Having introduced wicking beds and why they can be a great idea, let us get into some details. Check out the below diagram of a VEG-style wicking bed and then let’s go through each bit, step by step, in the order we actually install them.
You’ll notice that we add this that and the other.
The first thing is to create the edging or container the water and soil etc will sit in. In our case this is a raised bed 40 or 60cm high made from cypress macrocarpa timber sleepers (yes, as a matter of fact we do sell kitsets!).
The next thing is to get the liner in. Real careful like. We talk about the liner (drinking-water certified) we use below, but if the liner you use is thin, maybe double it up, or put a cushioning layer of sand or similar below it. Here and in the next step more than anywhere take your sweet time.
Now we usually add the outlet by drilling a hole through the bottom of the bed at one end then cutting through the liner and inserting the three-way water level indicator, overflow, and drainage outlet system we invented all by our little selves! With a little swivel of the pipe you can drain the water out periodically to prevent stagnancy.
Add the stones and 50mm diameter perforated inlet aggy (agricultural) pipe. We have found 7mm or 1/4inch bluestone screenings are the ticket, or pea gravel (though it’s harder to get and costlier), having experimented with sand and various other mediums. The main issue with sand, we have found, is that it compromises the speed at which water can move into the reservoir when filling, which can create a back-flow issue and mean you have to have the hose on trickle. A secondary issue is that sand usually carries fine particles with it which can clog the system over time.
Note that the inlet aggy pipe needs to be longer than the outlet aggy as per our diagram. The reason is you want to get the water in fast as possible (as in a hose on full tilt) without backflooding up the inlet pipe, so we make the inlet aggy section as long as the bed itself (often winding about – the pipe has curve memory – which is great as it means more inlet run length), then a short outlet section adjacent to this. The stones in between inlet and outlet aggy’s prevent water rushing through and out the overflow before filling reservoir, but allows fast fill without backflooding which is really important. There’s no need to have a long outlet aggy pipe, as it will flow fast enough even with hose on full without causing problems.
Finally you want the inlet pipe (what you stick the hose into) on the same side or end of the bed as the outlet/overflow riser/water level indicator so you can see the water level, or at least see it spilling whilst filling so you know when to turn the hose off.
Test with water and ideally leave overnight to check for any leaks.
Add the geotextile fabric layer (some folk use a double shade cloth – this layer needs to let water up but prevent soil moving down). Now add the soil as you would a normal raised bed (ideally a fairly porous loam that is not too heavy in clay or organic matter). You are also now ready to plant your plants. Also don’t think that just because it is a wicking bed that you can plant small seedlings then forget about them – seedlings will often still need supplementary watering for the first week or two until they get over any transplant shock and get their roots down into the soil moisture zone.
Detailed Video Clip of the Whole Process
VEG’s Carey talks through the entire start-to-finish process of building a VEG wicking bed and all the above steps. Note here we used scoria which is what we had even though it is not as ideal as the alternatives we mentioned above. Also, you don’t have to staple the liner to the inside of the bed if you’re careful when filling with stones; this reduces the chance of tearing the liner – you can just let the weight of the stones and soil hold it in place):