Last night I was working with the wife to make pots for her larger African Violets (they have outgrown most regular pots) and I bought her some self-watering pots for Christmas. The best looking (size wise) does not use a wick, wife wants a wick so I ordered some ($11/100') and will fix her pots next weekend..
This morning I woke up with a vision of LARGE (5-15 gallon) self-watering pots. The basic pot would be an old 5 gallon bucket for the inner pot supported by a simple cross made of 2 X 8? (Join them using a set of notches cut half way through the center of each) inside another 5 gallon bucket. Drill some (4-8) 5/16 holes around the inside edge of the bottom of the inside bucket, make the same number of wicks as you made holes (16" should be fine) and tie them together in a knot at one end, and lace 1 wick through each hole in the inside bucket, the result would be a bucket with what looks like a wagon hub (knot) and spokes going out to where the wicks pass out the bottom of the bucket.
The cross - I had to think about this a lot, it needs to be cheap, easy to build, sturdy (must hold the inner container without crushing), and reusable: So 1st step measure the inside diameter of the bottom of the bucket and subtract 1 inch; this is the length of your cross (Cut 2), next find the center of the pieces you just cut and make a notch the width of the board half way through the board (Do both boards), now you should be able to slide the boards together making a cross that will sit inside the outside bucket and be as tall as the widest dimension. This sets the size of your water reservoir. Fill the outside bucket with water to the height of the cross (it will want to float, so mark the bucket with a sharpie before you start so you will know what your fill line is).
Now set the inside bucket down on top of the cross the wicks should be hanging free into the water. After a few minutes check to see if the knot is wet, if it is then everything is working right. Now add soil to the inside bucket until it settles down. Put a ring of insulation rope (they call it Polyurethane Caulk Saver) all the way around between the inside of the top of the outside bucket and the inside bucket (this will control mosquitos) a 1/2" gap between the ends should be ok this could be used to fill using a drip irrigation line or to measure the water level using a dip stick.
Finish filling the inside bucket with soil and plant your plants or seeds, water lightly.
My estimate for the cost using only new materials is $33 for 1 or about $16 @ for 4: $10 for the 2 new buckets (or 2 of what ever container you are using per self watering pot), $8 for an 8' 2X (should make 4), $4 for 25' of the insulated rope (should make 8), and $11 for 100' of 1/4" wick (should make 12). If you have old materials lying around the cost could quickly come down.
Anyway I think the method could easily be used for larger square containers, I have a lot of containers sitting around and we have problems here with heavy rains followed by drought, so if the inside bucket can fill and drain into the outside bucket, the inside water level will be limited to the height of the outside bucket (hydraulic pressure will limit the inside bucket water level)
A year ago I tried something similar by just sitting 1 pot inside the other, but the inside pot sat too low in the outer pot (drowned the plant) now I think I can control the water level much better.
Sorry I rambled, but I wanted to document the idea, I now have drawings in front of me now and I know what supplies I will need to made a dozen of these in different shapes using containers I have sitting around.
I figure if I use old containers and scrap wood I can make mine for about $3 each. That sould be a good investment if it gives me some flexibility in watering requirements and uses more of the rain water we get.
