The bigger growing space using the wicking approach is to come from a wicking bed. The approach is exactly the same as for the tub: a reservoir filled with stones with an overflow hole.
The bed is a basic frame of treated pine sleepers (200mm x 50mm). It is 1.8m x 1.2m, two layers high so was built from four 3m lengths. The length of this bed was dictated by the distance from a paved area to a doorway. The width was then just convenience. The volume also came nicely to exactly a m^3 with the 100mm reservoir. I'd not want it any wider as it would be too difficult to reach the middle for planting, weeding and picking (hopefully lots of that last one!). Probably narrower would be better but at only 1.8m long it wouldn't be worthwhile.
The frame was dug to level which involved cutting away a small slope. This is to ensure even water distribution. I edged the frame with old bricks so I have something to run the mower wheel over and an edge to Chuffle along. Once the frame was in place a further 100mm of soil (read: sticky clay) was removed so that the reservoir does not eat into the 400mm of wall height.
The frame is attached widths to lengths simple with large gauge batten screws. I pondered over this for a while but resolve to the fact that now matter how carefully I build a frame the sleepers are guaranteed to twist about. The top layer is attached to the bottom with short pieces of galvanised strapping. This should prevent any significant separation.
The next step was to lay plastic sheet in the bottom to form the reservoir. I first put down two layers of weed met to avoid puncture. The plastic was the heavy (200 micro?) builders plastic from Bunnings. The roll width was actually a perfect match for the bed width. I put down two layers of this as it wasn't too expensive and one little hole makes the exercise futile! It was stapled to the frame (obviously only at the top edge...).
After the plastic sheet is laid it is covered with cheap weed mat to protect it from the stones.
The next step was drilling an outlet hole in the wall. It is just above the external ground level, at the top of the reservoir depth. I really wasn't sure if one was enough but I settled on that. If the bed gets too inundated with rain the water can always overflow the plastic which is almost 200mm below the top i.e. the whole bed won't become a soil soup.
On the inside of the outlet hole I placed a small piece of 90mm pipe with a cap. Into that is drilled a 20mm hole. I then have two PVC threaded pipe joints, one in the pipe and the other in the timber wall. There are any number of ways to do this (just a hole in the timber may suffice) but my thinking was that I need "free space" on the inside of the outlet to avoid it clogging. I can dig down to the pipe and lift the cap clear a blockage if I think there is one. It probably would have been better to have the inspection pipe come all the way to the surface but I'm taking a gamble.
The picture I'm missing is the laying of the slotted ag pipe and the placement of the filling pipe. I used a piece of 90mm pipe and drilled two 50mm hole in the bottom. The 50mm ag pipe is inserted in one hole and then snaked around the bottom of the bed with the end being inserted into the second hole. This avoided the need for T-junctions and the like. I put the filling pipe right in one corner.
After the pipe if laid the stones are shoveled in. I used 20mm scoria as it is cheap. The bed is filled with this to a level of about 90-100mm. The stones are spread to level.
On to the stones the non-woven weed mat is placed. The non-woven has better wicking properties than woven. This weed-mat comes up the sides of the bed most of the way to the top. Again it is stapled but the placement does not matter so I folded the corners as needed.
Before the soil went in I put in place a worm tower. This is my third, the other two are in an existing veggie garden bed. It is simply 100mm pipe with plenty of holes drilled. It gets filled with compost worms, although I didn't have may to start this one. I'd bought a "booster box" of 500 worms to start all three towers and it had been several weeks before I got to this third one.
The finished bed, filled with soil looks good. As with the tubs the soil was a 2:1:1 mix of mixed soil, mushroom compost and organic compost. One delivery of 1/4m^3 scoria and 3/4m^3 soil filled the bed and three tubs, with a little scoria left over to fill in the space left between the bed wall and the paving.
I don't want to rewrite all the wisdom that exists on the blogs that I've read about growing vegetables in these beds but a few notes are:
- They will allow more consistent water to the (established) plant with less frequent manual watering.
- The water level in the reservoir can be checked by looking in the filling pipe.
- The surface soil may be dry while the deeper soil is moist. This should reduce weed germination.
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