What is a Wicking Garden Bed?
A wicking garden bed is a raised garden bed with a water reservoir below the level of the soil.
Water is drawn up from the reservoir to the soil above via capillary or wicking action. Moisture level in the soil is maintained at a constant rate over extended periods. This minimises the need to add water whilst evenly distributing moisture, providing better growing conditions for plants.
- Water need is reduced by as much as 90% compared to ordinary raised garden beds. This results in a significantly lower impact on the environment and less money spent on water bills;
- Increased water retention of the soil = Low watering frequency: only every 2-6 weeks. Garden beds can be left unattended for extended periods of time without any risk of damage to plants;
- Plant roots grow deeply into the soil attracted by the underground water source. This results in stronger, healthier and more productive plants;
- Soil nutrients do not leave the garden bed by being washed away during top watering. There is more food available to the plants and less need to add extra fertilisers;
- Water is added directly into the water reservoir until it reaches the overflow level. No risk of over-watering the plants;
- Reduced loss of water due to evaporation. Topsoil in a wicking garden bed remains fairly dry due to the soil below retaining the moisture, and as such topsoil evaporation is very minimal.
Possible Downsides of a wicking system
- During high rainfall the soil in a wicking garden bed can get waterlogged causing an anaerobic environment, that is lacking in oxygen, for plants.
- Not all plants like a moist soil. Garlic for example will thrive in a wicking garden bed in it’s growing stages, but requires a fairly dry environment during last part of growth so as to avoid mould growth.
How a wicking garden bed works
There are two main effects in a wicking garden bed:
- Capillary action (capillarity, wicking). The small pores of the soil act as capillaries, allowing the water to be drawn upwards from the reservoir into the soil above.
- Reduction of excessive drainage. The down-flow of water is interrupted causing the water to reside in the soil for an extended amount of time.
Inside a Wicking Garden Bed
A Wicking garden bed is composed of three distinct ZONES (from the bottom up):
– the Water Reservoir
– the Zone of Saturation
– the Zone of Root Growth
The Water Reservoir
The water reservoir is designed to store water for between 2 to 6 weeks. It also captures the nutrients that are washed off the soil during top watering. The height depends from the total height of the garden bed and can vary from 50mm to 300mm. It is not advisable to build a water reservoir with a height greater than 300mm because the water will not be entirely be used within a 6 week time frame. This will facilitate growth of algae that can potentially be harmful for the garden ecosystem.
The Zone of Saturation
The Zone of Saturation is the heart of the wicking system. It is here that the soil meets the water reservoir and overlaps. The Zone of Saturation moves the water (by capillary effect) from the water reservoir at the base of the garden bed to the growing medium on the top. The height of this section is critical for the functioning of the whole system and determines how long you can leave your garden bed before you need to water it again. It ranges between 20-100mm of height.
The Zone of Root Growth
Most edible plants will develop roots systems within the first 200-300mm of soil. In this zone, water and air need to be in the right proportion for the plant requirements. It is for this reason that particular attention needs to be given to the composition of the soil mix. Failure to do this means the whole system will not work properly causing waterlogging or a non-wicking garden.
A soil mix designed for this purpose has the following composition at a volume ratio of:
40% sandy loam.
40% potting mix.
ModBOX™ Wicking System Kit Components:
- Water Inlet Pipe
Delivers water to the water reservoir.
- Slotted AGI Pipe
Connected to the Water Inlet Pipe, the Slotted AGI Pipe distributes water evenly along bottom of the garden bed.
- Water Overflow Pipe
Outlet positioned at 300mm below the soil surface. Prevents the water level from raising into the Zone of Root Growth.
- Food Grade PVC Liner
Lines the bed to make it waterproof. Needs to be food grade to prevent chemicals from leaching into the water.
- Drainage Scoria
The medium that creates the structure of the water resevoir. It has a high water retention capacity due to its porosity.
- Geotextile Fabric
Positioned between the Water Reservoir and the growing medium, prevents soil silt from ‘clogging’ the scoria.
- Tank outlet + Drainage Valve
They provide a drainage for the water reservoir for the high rainfall season.