While most gardeners set them up slightly differently to each other, here is my simple step by step guide on how to build one.
1. Choose a site – ideally level and sunny, suitable for growing vegetables.
2. Build a vegetable bed. While most people use wooden sides, you can use an existing bricked or corrugated iron raised bed, however as you need to line the bed with good quality plastic or a pond liner up to the top, you need to consider how to secure the liner. The total depth of the bed should be around 50-60cm high, allowing 20-30cm for the water reservoir and 30cm for the soil. If creating a bed on soil, you may choose to dig it 20-30cm into the ground and only have 30cm standing out of the ground.
3. Line it. Put a soft buffer of sand or carpet on the ground and fit the liner so there are no tears or holes. To prevent any damage to the liner add a buffer of sand, old carpet or an old blanket on top of the plastic. Then add a 2.5cm layer of around 12mm scoria or gravel to the base. With my new IBC beds the liner is not required as they are watertight.
4. Create a fill pipe. Install a length of 50mm PVC pipe vertically, attached to a PVC 90 degree elbow that will sit near the base of the bed on top of the scoria you have just placed. Next, attach a length of 50mm slotted AG drainage pipe (or 50mm pipe with holes drilled in it) to the elbow, and this run along the middle of the bed, adding a cap on the end. Finally add a cap to the top of the pipe to prevent mosquito larvae getting in.
5. Cover the pipe and the bottom of the bed with scoria, to a depth of 20-30cm. Cover this scoria with a geotextile material which acts as a wick while stopping fine soil particles from filling the reservoir.
6. Fit an overfill pipe. With the first 10 beds I made, this was inserted at the level of the fabric (the join of the soil and scoria), on the opposite side from where the water inlet is. This allows excess water from rain to run out, thus preventing water-logging. With my new beds I have actually put this overflow valve at the bottom of the tank with an elbow up to the join of the soil and scoria. This means that I can twist the pipe around to let all the water in the base of the reservoir drain out if required.
7. Fill the top half of the bed (approx 30cm) with a blend of 50% good quality organic soil and 50% compost. For the wicking to work properly it is essential that the soil medium is high in organic matter.
8. Top the bed with a layer of 5-7cm of straw mulch, taking care not to cover the PVC pipe opening.
9. Using a hose, pour water into the PVC pipe to fill the wicking bed water reservoir at the bottom. You can fill the reservoir to about 20cm, using a garden stake as a dipstick to see how deep the water is. Initially you may also need to water from the top too to fully wet the soil and start the wicking process.
10. Once the soil is damp, the bed is ready for planting.
If you are considering building a new vegie bed or even adding some more beds to your existing garden, seriously consider setting it up as a wicking bed.