What is air pruning?
Air pruning is a method of growing trees in specialized beds or containers that are designed to allow the roots to exit the walls of the containers and when the root tips are exposed to air they dry up and die. In other words they are pruned at that point by the air. This pruning stimulates secondary lateral roots to form behind the pruned root tip, creating a dense and fibrous root system. This process is very similar to what happens when we prune branches above ground, where new growth is spurred below the pruning cut. This happens in branches because the apical bud (the bud at the tip of the branch) produces a hormone called auxin, which suppresses the growth of the lateral buds further down on the branch. When the apical bud is removed so is the auxin production and the lateral buds below are no longer suppressed and can push out new branches. Roots don’t have apical buds and I’ve not been able to find good information on whether or not auxin is at play in the same way, but I find it a helpful comparison as the outcome of pruning roots is functionally the same as pruning branches.
Air pruning originated as a solution to the root system issues that growers were experiencing in traditional smooth walled pots. In those containers roots will run into the smooth walls of the pot and continue circling endlessly around the pot, creating the potential for girdling roots that can end up wrapping around themselves or the trunk of the tree and as they grow restrict vascular function, which can lead to poor overall growth or even the death of the tree. To avoid this trees in traditional pots must be up-potted frequently or the roots need to be manually pruned, both processes are time and labor intensive, not to mention manually making pruning cuts on roots unnecessarily opens up pathways for pathogens.
The ability of air pruning to prevent circling roots and other root deformations by gently pruning the root tips via exposure to air and encouraging secondary and tertiary roots to develop is one of its primary benefits. However, there are several other wonderful reasons to use air pruning.
- The abundance of small, fibrous roots that grow as a result of the root tips being pruned means there is a great deal of root surface area and these root systems can take up a lot more nutrients and water than can a root system whose roots are fewer, larger and more woody. This can translate into faster, more robust growth and the ability to quickly adapt to the final planting location. Because of the robust root systems that have good structure, trees that have been air pruned at the nursery stage are set up for long term success especially when compared to traditionally potted trees whose root systems may be circling, pot bound or otherwise compromised.
- Air pruning can allow for successful transplant of strongly tap rooted species like oaks, hickories, walnuts and chestnuts. When grown directly in the ground these taproots can dive incredibly deep into the soil, which makes transplanting anything larger than a very small seedling very difficult or impossible. Air pruning removes that roadblock by terminating taproot growth and converting the root system to a more fibrous root system, which opens up the door to growing and transplanting these wonderful species much more easily.
- Air prune beds can be very densely planted and still allow for easy harvest of the bare root trees in the fall. I use 2’x4’ beds and grow anywhere from 100-200 trees in each bed. Additionally, since the beds need to be filled with a growing media, I am free to choose whatever I want. The soil on our farm is very heavy clay soil and growing and harvesting bare root trees in soil like that is a non starter. Using air prune beds I can fill them with healthy, friable (a fun word that simply means easily crumbled or pulverized) soil that produces healthy trees and allows for harvest that is easy for me and safe for the trees.
All that is wonderful theoretically, but what does air pruning look like physically? At its most basic version, an air pruning container is one that allows plenty of air to access the root tips as they get close to the perimeter of the container and thus be pruned. This can look like manufactured plastic air pruning pots with holes or slots on all sides and that are meant to prune the entire root system of an individual tree or fabric grow bags that function similarly to plastic air prune pots, but create the pruning effect by using a highly breathable fabric that desiccates root tips as they get close to the walls. Air pruning can also be accomplished in beds that can house (as mentioned before) many trees. In this case, the beds are usually a wooden frame with some sort of mesh bottom. This mesh bottom is where the air pruning happens. Since the pruning effect is only happening at the bottom these types of beds are especially helpful for species with taproots. Below is a picture of one of my 2’x4’ air prune beds. I use 1/4" hardware cloth to line the bottom. This allows for lots of air access, but the openings are small enough that it keeps the soil in place and prevent rodents from accessing the seeds nuts from the bottom.
Air prune beds are fairly easy and inexpensive to build and are a wonderful option for the production of lots of trees. Air pruning pots, on the other hand, are a good option for people who would like to grow single trees in containers long term, someone wanting to grow a citrus tree in a northern climate, for example.
When I was getting started in the tree world I ran across the term air pruning and I had no idea what it was. Learning about it was an exciting revelation for me and I wanted to provide a brief intro to it here for folks just starting their growing journey. I hope it is helpful to that end.
If you would like to see what we offer, check out our selection of bare root trees. All of our nut trees are grown in air prune beds so we are able to deliver healthy trees with abundant roots that are easily transplanted. Thanks for reading and feel free to reach out with any questions or feedback!