A wicking garden bed is basically one that works much like a self-watering pot. There is a water reservoir underneath the bed, and water wicks up from it through the soil to reach the plant roots. They can be any shape or length you want. The idea is that watering from the subsurface means the soil stays moist and evaporation rates are lower, so you get better results for less water and better plant survival in hot conditions. The people I know who’ve used them say they do get noticeably better production for vegies than with normal garden beds. They are more work to construct, and more expensive than a standard garden bed because of the extra materials needed, though total work and cost will depend on what materials you use and whether you recycle found materials or buy the top-end commercial products. But depending on your conditions and water availability, results may be worth it.
The basic principle is that regardless of shape or total area, your bed will have a frame or external support – either raised walls or a hole dug into the ground. Over this is a sealing layer – most often a plastic liner. Then there is a layer of gravel to form the water holding reservoir, which has two drainpipes coming out of it at the side – an overflow at the top of the reservoir, and a drain at the base of the reservoir should the reservoir need flushing clean. Above the gravel is a layer of shadecloth or geotextile fabric – something that will allow water to pass through it but stop soil falling down into the reservoir. Then above that you have about 30 cm of soil – that much, because that’s the depth a lot of vegies need, but no more, because that’s about the limit that water can wick upwards so anything above that will stay a bit dry. Most wicking beds also have a vertical pipe reaching down to the reservoir so that you can fill it easily from above, but tiny ones might still be top-watered.
There’s lots of info online. The phrase “wicking beds” gets you home garden beds on this principle. You’ll also find some material under “self-watering pots”, especially if you’re working at planter size or building a wicking bed in (for instance) a styrofoam broccoli box – there are some effort- and cost-saving tricks you can use at this size that you wouldn’t in a full-size garden bed. The last phrase of use is one I only came across a few days ago – “sub-irrigation”, which so far seems to be used by more commercial systems (and not necessarily in this country). I have yet to work my way through the sources I’ve found on that phrase and see which are worth the read, and how much difference there is in the sub-irrigation systems to the wicking bed systems, so there’ll be another post on this sometime. Here are some of the links I’ve found good, with descriptions and details of how to build, diagrams and photos of a few useful variants:
My Smart Garden wicking bed fact sheet (produced in Victoria, Australia)
Wick gardening at Childs Play Permaculture (Australia)