What root pruning containers should I look to purchase (if that’s my best bet for high success rates)
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One key to air pruning is to prune at the right point. Research shows that when a root is pruned, most of the upstream branching occurs in the first 4" from the pruning point. So, the first container should prune the tap root at about 4". These first stage containers are good for about 16 weeks. Each subsequent stage should allow about 4" on all sides of that root ball. There is a trade off with what is optimal for root development and what is practical.
The containers I like best for the first stage are Rootmaker Express 18s. The cells are heavy duty and can be removed from the tray and reorganized. This makes it easier to keep the lights about the same height from all the seedlings. They are more expensive than the basic Rootmaker 18s. They work just as well but the cells are not removable and the plastic is more brittle. They are good for several seasons but eventually wear out unlike the Express cells.
If your budget is very tight, you can use the DIY containers I experimented with here:
http://www.habitat-talk.com/index.p...-transfered-from-qdma-forum.5542/#post-107862 These are very inexpensive to make. They dry out more quickly than rootmaker cells so you have to keep a closer eye on watering but they work just as well. Some trees like chestnuts are much more sensitive to water than apples. If you decide to go this route, let me know. I have many of these that don't get used since I have plenty of express trays now.
I use 1 gal RB2s for the second stage and finish with 3 gal RB2s (Rootmaker Rootbuilder II). If you look at actual dimensions, the 4" rule lies in between. Someone else was just asking me about this. I think you could probably skip the 1 gal stage if cost is a real issue. The root system may be a bit less dense but I'm not sure one would notice it with the final result. I like the RB2 style container because they are reuseable for many years and they unwrap. The diameter is perfect for matching my tractor auger for planting. I have heavy clay so my planting technique may be different than many.
Many folks use air pruning style bags for the second stage. They are less convenient but if the size follows the 4" rule they work just as well. They can dry out faster so again keep an eye on watering. They are less reuseable but less expensive than RB2s. Some are biodegradable and intended to be planted along with the rootball for one time use.
I would avoid smooth sided containers because you have root circling and j-hooking issues. Direct seeding can work well too, you don't get that early jump start or accelerated early growth.
One last thought: I don't provide supplemental water after planting my trees. By far, trees planted after they have filled a 3 gal RB2 do much better than trying to plant them from 1 gal and planting directly from 18s is a complete failure for me. If you don't provide supplemental water after planting, your climate is a factor in success. While a root system from a root pruning container system is much more dense and efficient than a native seedling and can make much more efficient use of nutrients and water, it can only use what it can reach. A native seedling sinks a tap root for a reason. It is kind of insurance against drought before the tree is fully established. The tap root gets deep enough that there is some moisture available for the tree. I would not recommend root pruning for arid regions where supplemental water can not be provided during drought. Once a rootmaker tree is in the ground for a few years, it has plenty of reach in most regions to get enough water. They work great in my climate. The more prone you are to drought, the more a natural tap root will benefit you.
Thanks,
Jack