What on earth is a wicking bed I hear you ask *holds hand to ear* I thought so! Wicking beds are a method of growing plants, in this case edible ones, that provides a reservoir of water that then wicks up to the root zone of the plant by capillary action. The use of this system may reduce water usage by up to 50% over conventional irrigation techniques and so is ideal for producing food in the warmer drier Australia we may be facing due to global warming. There are two types, open and closed but it is the closed system that is most suitable for the sort of intensive back yard food production that we practice here, the open style being more suitable for broad scale agriculture.
A closed wicking bed is similar in principle to a self watering container as covered in other articles on this site and consists of a lower water reservoir, a means of inspecting and topping up the water supply and an upper growing bed. The may be constructed above ground or under a conventional vegetable bed. In its simplest form an above ground wicking bed could be a broccoli box half filled with sand, with a length of 50mm pipe inserted vertically into it and the rest of the volume of the box topped up with growing medium. Ours is just a little bigger.
Construction Method
1. Get hold of something to make the sides out of, we used timber that had been preserved with Alkaline Copper Quaternery (ACQ) in the form of DIY garden beds 1200mm x 1200mm x 310mm high. We got two; the reasons will become obvious a bit later on. If you are going to use treated timber, avoid copper, chrome, arsenic (CCA) treated timber so that the arsenic cannot be leached into your edibles.
2. We made up the first bed and set it up in the area where we wanted it to go. It looks quite nice so we put it in the front yard.
3. We got hold of some builders black plastic sheeting to line the bottom bed to form a watertight reservoir. The ground under the bed should be smooth and without any projections that could puncture the plastic so you might want to put a layer of sand inside for the plastic liner to sit on. 
4. To form the reservoir I place the plastic inside the bed and folded the edges to make a rectangular shape inside the bed, then using 20mm flat head tacks I secured the plastic to the top edge of the timber bed. 
5. I got hold of some agricultural piping, the slotted stuff used to drain areas of excess water, and ran it along the bottom of the bed, with enough overhang on the short side to go up to the top of where the second bed would sit. 
6. Once the agpipe was in place I filled the reservoir with hardwood chips that I had bought in bulk to do some mulching with and It took not quite half cubic metre of the wood chips to fill the reservoir. The easiest way was to shovel and barrow the wood chips from the stockpile where they were delivered directly into the reservoir.
7. Once the reservoir was set up, I put the second bed together and screwed on a piece of timber about 200mm x 100mm into the centre of each site to make sure the top section did not part company with the bottom when I started putting in the growing medium.
8. To make sure the agpipe filler tube did not get caught up when placing the growing medium I pulled it up so that it was level with the top of the top section, cut it off flush with the top and tacked it in place to the top section.
9. With this done I needed to put in the growing medium, which was a bulk potting mix that I picked up (with the help of a friend with a trailer) from a local supplier. I intended to us the same technique as the wood chips but with the top section on it was too high for the wheel barrow to dump into, so I made up a small ramp that got it high enough and problem solved!
10. Once the growing medium is in you can plant your wicking bed and you are ready to go. I wanted to grow asparagus, our rotation in the backyard is not sympathetic to perennial crops so I planted up the wicking bed with two-year-old crowns, which by the way looked amazingly like small tentacle-ly aliens. Anyway once we planted the aliens we applied more wood chips as mulch then it was time to fill up the reservoir.
You could drill a small overflow hole so that you can tell when your reservoir is full but in practice the water starts leaking out between the joins in the timber once it gets over the waterproofed area so I didn’t bother. Another advantage of using 50mm ag pipe as your filler pipe is that you can see down into it and get a rough idea where the water level is and if you need to top it up. It has been in for a few weeks and there does not seem to be much drop in the water level, the reservoir is comparatively big and with the mulch and small size of the plants there is not much evaporation.
I hold out great hopes for the coming summer!
