How Deep do Raised Garden Beds Need To Be?

Raised beds are a great option for gardeners but how deep do they really need to be? The depth of a raised garden bed is an important factor to consider because it can greatly impact the health and productivity of your plants. The true answer to How deep do raised gardens need to be really is it depends!

What the Soil Like Underneath Your Raised Beds?

Ask yourself a few questions before building your raised beds:

• What will the soil be like under my raised beds?
• Will it be good friable earth perfect for bumper crops of vegetables?
• Or is it more akin to rocks and stone like we seem to have here in Tennessee?

When determining the height of your raised garden bed, it is important to consider the soil substrate and the condition of the ground underneath. If the soil is poor or compacted or not even soil but rock or concrete, you may need a deeper raised bed to provide enough space for healthy root growth. On the other hand, if the ground underneath is already fertile and well-draining, a more shallow raised bed may be just fine.

Below are some old raised beds made from 2″x10″ lumber

There are a couple things you can do.

Take a shovel out to the garden spot and do some test digs in a few places. Are you hitting any rock? Is the ground coming up sandy? Too much clay? If you are coming up with issues that may be harder to deal with plan on building taller raised beds.

I’ve seen this question come up in gardening discussions as well: “can you build a raised bed on concrete?” You certainly can, just plan on a taller raised bed in these situations to allow plenty of room for plant roots to grow.

If you are Designing a Raised Bed Garden Be sure to check out these tips: Designing a Raised Bed Garden Layout 11 Things to Think About

Advantages of a Taller Raised Bed

There are several advantages to having a taller raised bed. A deeper bed allows for better drainage and aeration, which can prevent waterlogged soil and root rot. It also provides more room for deep-rooted plants and allows for better control over soil quality and amendments. Think about the root structure of the plans you want to grow and add a couple inches to that depth.

Taller raised beds can also be great option for aging gardeners and those with mobility issues. One of the main benefits is that taller raised beds are easier to work with for people who may have difficulty bending or kneeling for extended periods of time. By raising the height of the bed, it reduces the strain on the back and knees, allowing the gardener to comfortably tend to their plants.

Disadvantages of Tall Raised Beds

The biggest disadvantage I see to taller raised beds is that they require more soil and amendments than shorter ones. This makes taller raised beds more expensive and a bigger workload – at least initially. Soil can be pricey and it never goes as far as you hope. Even if you are the most avid composter you are probably ALWAYS looking for more material for the bin. There is never enough compost! So you can imagine if you simple double the height of your bed you will essentially double the cost of filling it and sourcing all that organic matter. Organic Matter Matters!

There is a trick you can use to fill tall raised bed at a lower cost: use logs to hold some of the volume (a quasi-hügelkultur method). This still increases the work load factor but can save you money. Ideally you can locate a good local source of compost material and maybe someone to bring you some good logs to use. I have also used well broken down wood chips to fill raised beds. The downside to the wood chops is they will need to compost for 6 months or longer to break down well enough to use as raised bed filler.

Check out this article for more on filling raised beds: How to Fill Raised Beds CHEAP!

Spread out the cost and add a little each year.

To offset these significant costs for taller raised beds you can simply plan on adding a little soil more each season. The first year put as much as you can in the raised bed then each year add a little bit more compost on top. Root and soil depth will be the ultimate determining factors to how much soil you actually need in the garden bed. For example if you are planning a root crop like carrots you want enough raised bed height that you can allow them to develop to their fullest potential. That will usually be a couple inches deeper than your crop will grow.

Weight and Stability

Another potential disadvantage of taller raised beds is the increased weight and potential stability concerns. The additional soil required to fill the deeper beds can make them heavier, which may pose challenges during construction and maintenance. It’s important to ensure that the structure of the raised bed is sturdy enough to support the weight of the soil and plants. You may need to add cross braces to hold the middle of each raised bed together and prevent bowing.

The Problem with Short Raised Beds

While taller raised beds have their advantages, it’s important to consider the concerns associated with shorter raised beds as well. One of the main problems is the limited depth of soil available for plant roots. Shorter raised beds typically have a shallow soil substrate, which can restrict root growth and nutrient uptake for plants. As I mentioned before this can be especially problematic for crops with deep root systems or those that require a larger volume of soil.

Soil Compaction

Another concern with shorter raised beds is the potential for soil compaction. With a shallower soil depth, there is less room for the soil to naturally aerate and drain excess water. This can lead to compacted soil, which hinders root development and can negatively impact plant health and productivity.

More watering…more fertilizing…

Shorter raised beds may require more frequent watering and fertilization. The limited soil volume means that water and nutrients can be quickly depleted, requiring more frequent maintenance to ensure optimal growing conditions for plants. You can offset this by adding an irrigation system or using a watering technique like Ollas near your plants.

All that being said if the ground underneath your raised beds is good you shouldn’t have to worry about the height of the raised bed. The raised bed merely acts as an extension of the ground with improved nutrition from the amazing soil you are developing!

My Recommendation for the Ideal Raised Bed Height

After many years of growing raised beds (since 2007) my recommendation for the ideal height of raised beds is 6 to 8 inches tall if you have good soil underneath. Otherwise I would double the height to 12 to 16 inches tall for your raised beds. Taller is generally better – to a point. I once built a tall metal sided raised bed. I used 3ft wide metal roofing material which I framed with lumber and screwed all the sides together. The 3ft of height on the raised bed was a challenge to fill.

The materials I used to fill the raised beds broke down very quickly and the garden bed continued to sink as the filler material I used (leaves, grass clippings, branches, logs) composted. I eventually took apart that raised bed because of the difficulty in filling it. In realty the plants didn’t need a bed that deep anyway. Plants will take their nutrition from as deep as their roots will grow. In many cases that isn’t very deep at all.

Consider the Root Systems

For tomatoes and peppers (which I like to plant deep), I will dig into the soil underneath the raised beds when planting them. I’ve been very lucky with good soil under my beds. This year we’re in a new location and hopefully will have similar soils but I know rock is present in some locations. My goal is to start with 8 inch tall beds then add a second level to them later. By using a 3 2×8 pieces of lumber I can easily make an 8″ tall raised bed that is 4ft x 8ft in size. Then it can be stacked by doing the same exact layout the following year. By adding the garden beds gradually I’ll be spreading out the workload and the cost of building the garden.

A 24 inch raised bed would be the maximum height I would recommend because any taller would make it harder to load more soil into it with a wheel barrow. Personally I would favor 16 inches because of the ease of filling it and the ability to more easily reach plants for weeding and harvesting. 16 inches is a good blend of raised bed height without going overboard.

Unfortunately no one can tell you the exact right height for your raised beds but as a general guideline at least go 6 inches tall with great soil underneath and try to double that if your soil isn’t so great. Remember you can always get a soil test from your local agricultural extension office and see just how great your dirt is!

For Soil Testing in Tennessee: University of Tennessee Agriculture