Air pruning is a rather nebulous term. How would a plant be pruned by air anyway? Well, first off, the term doesn’t refer to clipping leaves. It actually refers to a controlled drying out of root tips to encourage lateral growth in the root zone, resulting in improved growth up top. Perhaps a better term would be “air-assisted root pruning”, although, admittedly, it doesn’t sound as catchy.
That said, the principle of air pruning is a lot like what happens with the green shoots above ground. If you pinch off the growing tip of a plant, it will respond by making all the lower growth bush and branch out considerably more. Usually, two or more fresh growing tips appear at the site where you cut off the original sacrificial shoot. (See More: What Is Air Pruning?)
The same thing happens with roots. If you cut the tips off, it stimulates lateral growth further up the root shaft, thus filling the space with lustrous dense roots. (Quick note: cutting root tips off with a blade can introduce infection to the wound. Air pruning, on the other hand, avoids this.)
Left to fend for themselves in a pot, roots don’t usually reach full potential. The plant will send out one main tap root, which will hit bottom and circle round endlessly at the bottom of the pot—the wettest place in the rhizosphere with the least available oxygen. This can lead to all kinds of problems in the root zone that will be reflected up top. It’s also an inefficient use of space as most of the grow media is unused and takes a long time to fill up.
Roots that have been air pruned will fill the entire container, giving the plant maximum efficiency and an enormous surface area to absorb all those yummy nutrients. Drainage will also improve, increasing the amount of oxygen in the root zone and reducing the pooling of water, which in turn reduces the risk of root rot. You will notice faster growth up top, healthier plants, and significantly increased yields. After all, massive roots equal massive blooms.
To air prune, you must go against the pervading wisdom about roots—that is, keep ‘em dark and keep ‘em moist—and expose their tips. Again, just the tips; roots don’t ever appreciate bright light on them.
My favorite method of air pruning is to start early. When a cutting or seedling has rooted, you can transplant it into a three-inch stonewool (rockwool) cube. I prefer the smaller three-inch size as roots can fill the space quicker than the four-inch size, and you will soon transplant the roots to a larger space anyway.
Then, sit the cube on a metal grill, allowing the air to circulate underneath. You can either make a bench for the grill to sit on or just rest it on top of your planting tray. Top-feed as usual and allow the nutrients to drain freely through the base of the cube.
After a few days, you’ll see some white roots poke out of the bottom. At this point, some people might panic and race to re-pot the plant to avoid the roots drying out. Don’t panic! Let those roots dangle. They will dry out and die back—which exactly what you want. Give it a few days and you will see many little root clumps burst through instead of one or two stragglers. Hang tight and let these ones die back, too. Before you know it, the whole base of the cube will be a monstrous carpet of luxuriant white roots.
Do not even think about transplanting the cube into a bigger pot until the roots are bursting out of the sides like crazy. Bear in mind that more roots you have in the cube, the faster it will dry out, so adjust your watering schedule accordingly. (See Stonewool Do's and Don'ts for more tips on using this medium)
If you are using soil, it’s the same principle. Put your rooted cuttings and soil mix in a cheap cloth or plastic pot that will allow the roots to grow through and die back. When the roots are totally filling the small pot, it’s the perfect time to re-pot into the larger container that the plant will spend the rest of its life in. The roots will colonize the new space vigorously and the plant will grow at an enhanced rate. You can keep this principle going with the larger pot, too, thus maximizing the root growth all the way to harvest.
Personally, I use air pruning on every plant I grow and have only seen beneficial results. Try it yourself; you won’t be disappointed.
Check out The Top 4 Recommended Transplanting Mediums and Methods for more tips on growing in containers.