Wicking beds are self-watering raised garden beds, that operate on the principle of sub-irrigation. Water stored in a reservoir located at the bottom wicks water upwards to the soil above, watering the plants from below.
When constructing wicking beds, it’s important to understand how they operate, and the needs of the plants that will be grown in them.
Good quality soil in a wicking bed will wick water up to a height of around 30cm. Wicking beds are most suited to growing vegetables and shallow-rooted herbs. Most vegetables have 80% of their roots in the first 30cm of soil, and will grow perfectly well in a raised bed with only 40cm of soil.
Making a wicking bed too deep is counter-productive, as that will require more materials construct and fill it, making it more expensive. Also, any seedlings planted will need to push down their roots much further into the soil in to reach the water.
The optimum depth of soil in a wicking bed is around 40cm, as it provides sufficient soil depth for growing vegetables, while allowing newly-planted seedlings easy access to water. Using a soil depth of 40cm, with the standard water reservoir height of 20cm requires a raised bed height of 60cm. When building raised beds though, it’s important to allow 10cm of empty space to hold mulch and stop water runoff. This brings the height of a wicking bed to 75cm high when using 40cm of soil, which is a manageable height to work with.
For more information on wicking beds, please see the article – Wicking Bed Construction, How to Build a Self-Watering Wicking Bed