Simple 1' x 2' air prune beds.
Have you ever thought about starting plants for your own food forest, or growing trees and shrubs with the intention to plant them out somewhere else? Have you ever wanted to grow hundreds of trees and shrubs in a small space, then gift them to families and friends? Or maybe you’ve thought about starting your own nursery? The thing with tree seeds is that they can be easily planted in pots, but they then become root bound, making for tough times down the road. They will recover in time, but the roots will need a period to adjust, which can set the growth back quite a bit. This is where air pruning or root pruning comes in handy! This has nothing to do with making actual cuts to the roots, but it has everything to do with the natural processes of tree growth. Here, we’ll talk a bit about how to build an air prune bed and to grow hundreds of easily transplanting plants!
The Growth Process of a Tree
If you know anything about the growth process of a tree or a shrub--or most plants that aren’t annuals, or even some annuals as well--you’ll know that one of the first things that a seedling undergoes after germination is sending out two shoots. One becomes the trunk or main foundation of the tree, and the other is the taproot. A taproot is a long and strong central root, sort of mirroring the trunk of the tree. Taproots help to keep a plant in place, anchoring it so it doesn’t get washed or blown away. They also dig deep into the Earth, retrieving trace minerals and nutrients that aren’t accessible at the surface, but instead lie far beneath in the substrata. These are ingenious adaptations made by many plants, no doubt ensuring their survival and a main part of why they are here today! However, if you are trying to transplant a tree, the taproot can be quite the nuisance. They can grow quite fast, and are usually longer than the main stem in the younger ages, and if you break one while trying to dig up the plant, it will severely damage the plant, stunt the growth and in some cases it will prove fatal to the plant.
Air Pruning or Root Pruning
If this is sounding a little bit daunting, or if you’re thinking you may have to dig a very large hole, don’t fret just yet! You may be lucky enough to have very loose and friable soil, or even very sandy soil, or maybe you’ve built your soil up enough to the point where it is rich and loose. If so, then that is great! You may be able to dig up and transplant trees quite easily. Although not everyone is that lucky. We’ve learned the hard way, our hard clay soil is not ideal for most trees. Trees will grow and survive, but they won’t really thrive here, especially in the young stages of growth. Here’s where air pruning comes into play.
When a tree root (or any plant root for that matter) comes into contact with oxygen, it slows down growth, and the plant puts energy into other roots, or the above ground portions. Plants are smart, and they know what to do and when to do it, they know that if they hit a pocket of air, there’s not likely to be much soil after that. Root growth will still continue, albeit at a much slower rate. Now we can use the intelligence of plants to our advantage, much like pruning undesired or unproductive above ground limbs can influence the plant to put more energy into fruit development. The key to working around the deep taproot is to introduce the roots to oxygen without digging up the tree, and the way to do this is to build some air prune beds and plant some seeds in them! Oh, and you have to build them, because they won’t be available at your local gardening supply store.
Pawpaw seeds being planted in an air prune bed. These trees have long taproots that need to be pruned by the air for easy transplanting.
A friend and fellow nursery owner turned us on to air prune beds. These are beds built with the intention of introducing the roots of trees and shrubs to oxygen at a young age in order to form bushier and easier transplanting roots, or root balls. The first thing you want to do is pick out the dimensions of the beds you’d like to build. This can be any dimension you want, 1’ x 2’, 3’ x 10’, or even 5’ x 100’! It’s all about personal preference and what works best in the setting and situation you find yourself in. We like to build smaller beds, they end up being more manageable. Usually about 1’ x 2’ and 6” deep. Keep in mind that the larger you build your bed, the harder it becomes to hold the weight of the soil without popping out your staples, you may need to build additional support.
Next, you’ll want to acquire your materials. You’ll want to use untreated lumber if you can--pressure treated lumber is treated with chemicals so you’ll want to steer clear from those with your plants--so maybe rough cut from a sawmill, or maybe some scrap wood you have lying around. We usually use off cuts or even slab wood from mills. Then, you’ll want to get yourself some ¼ inch hardware cloth. This can be bought at any hardware store, and this will form the bottom of your bed, retaining the soil and exposing the roots to air. Build your bed in a square or rectangle (you can be as fancy and as rigid as you want here, we find that just 2 or 3 screws attaching each piece of wood is plenty fine) and then fix the hardware cloth to the bottom using heavy duty staples or nails or screws. Be sure to use caution when cutting the hardware cloth and handling the beds, the edges towards the bottom can be sharp! We bend them with some persuasion using a hammer.
Fill with soil and voila, you have yourself an air prune bed! Another thing to make sure that you do is leave an air space between the bottom of the bed, and the ground. You can do this by building legs for the bed, though we find that some rocks or cinder blocks work just fine, you want the roots to hit air, not keep growing into the soil!
The wonderful thing about these beds is that they can be mobile, they are easy to build using commonly found, or even recycled materials, and they get around the issue of the taproot. Another lovely thing is that because most tree seeds that grow naturally in a Northern or temperate climate require a period of cold stratification, you can use these to stratify your seeds! Cold stratification is when a seed undergoes a period of cold temperatures to wake it from it’s dormancy and signal it to start growing in the Spring. You’ll want to add an extra layer of hardware cloth on the top part of the bed to protect your seeds from visitors (squirrels, chipmunks and the like love nuts and seeds, but you probably knew that) and maybe some mulch for a little bit of insulation. You can also stack them when not in use, or use them if you’d like to get a head start on trees and shrubs, but you don’t currently have the space to grow them out, say you’re in an apartment looking for land, or you just haven’t made a commitment to where you’d like a tree to grow. Design is important!
All in all, this is a fantastic way to grow lots of high density, high value plants in a small space, and make sure that they have the attention they need to be easily transplantable. They will be happy when you plant them into their forever home and you’ll be thankful you don’t have to worry about damaging that taproot! And don’t worry, it will pick up the pace once it realizes it’s in some nice soil again. Plants are smart, remember?
Plant trees, and live well my friends!
Michael Perry and Schikoy Rayn operate Sacred Circle Homestead, a small-scale, low-tech perennial nursery focusing primarily on medicinal and edible species utilizing principles of permaculture. Learn about the classes they teach at their website or at The Trillium Center, a healing center where they hold workshops in Burlington, VT. You can find the articles they've written on their website here, or the articles they've written for Mother Earth News here.
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