How We Built Our Hügelkultur Keyhole Garden

For this project we combined two techniques – the “Hügelkultur” gardening technique which involves creating a raised bed filled with a variety of organic materials, and a “Keyhole Garden”, which is a raised bed with a compost basket built into the center. This combination of techniques is gaining popularity as they are easy to construct using found materials and provide a long lasting self-feeding bed for growing vegetables.

Keyhole Gardens are large circular raised beds about 6 feet across that usually have a stone wall, rather like an old fashioned well, that runs about 3 feet high.

A small notch in the circle (the keyhole) is cut into one side so that the gardener can reach the composting basket at its center. The basket runs all the way down to the base of the garden; scraps and other compostable materials can be dropped directly in. This feeds the raised bed continually, thus saving the gardener space and time. Keyhole gardens originated in South Africa. The stone wall helps conserve water in dry environments and the raised beds allow older gardeners and gardeners with disabilities to access the garden from a standing or seated position. In Africa they were usually positioned next to the kitchen and planted with leafy greens, herbs and root crops; plants with larger root systems such as tomatoes are not as effective in these.

We began ours by marking out a six-foot circle in a leveled area of our garden. We then marked out the “keyhole” and a one-foot-wide inner circle for the composting basket.  To construct the basket, we drove pieces of rebar into the ground and wired them to chicken wire.  We then repeated the process for the larger circle to mark the outer wall.  Around the outer wall we created a “stone” wall of cinder blocks. Almost any stone-like material can be used – actual stone, bricks and pavers also work – and these materials can often be found on Facebook’s Marketplace free to anyone who is willing to pick them up and take them away.

We made our first mistake at this stage – it is generally a good idea, if using cardboard to kill the grass, to place that cardboard BEFORE you build the wall up. Lesson learned!

Notes on using cinder block: before we placed them, we dug a small trench on the footprint and filled it with sand and leveled it.  We also used the level as we went around the first tier.  This meant the succeeding tiers also remained level which is important as they will be weight bearing.  You can also drive rebar down the interior of the cinder blocks or fill them with dirt or sand to help maintain strength and stability.

When the garden reached the desired height, we finished off the “keyhole” entrance with some leftover wood, and voila! Our structure was finished.

Next came filling the structure, and that is where the Hügelkultur method comes in.  Large, raised beds such as keyhole gardens require an immense amount of soil.  Hügelkultur utilizes compostable materials such as wood to fill in the bottom. On top of the cardboard layer (which you definitely placed BEFORE you built your wall), you can begin filling the garden with wood.  For the first layer you can use logs up to 40% of the height of your overall garden (about 15 inches for a three-foot high garden). The next layer can be twigs and sticks, followed by a layer of leaves, grass clippings and garden scraps. Grass clippings and leaves are especially important for replacing the nitrogen that the rotting wood on the base layer will use. Next comes a layer of compost, and lastly a layer of topsoil. The bottom layers will slowly rot, providing a healthy environment for your crops, as well as acting as a sponge to retain moisture. Be sure to water your layers as you build them to help prevent air pockets. Also, be mindful of the type of wood you use; hardwoods are best. Avoid allelopathic trees such as black walnut and sycamore as they have naturally occurring chemicals that inhibit rot.

There you have it! This project was fun - not difficult to construct and fairly easy to find free materials for. Ours was built for next to nothing. Other than a couple of $6 bags of sand, we used materials left over from other projects and acquired the cinder block for free by someone trying to get rid of it. The only time-consuming part was leveling that first layer of cinder block; after that construction went very fast. While it can be done by one person (as in this case), a second set of hands would be much appreciated for all those cinder blocks!

Below are some resources on constructing a keyhole garden and hügelkultur methods. Please note there is no “right” way to do this project. These videos show a variety of construction methods, materials and sizes. Try watching several and then feel free to experiment based on what works best for you. We hope you enjoy learning about this as much as we did.

This video by El Paso Water gives a short but thorough tutorial on how to build a Keyhole Garden.

This video is far more involved and was produced by a personal favorite of mine, GreenShortz DIY.  We took our inspiration from him. He really likes working with found materials and tying his projects into other projects he has going on in his garden. Very detailed in his description of his thought process.

I’m also linking in a discussion from the Permies.com website on first year plantings for Hügelkultur gardens. They have some interesting advice.