Building an Air-Pruning Raised Garden Bed

If you have limited space or poor soil conditions, raised garden beds are a viable option.
Raised beds give you more control over your growing medium, provides better drainage and can even allow you to get a head start on the growing season since the soil inside a raised bed usually warms up faster than in a conventional in-ground garden.

Most raised beds are built inside a wooden or metal box, but we came across a method that is perhaps the most unique we have seen: air-pruning raised beds.
The advantage of this design is that it allows air to penetrate directly into the soil promoting soil health and air pruning the roots of those plants growing closest to the side of the bed. And the fact that it is 24 inches tall makes it easy on the back!

We got this idea from Mark with the I AM ORGANIC GARDENING YouTube channel. Mark is a market farmer based in New Jersey who uses regenerative methods to grow his fruits and vegetables. If you want to learn more about this natural growing method that yields more nutrient-dense fruits and vegetables, Mark does a great job of explaining it. We highly recommend his YouTube channel as a great resource for chemical-free gardening.

Mark has two YouTube videos that take you through the process building his air-pruning beds: “How to Build a Raised Wood Chip Organic Gardening Bed for beginners, Cheap Designs – Part 1” and “How to Build ( NATURE’S Healthy Soil ) in Raised Wood Chip Organic Bed Gardening – Designs – Part 2.”

The frames are built from a single cattle panel that is cut in half. You can find these panels at most big box home improvement stores or farmers supply. Each panel is 50 inches high and 16 feet long, allowing you to build two air-pruning beds. Each half of the panel can be used to build a 4 foot x 4 foot square and/or a 5-foot radius circle garden bed.

In addition, you will also need 32 feet of hardware cloth or screen that its 24 inches wide. This goes inside the bed frame to hold in your growing medium.

Building the Frames

Measure 25 inches to find the middle of the height of the cattle panel. Use a grinder, bolt cutters or hack saw to cut the panel into two pieces that measures 25 inches high and 16 feet long. If you have a pair of bolt cutters around, they work great!

Once the panel is cut, bend each half into the configuration you want, either a 4×4 square or 5-foot circle. If you go the square route, make marks every 4 feet and then get a board to lay across the panel to give you support to bend the corners (Mark shows how to do this in the video). These metal panels don’t like to bend easily so expect to wrestle with them a little!

After you have configured your bed frame, you will notice one side has spikes on it. This is the side the will go in the ground to help support the frame.

Connect the ends of the panel using clamp or wire, and then line the inside of the frame with your hardware cloth or screen. Either ½-inch or ¼-inch hardware cloth will workd. You can zip tie the hardware cloth/screen to the bed frame to hold it in place as you begin to fill it.

Filling the Bed

Mark lines his air-pruning raised beds with a 5-6 inch layer of wood chips along the inside of the frame. To accomplish this, you will need to will need pile the wood chips against the inside of the bed frame about 8-12 inches and then fill the inside of the bed with your growing medium (Mark goes over this in the video). Repeat this process until you have filled the bed.

In the video, Mark uses leaf mold and compost for his growing medium. He also incorporates some of his best garden soil into the bed to help introduce native microbes. Once he fills the bed, Mark gives you a combination of crops he plants to help promote microbial life inside the bed.

A Hybrid Method

Filling a 4×4 square or 5-foot circle garden bed that is 24 inches tall can take a lot of soil!
Les Walz with the University of Arkansas Division of Agriculture Cooperative Extension Office developed a hybrid method that incorporates a couple of raised bed growing methods into one bed: hugelkultur and lasagna gardening.

Hugelkultur is a centuries-old growing method used in Germany and eastern Europe that essentially establishes a garden bed over the top a decaying pile of wood. The benefits of hugelkultur is that the wood soaks up the water during the rainy season and then releases it throughout the dry spells. In addition, as woody material begins to decompose, it feeds the garden over time.

Meanwhile, lasagna gardening uses alternating layers of “greens” and “browns” to build what is essentially an active compost bin beneath your garden.

The “green” layers consist of nitrogen-rich materials like fresh grass clippings; vegetable scraps and other plant kitchen scraps; coffee or tea grounds; fresh plant cuttings; and manure from plant-eating animals.

The “brown” layers consist of carbon-rich materials like dry leaves; straw or clean hay; shredded newspaper or plain cardboard; wood chips, bark, or small twigs; lightly used sawdust, and pine needles.

Avoid putting meat, dairy, fats, and bones; treated wood; weeds that have gone to seed; and grass or plant material sprayed with herbicides or weed killers into your lasagna bed.

The Hugelkultur Layer

Start with the hugelkultur layer at the bottom of your bed. This can include tree stumps, limbs and twigs from tree species that will rot over time. Avoid tree species like cedar, black locust, redwood and white oak that resist rot or decay. This layer of woody material needs to be about 8 to 12 inches deep.

I like to include pieces of wood in various states of decay in the hugelkultur layer, especially native wood that has been in contact with the ground for several months or even years. That contact with the ground means the microbial life is already at work inside the wood.

Once the wood is in place, fill in any gaps you have in the hugelkultur layer with wood chips, shredded leaves or compost. You want this layer to be like a sponge to soak up water.

As you build the bed, water in each layer, including the hugelkultur layer at the bottom. If possible, use non-chlorinated water since the chlorine can kill some of the good bacteria in the soil.

Add the Lasagna Layers

Once the hugelkultur layer is in place, throw in an inch or two layer of compost. You might even want to include some native soil to help introduce the native microbes into the bed. A great source for native soil would be some undisturbed woods. I rake back the leaves that are covering the forest floor and a scrap off some of the natural compost that is beneath it to include the bed.

The next step is to begin the lasagna layer. You will alternate between “brown” and “green” layers, starting with the brown layer first. The brown layer needs to be about two to three times deeper than the green layer. In other words, if you have a 3 to 4 inch brown layer, the green layer will need to be 1 to 2 inches deep.

Continue alternating the brown and green layers, keeping a ratio of roughly 2 or 3 parts brown to 1 part green. Remember to water in each layer! This encourages the decomposition to begin.

Once you’re about 6 inches or so from the top of the bed, finish it off the top layer with at least 4 to 6 inches of compost and/or topsoil. This will serve as your planting layer.

Closing Thoughts

Keep in mind that both the hugelkultur and lasagna gardening methods are essentially ways to turn your garden into a compost bin. As a result, those layers beneath your growing layer will begin to decompose and you will notice that the growing medium inside the bed will begin to shrink. Don’t be surprised to see it fall by about 12 inches over the course of the year.

This will allow you to add fresh compost for the next growing season to help maintain the soil health inside the bed.

While there is some initial investment in building these beds, we have found them to be long lasting and we have been impressed with their overall performance.