I saw this a few years ago and decided to try it. A variety of root pruning pots are available commercially but I like to try things for myself. And I'm cheap.
Air pruning occurs when roots are exposed to air. The roots dry and die causing the plant to grow new branching roots. If plants are left in pots too long the roots will continue to grow around the container in a circle, making effective transplanting difficult later. Unless roots are well spread at planting the plant may not reach its potential.
Hardwoods like oak and walnut are sometimes difficult to transplant since the long taproot is easily broken when the seedling is dug, and the plant often dies. By air-pruning the taproot I hope to get a mass of non-circling roots.
Step 1: The Theory and Getting Started
This design, again not all mine, should air-prune the taproot and stimulate lots of side roots, and should permit removal of the entire seedling and root ball without damaging the taproot.
4" diameter PVC drain pipe (I got 10' sections of the perforated kind with drainage holes pre-drilled)
Heavy steel wire* (or just use coat hangers)
12" x 2" x 4" piece of scrap wood
6" x 6" window screen (1 per tube)
3-3/4" diameter plastic container (chocolate drink container maybe)
1" x 2" x 18" wooden "ramrod"
1 roofing nail
Compost, sand, vermiculite, etc. for a soil mix
Acorns, walnuts, hickory nuts, etc.
Measuring tape & marker
Saw (crosscut or table)
Safety glasses and hearing protection (if using a tablesaw)
2 clamps (helpful only if you use a tablesaw)
Vise and a workbench
1/4" drill bit
Pliers (I used a needle-nose)
Large plastic buckets
Scoop or trowel
* the jury seems to be out on whether or not copper nails kill trees but I'm going with the steel, not copper wire
Step 2: Prep and Assembly
Mark the PVC drain pipes at 12". Cut the pipe into sections. Keep the cuts as square as possible. If you are going to use a tablesaw, clamp a couple pieces of scrap wood to the table, one on each side of the blade, back from the cutting edge far enough to just cut through the PVC . These blocks will help you to safely make nice square cuts.
Put on safety glasses and hearing protection. With both hands, move the PVC pipe against the blocks to just cut through the pipe. Slowly rotate the pipe as you cut through the wall completely around and separate the section. Repeat until all the tubes are cut.
Clamp a 12" piece of scrap 2" x 4" horizontally in a vise and slip a tube over it. Drill a 1/4" hole through the tube about 1/2" from the end at the 12:00 position. Rotate the tube 90o towards you so the hole you just drilled is at the 3:00 position. Drill another hole down from the top. repeat until you have 4 holes, each 90o from each other.
With all the holes drilled, reclamp the 2" x 4" vertically. Slip a drilled tube over it so the holes are towards the top. Insert a wire through two opposite holes and bend one end 90o. Bend the other end 90o also and trim excess wire. Repeat with another piece of wire for the other two holes.
Step 3: Final Assembly
Measure, mark and cut a 6" x 6" piece of screen* for each tube. Assemble your ram by nailing the 3-3/4" diameter plastic container to the 1" x 2" x 18" wooden "ramrod".
Stand the tube upright with the wires at the bottom. Center a piece of screen over the top of the tube. Center the ram on the screen. Force the screen to the wires at the bottom of the tube. Repeat for each tube.
Now you are ready to plant.
* from old window screens bought at a garage sale
Step 4: Planting
Put the completed tubes in a bucket. The bucket is to catch the soil mix that misses the tube. Fill each tube to 2"-3" from the top. Fill all tubes. Put the tubes in a bucket and add a couple of pints of water to each tube. The bucket catches the water that runs through.
Place a nut in each tube and, with the tubes back in a dry bucket, fill the tube with soil mix. It is mid-November and I am putting the tubes in the shade along the north side of an outbuilding where they won't dry out. I can see them and will remember them next April. At that time I will move them into a sunny location and make sure they don't get too dry. Be sure to mark the tubes with what is planted in it.
Gardening, and forestry especially, are not activities for the impatient and pessimistic. I will be happy if I get half of the tubes to reward me with a seedling. The young trees will grow in the tubes all summer and will then be transplanted either next Fall or the following Spring.
Do some research and try this yourself. If trees aren't your thing you could try the air-prune idea on a couple of tomato or pepper seedlings you start very early and see if you can get some really good root systems going.