy to see that this bed is above ground, some beds that sit directly on the dirt are also above-ground raised garden beds. These structures use a liner or a flat bottom to contain the soil, preventing your plants’ roots from digging deeper.
Naturally, the types of plants you want to grow in your vegetable garden will impact how deep they should be. Plants’ roots that need more space to grow, such as root vegetables, will do best in a deeper garden bed.
As a general rule, one foot of space is an adequate depth for root crops, such as carrots, beets, and potatoes. Other vegetables with shallow roots, such as lettuce, must have at least six inches of space to root. However, the ideal depth for any vegetable garden is 36 inches.
Garden plants will always grow best when they have plenty of space. Providing more depth allows them to develop deeper roots and access nutrients and water deeper in the soil, helping them mature more quickly and stay resilient all year.
Think of it this way – just because you can fit into a pair of jeans two sizes smaller than you would usually wear doesn’t mean that you can move or be comfortable in them. The same concept applies to plants in containers and garden beds.
Let’s look at tomato plants, for example. While a tomato plant can grow in a raised garden bed that is one foot deep, it won’t ever get very big in a bed of that size. For your tomato plant to flourish by harvest season and reach its ideal root depth, your above-ground raised garden bed should be more than twice as deep.
Still, when considering in-ground raised garden beds, as long as there are 26 inches of soil beneath your raised bed, your tomato plant should grow well.
How Deep Should Soil Be for a Vegetable Garden?
But what if you want to grow smaller-rooted plants like leafy greens in your vegetable garden?
Well, here’s a helpful guide to help you determine your plants’ soil depth requirements:
|Plant Type||Examples||Minimum Soil Depth Requirements|
|Shallow-Rooted Plants||Lettuce, fennel, chives, spinach, onions, garlic, strawberries, cranberries, basil, cilantro||6 inches of soil depth|
|Moderate-Rooted Plants||Broccoli, cabbage, cauliflower, beets, carrots, eggplant, kale, okra, peas, peppers, potatoes, turnips, sweet potatoes||18 to 24 inches of soil depth|
|Deep-Rooted Plants||Asparagus, lima beans, melons, tomatoes, squash, corn, pumpkins, parsnips, grapes, blueberries||26 to 36 inches of soil depth|
We always recommend that a garden bed have at least 36 inches or three feet of soil beneath it, especially when growing a vegetable garden. That way, you won’t be squeezing your plants so tightly that they can’t grow to their full potential. Ample space generally means bigger, healthier plants and better crops.
Still, if you only want to grow a few shallow-rooted leafy greens like lettuce, you may be able to get away with a very shallow garden bed. But, generally, even lettuce greens will grow better in a deep garden bed since deeper soil means better drainage and more soil nutrients.
Plus, any additional space can help you maintain the proper amount of soil moisture. Some well-needed drainage space below your plants’ roots can prevent your raised garden bed from becoming a cesspool of root rot. That really doesn’t sound fun.
Your Ability to Bend Over to Reach Your Raised Garden Bed
While raised garden beds are an excellent tool for preventing weeds, concentrating water and fertilizer in one area, and keeping your plants in one spot, they are also great for those of us with back problems or other mobility issues.
Raised garden beds, as long as they are tall enough, can give people with limited mobility the opportunity to do garden maintenance and offer access for weeding. That’s a great benefit!
If you have any bodily discomfort when gardening or limited mobility, raise your garden bed height to at least 36 inches in height. Raised garden beds of this height allow your plants to reach their ideal root depth while also being tall enough to prevent joint discomfort and accommodate people who use wheelchairs.
Still, if that’s not a concern for you, just be sure there are around 36 inches of soil underneath your garden bed.
How Wide Should a Raised Garden Be?
A raised garden bed should be around four feet wide and three feet deep. Four feet of soil will give your plants plenty of room to sprawl. In addition, it will allow you to reach every plant in the garden easily without stepping onto the soil or bending over excessively.
You can make your garden bed however long you like. Just remember to follow planting instructions for whatever you choose to grow, giving each seed or seedling plenty of room to expand over time.
Build the beds in the direction that works best for your property. You might like to build one along a walkway or along the side of the house, for example. If you’re building in an open space, face them north-south. A location that has sun exposure to the north and south gives the most sunlight to each plant in the bed.
Why Raised Garden Beds?
Raised beds can be beneficial in some circumstances.
One of the main reasons to grow in raised beds is poor-quality soil. If your soil lacks nutrients or is very sandy or full of clay, a raised bed can solve your problems. Raised beds can also look great, and you can put them wherever you like, even on stone or concrete.
However, raised garden beds can be more work than gardens in your native soil. A garden bed effectively cuts your plants off from your backyard ecosystem, which, in some cases, can be beneficial. However, in other cases, it means you’ll have to work harder to fertilize, prevent pests, increase drainage, and ward off infections.
So, as with anything, raised garden beds have a unique set of pros and cons.
Advantages of Raised Bed Gardens
Some of the biggest advantages of using a raised garden bed are:
- Raised garden beds allow you to focus on soil quality in a small area. Raised beds are clearly defined gardens, so you can concentrate on that area to improve the soil. You can fill the beds with good-quality soil and add lots of organic matter to create a healthy environment for plant growth. Additionally, when you are using an in-ground bed, you are improving your native soil by layering fertile garden soil on top of it.
- Raised beds offer frost and sun protection. Raised beds are easier to insulate or cover than an in-ground garden. With a raised garden bed, it’s easy to cover it with shade cloth for the sun in hot summers or with some insulation in the cold winters. Additionally, raised beds generally have a higher soil temperature than ground soil since beds have better drainage.
- You can plant earlier with a raised garden bed. Raised beds generally have excellent water drainage. Because of that, your soil warms up faster in spring, giving you a head start to planting.
- Raised garden beds can improve your landscape. Raised beds can look fantastic, and you can place them anywhere, even on concrete.
- Raised garden beds give you better access to your plants. Raised gardens are great if you have back problems or other issues bending down. You can make them quite tall so they’re very easy to get to.
- Raised garden beds can go in small, otherwise, unusable outdoor spaces. An above-ground garden bed can go anywhere, which makes these gardens very useful for people who don’t have much soil space in their backyards. For example, these beds can go on paved surfaces, allowing you to use your patio as a garden.
An Unexpected Benefit
An unexpected advantage for me became apparent just the other day. We received over 40″ of rain in one week. The farm was a lake! I lost a ton of topsoil due to runoff in my ‘regular’ gardens. The raised beds, however, stayed put, and even though they lost some soil, most of it remained. I was able to put some new compost on top and they were as good as ever.
The plants in the raised beds also fared better. Once it stopped raining, many plants in the garden went downhill. This is because the soil, after being so soaking wet, contracts and compacts when it dries out. In the raised beds, soil compaction is much less of an issue. Most of my bed-plants recovered nicely.
What Type of Garden Soil Is Best for Raised Garden Beds?
The best type of garden soil for a raised garden bed is a loam-based fertile soil that drains well. You can use your existing soil in a raised bed or buy soil to fill it. You can also use soil dug up from somewhere else. The one thing you want to avoid is a “soil line of change.”
A soil line of change is a distinct line where one soil ends and another starts. This line creates problems with root and can interfere with the dirt’s ability to offer consistent soil moisture.
If you’re adding different soil to the raised bed, mix a bit of the natural soil with the garden soil, so there’s a gradual change.
If your aim is to solely grow in an above-ground raised garden bed, without the plants’ roots growing into natural soil, you should aim for a sandy loam soil to assist drainage. Add lots of organic matter nonetheless!
Before you buy soil, remember that just because you can buy it doesn’t mean it suits your purpose. Commercial soil that’s made with aged manure can be high in salt, for example.
Some commercial planting soils are too light for raised gardens and won’t hold water at all. That causes a nightmare situation where you’re constantly fighting against your soil drying out. Once it dries out, there’s no re-wetting it without serious intervention.
Always mulch deeply in your gardens. Placing a thick layer of mulch on the soil surface is one of the most important things for your raised garden beds, as it can maintain consistent soil moisture for you. Plus, it adds nutrients to the soil over time, minimizing the need for other fertilizers. Offer your plants at least three inches of mulch, but more is better.
Should I Till the Soil In My Raised Garden Bed?
You should till the soil in your raised garden bed to break up clumps and mix the soil you are adding to the bed with your native soil. Mixing your garden soil into the native soil will ensure your plants can develop deep roots beyond the bottom of your raised garden bed.
I’ve always built raised gardens with the intention that eventually, plants’ roots will grow into the natural soil. In this case, it’s best to use garden bed soil that’s as close to the natural soil as possible.
Double-digging is a very helpful technique when establishing raised garden beds. In double-digging, you dig up the top 6” of soil and move it aside. Mix organic matter into the rest of the soil, then add the 6” back on top. The top 6” is your topsoil, the most nutritious part.
Frequently Asked Questions (FAQs)
Raised garden beds cannot be too deep. The deeper your garden beds are, the more space your plants will have to develop healthy, robust, deeper root systems. Additionally, deeper soil will only add soil drainage, keeping your plants healthy and free of infections like root rot.
You can put any sort of organic material in the bottom of a raised garden bed, including dried leaves, mulch, paper, twigs, grass clippings, straw, or cloth. Whatever garden bed materials you choose, be sure to make it nice and loose to increase soil drainage and promote deeper root development in your plants.
Above-ground raised garden beds, which have a barrier between your native soil and your garden soil, need drainage holes in the bottom. When plating in an above-ground garden bed, it’s also best to incorporate some sand or perlite into the soil to increase soil drainage and prevent water from building up inside.
A no dig raised bed should be at least six inches deep, but your plants may need more root space. Lettuce, spinach, onions, and garlic may grow healthily in a six-inch-deep garden bed, but other plants like potatoes, peppers, and tomatoes, need between 18 and 26 inches of soil depth. A depth of 36 inches can accommodate almost any vegetable plant.
Final Thoughts: What Is the Ideal Raised Bed Size?
So, the answer to: “How deep should a raised garden be” depends on a few things, but, hopefully, this has given you an overview of what to aim for.
Ultimately, the ideal raised bed size is four feet wide by three feet deep. However, you may be able to get away with a garden bed that is narrower or shallower if you are using an in-ground garden bed or only planting shallow-rooted plants.
If you have any questions or tips about raised gardens, please leave a comment!
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