Along with the herb spiral, keyhole beds are often among the first permaculture design methods that a student becomes familiar with. They are often used in permaculture designs because they maximize the available planting space in a bed. Planting in a circle is much more efficient than planting in straight rows, allowing you to fit a greater number of plants into the space and thus, increase your yield.
Keyhole beds can either be stand-alone structures or combined into a series, with keyhole access points branching from a central path (winding to increase edge, of course). This will depend on the amount of space you are cultivating as garden beds, as well as your mobility and access. For instance, if you are in a wheelchair you will need more keyhole space to access the bed, while the bed will be narrower to accommodate your range of reach.
Garden beds using the keyhole design can be at ground level or raised. Raised beds make tending the garden easier, and can prove very productive as they are established on lots of compostable material, and constantly fed more nutrients from a center point. This is also energy efficient as they are no-dig beds. Here are the steps to building a single raised keyhole bed.
Mark out a circle where you want to site your keyhole bed. As mentioned, the size of your bed will depend on a number of factors. The available space for cultivation, your mobility and reach, the materials you have available, and the number of species of plant you wish to grow will all play a part in deciding how large to make your bed. As a rough guide, however, work on a bed with a diameter of around 6 feet. This will give enough room to grow a number of different plants, and allow you to reach all parts of the bed for harvesting.
Mark out the keyhole access path. This is where you will stand to reach the bed and also where you will site the central composting pile. If using the 6-foot diameter circle, mark a center circle of around a foot across, and then mark a path – either straight or in a wedge shape depending on your mobility needs – to the edge of the main circle.
Construct the outer wall of your keyhole bed (remembering to leave the access path open). One of the benefits of this style of garden bed is that you can construct it from any number of recycled materials, as you are essentially building a retaining wall to hold the growing medium. Broken bricks, rocks, timber or metal are all options, depending on what sort of recycled material you can source. You might even consider using discarded plastic bottles filled with sand and bound with clay, as a way to make use of waste products that do not biodegrade. Any material is feasible as long as it is strong enough to hold in the growing medium. Different materials will however, create different microclimates within and around the bed (for instance, rocks retain heat more than timber) but this can provide a wider variety of growing opportunities.
Create the center compost provider. This is a wire mesh tube into which kitchen scraps and other green composting material is placed, providing the keyhole bed with a constant supply of nutrients and moisture. Roll up a piece of wire mesh around 4 feet tall into a tube about 1 foot in diameter and place in the center of your bed.
Fill in the keyhole bed with layers of compostable material. Ideally you want a ratio of about three to one of brown to green materials. You brown materials could include fallen leaves, straw, sawdust, newspapers, cardboard and prunings from the garden. Green material may include scraps from the kitchen, composted animals manure and grass clippings. Water each layer of material well as you go. At the top add a few inches of potting soil and some compost. The heat generated by this layering of material will serve to break it down and turn it into rich soil.
Fill the center tube with green composting material. Paper, scraps from the kitchen, coffee grinds and manure can all go in here. Keep it topped up regularly with material to ensure that the soil in the bed remains high in nutrients. It should also, in combination with natural rainfall events, supply sufficient water to keep the plants happy (only water the bed and center compost pile if the plants are visibly in need of extra moisture). This centerpiece also promotes strong root growth in the plants sited in the keyhole bed, as their roots are attracted to the nutrient rich area and so penetrate more deeply into the soil.
Plant your keyhole bed. The species you choose will depend upon your personal taste and the climate conditions on your site, but it can be useful to approach planting your keyhole bed as a zone system in miniature. For instance, the area closest to the center would be planting with fast-growing species that you would harvest often, such as lettuce, salad greens and herbs. The next circle out would comprise plants that you harvest slightly less often, such as tomatoes, peppers and peas, while in the zone beyond those you could plant slower-growing crops like potatoes, carrots and onions. It is also worth thinking of the bed as a guild, so combining species that benefit one another through their proximity – be it providing shade, fixing nitrogen, repelling insects, and so on – can prove very beneficial. Placing taller plants on the edge of the bed can help protect lower-lying plants from damage by sun or wind, and can generate microclimates within the bed.
Harvest your crops when they ripen, and return any scraps from the kitchen that result from their use to the center compost pile. This will ensure a sustainable keyhole garden bed that is energy efficient and productive.