The keyhole garden is very water efficient and has its own built in compost pile. The instructions said that the circle should not extend out more than 6 feet because the central watering basket becomes less efficient as the circle expands. Geometrically the circle makes the best use of the space. It allows for good access to the growing space. A raised garden is easier to work and since it is a permaculture design, compost is added to the basket and on top and it need never be tilled. Some plants were more suitable than others for the design. It can be built out of a variety of materials and was intended to be built out of whatever materials were readily available. The African design has a central wicker type basket but needs to be rebuilt every couple of years since it does break down over time. A cover keeps out critters and the hoop structure which is optional can provide shade or extend the season. When other more durable materials are used for the construction, it can last longer.
In Africa, water is a scarce resource so the central compost basket was designed to make use of gray water.
It is essentially sheet mulching or raised bed with a built in compost pile.
Some keyhole designs don't have a central compost basket but use the circular design to create gardens with easy access and efficient paths that maximize garden area for planting.
The keyhole allows access to the central compost/watering basket and access to the middle of the bed.
The plants are planted close together so there has to be good pest control and the plants have to be selected and rotated so the nutrients will not play out too soon. Space and nutrient hogs are not recommended and green leafy vegetables and root crops will do best. Plants that require a lot of water do not do as well unless you provide enough water and mulch heavily. Sprawling plants will take over so you will get more efficient use of the space if you use short cycle plants that don't take a lot of space. If you are doing the garden organically, you have to balance what you plant to support the soil web and supplement with nutrients. If you just have a compost pile and do not plan for rotations of cover crops, eventually the garden will become less productive over time and will have to be rebuilt.
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Happy gardening in Hawaii. Gardens are where people grow.