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A wicking bed has been described as "a self watering pot on steroids".
It is a way of growing plants where water wicks up from an ungerground water reservoir by capillary action. Water use can be reduced by up to 50% from conventional growing systems, as evaporation is significantly reduced. There is no doubt that we need to be always looking at more efficient ways to use water in the garden.
Queenslander Colin Austin is credited with coming up with the concept of a wicking bed. Possibly the next big thing in our hot, dry summers as a water saving tool, it is worth having a look at the concept and assessing whether they are an option for you.
The essential feature of a wicking bed is an underground reservoir of water in immediate contact with soil in the root zone. The water reservoir is typically filled with coarse organic material or aggregate. They can be 'closed systems' or 'open systems' and as simple as a polystrene box or as complex as a large growing area incorporating worm farms and composting bins. Closed systems can be raised above the ground, and can be made from timber railway sleepers, old bathtubs, cut down water tanks or commercial corrugated raised beds - really, you are only limted by your imagination and budget! Closed systems can also be dug into the ground too, but have a waterproof membrane layer at the base separating the reservoir from the soil below. (Click here for a larger, downloadable PDF of the wicking bed illustration above.)
Open systems are incorporated into the ground, and are not completely closed off. At least one side is open, often with a slope incorporated to allow waterflow towards a particular direction, for example alongside tree crops, where they can access the moisture with feeder roots, without stoping the ability for them to form deeper root systems into the ground.
The first part of a wicking bed you need to have is the waterproof membrane layer. This can be black plastic (make sure it is UV treated if it will be exposed to any sunlight), or a layer of pond liner material. While more expensive, this is usually much longer lasting. Underneath the plastic you may wish to use a layer of cardboard or newspaper to prevent any stones damaging the plastic. (Obviously, in a bathtub or any container which holds water, a waterproof layer is not required!)
The reservoir layer - What is scoria anyway?
The second part is the reservoir layer. This reservoir layer can be made from either fine bluemetal or gravel, coarse woody mulch (eg. tree prunings) or coarse river sand. There are advantages and disadvantages of all options - consider which you think would work best for you. On Eastern states websites you will see them recommending 'scoria'. This is a lightweight 'gravel' that is mined and readily available over east - but it is not found in WA. Scoria doesn't have any magical qualities; it's just their local material. Its lightweight and porous, but it's certainly not worth trucking over the Nullabor. Here, our recommendation is to use 7mm blue metal chips as an alternative. (We normally carry it @ GLSC.)
The last layer in a wicking bed is the growing medium. The depth of growing soil above the reservoir should be between 200 - 300mm, which is deep enough for most vegetable crops. A good, friable organic vegetable mix (like ours!) makes an ideal growing medium. Good drainage of the topsoil layer is critical. Remember that a wicking bed is like growing in a big pot - plants are depending on you to make sure they have adequate nutrition. Don't skimp - use a good quality soil and you'll get good results.It is essential that between the reservoir and topsoil layers, you add an overflow drain. This prevents overwatering, allowing surplus water to seep out rather than waterlogging the topsoil, which would quickly kill your plants.
Greywater is not recommended for a wicking bed system, due to buildup of salts, fats, etc. in the greywater that will remain in the closed system.
(Pictured above right is a raised bed with bottom incorporated, which makes a perfect wicking bed. Kits are available, complete with pipes and accessories. Contact Water Installations for more information.)