DIY Air Pruning Beds – Grow Hundreds Of Trees With Well-Formed Root Structures In A Tiny Footprint
What Is Air Pruning?
Air pruning is a technique for developing healthy, fibrous, non-circling and non-girdling root structures in trees and and plants raised in a nursery setting that uses air to prune and shape root systems. Trees and plants can be grown from seed, cuttings or root divisions while still preserving a healthy tap root and creating a highly-branched root structure characteristic of a tree or plant grown from seed in-place. Below is an example of two Sequoia Redwood trees grown in an air pruning pot (left) and a typical slick-walled plastic pot (right).
How Does Air Pruning Work?
Trees and perennial plants, even annual vegetable starts, grown in a way that allows roots to air-prune have much healthier and effective root structure compared to those grown in slick walled pots. Consequently, they will grow and develop much faster and into healthy adult specimens capable of withstanding the rigors of their environment – without the coddling required by trees and plants with stunted root systems.
Air pruning systems take advantage of the natural response that root tips have when they contact air. A growing root tip will stop growing, and the tip will shrivel, effectively going dormant, when it comes into contact with air.
This event creates a hormonal signal that back-propagates up the root back towards the stem-root junction and initiates new root growth in the form of branching roots and new roots emerging from the junction itself. In this way, air pruning induces the creation of vast numbers of fibrous roots all heading in their own direction.
Whenever a root encounters the edge of bottom of the pot, it self-prunes and send that hormonal signal back towards the stem-root junction, inducing furthering branching. The illustration below shows how an air pruning style container for an annual vegetable start influences root structure as compared to a typical slick-walled container.
What Are The Benefits Of Air Pruning?
- Air pruned root systems have a much higher survival and thrival rate in comparison with slick walled root systems (stunted root systems).
- Air pruned root balls routinely exhibit growth in 1 year that will take slick walled root systems several years to attain.
- Improved drainage (no anaerobic pooling as often happens in slick walled pots) = healthier soil.
- Tap-root preservation (the tap root may air prune, but once planted the dormant root will resume growing downwards – this is important for tree and plant species that need strong tap roots to survive in harsh climates).
- Highly-branched, fibrous root systems gather more moisture and nutrient and anchor the tree much more solidly, reducing the need for staking.
- Trees are protected from ground-dwelling vermin (growing in pots or elevated air prune beds).
Why Trees Grown In Standard Pots Will Never Reach Their Potential
When a tree seedling germinates the first thing to emerge from the seed coat is the apical tip (growing end) of a tap root (in most species – not all trees or plants have tap roots). The tap root’s job is to dive deep into the soil in search of water and to anchor the young seedling in place. Tap roots of many oak trees, for example, will be as much as 18 – 24″ deep prior to any emergence of a shoot at the soil surface. If this young seedling were to germinate on its own in a natural environment this tap root would continue its growth undisturbed deep into the subsoil.
Now imagine the seedling was started in a typical black plastic slick-walled pot. As that tap root dives down, eventually it encounters the bottom of the pot, at which point it begins to circle along the bottom, continually searching for a way to escape and continue its journey downward. The tap root can even grow back upwards, sometimes winding around itself, creating a girdled root system. Circling and girdled root systems are ticking time bombs, for as the tree grows it can literally strangle itself with its own roots. The same thing will happen to the lateral roots as they find the wall of the pot and begin their circuitous search for water and nutrients.
Another tragedy of growing trees in slick-walled pots is the amount of life energy the tree wastes on creating an ineffective, stunted root system. Circling tap roots continue to grow despite being trapped in the container, and consequently the tree is left with an anemic, shallow root system with very little branching or lateral roots in addition to the stunted tap root. This means it will have to be coddled when planted – overly frequent watering, extra nutrient, staking and more will be required to keep the tree alive as it struggles to overcome its poor start in life.
An additional innate advantage that is lost when raising trees in slick-walled pots is the tree’s ability to anchor itself, with both a deep tap root and well-balanced lateral branching roots, so that it may withstand high winds. Trees with stunted root systems will often require staking as their root systems try to catch up to where they should be. Staked trees are prevented from moving as they normally would in the wind. A tree is designed to sway, lean and bend with the predominant winds, and this motion and continual stress actually creates a stronger and more well-adapted tree.
For an excellent overview of what makes a healthy root system, and how we can set up our growing systems to create them, we highly recommend watching Dr. Carl Whitcomb’s video on Root System Basics.
Air Pruning Methods: Pots vs. Beds
There are two ways to grow trees and perennial plants with air-pruned root systems, using pots or elevated air pruning beds. Growing in air pruning pots will create uniform, easy to transplant root balls, but tends to be quite expensive given that air prune pots are more costly to produce than their slick-walled counterparts. Growing in air-prune beds is a low-cost way to produce large numbers of air-pruned seedlings in a small footprint, though the root balls will not be as neat and tidy as in pots.
Air Pot Manufacturers
Below is a list of various air prune pot manufacturers. We have used Pioneer Pots, Air Pots and RootMaker Pots thus far, and have found the RootMaker brand to be our favorite, though they all work. Depending on what you are trying to grow, you may choose to go with one style of pot over another based on your target species.
DIY Air Pruning Beds
For the home propagator, budding nursery steward, homesteader or landowner looking to grown and plant a lot of trees without spending a fortune, we recommend building your own air pruning beds. In a single 10’x3′ air pruning bed it is possible to grow ~ 600 – 800 chestnut trees in a year. As far as cost savings go, air pruning beds are by far the better option. It will take longer to get your trees in the ground compared with buying trees (you have to grow them first), but this method has a whole host of additional benefits that make it out preferred method of growing trees at 7th Generation Design and our production arm Honey Badger Nursery. For us, air pruning beds have demonstrated the following advantages over growing in air pruning pots:
- Able to grow far more trees in a much smaller footprint (15 – 25 trees per square foot!)
- Way cheaper (air pruning pots are expensive!)
- No need to purchase more plastic
- Slower (yes, this is a good thing! You get to spend more time getting to know your trees and your land. You can even spend time preparing the planting sites with beneficial companion plants and fertility species while your productive trees grow to size)
- Protection from vermin (both above and below ground – see our video below!)
- Very low-maintenance (integrates well with automated watering system – you can leave for weeks at a time and your trees will still be growing just fine!)
How To Make An Air Prune Bed
An air pruning bed need be no deeper than 12” to allow for easy removal of seedlings when they are ready to be planted out. The soil containing box can be made with 2″x12”, 2″x10″ or (2) tiers of 2″x6″ boards, and should be around 36” wide to allow for easy reach access to the whole bed from either side. The root permeable bottom is shade cloth laid over 1/2” wire mesh laid over 6″x6″ concrete wire remesh, stapled on top of a table frame onto which the soil containing box is set. Together, these layers serve to limit sagging and prevent the loss of soil out the bottom of the bed while still allowing roots to air prune.
For a brief visual walkthrough of a few of our air pruning beds, watch our video on DIY Air Prune Beds below.
Tree seeds can be planted as close as 1 – 2” apart depending on seed size. Some may need scarification or stratification prior to germination, but once this process is completed they can be set into the bed to a depth of about 3 times the width of the seed and lightly covered with soil or wood chip mulch. Bed irrigation is via ¼” drip lines with emitters every 6” laid 6″ apart at the soil surface or even beneath the mulch. It may be helpful to hand water until germination to ensure proper coverage of the bed.
An 18 – 24” high wire mesh cage should be set on top of the air pruning bed frame until seedlings have set several sets of true leaves and the trunk has hardened somewhat. Lots of critters love tender tree sprouts (not to mentioned buried tree seeds)! This cage can be removed once they are tougher and less tasty.
Once the seedlings are ready to planted out, they can be gently but firmly shaken out of the air pruning bed. For trees that go dormant this would be easiest to do when they have entered dormancy. Keep the soil medium light and friable so they can be shaken out even when not dormant. The soil medium can be a range of different substrates, and should be tailored to whatever variety of tree(s) is being grown in the bed
Continued Learning
Want to learn more about the science and technologies behind Air Pruning? Get started with out curated playlist of Air Pruning YouTube videos.
We first learned about air prune beds following the work of Akiva Silver of Twisted Tree Farm. Akiva is doing awesome work in the nursery world and we highly recommend checking out his YouTube Channel.
~ Most of the larger problems we face today could be solved by having more trees around. Let’s get to it! ~
