I'm interested to follow the comments on this subject too, so I hope this is not too much of a tread-drift. As Jack mentioned, I like to baby bare root trees over the summer before transplanting to their final locations. Currently, I simply re-use black plastic containers I have saved from garden store purchases (probably 3 and 5 gallon sizes). Since I only add maybe 6 trees per year, I'd like to give them the best possible start, and if using some sort of air-pruning would help, I'd do that. However, are so many options... Root builder, root trapper, injection molded, fabric - and then in what size.... yikes, I always end up using what I have. In laymen's terms, for a one summer transition from bare root to orchard, is root pruning practical? And if so, what would be the best product/size? Thoughts appreciated.
First, let me address Mahindra's size question. For clonal root stock (I used M111 but it doesn't really matter), I've tried 1 and 3 gal RB2 containers. I had much better success with the 3 gal containers. These have no tap root to worry about. I found that it really comes down to size. Smaller M111 will fit in a 1 gal but larger won't. I've grafted M111 and immediately potted it. I got better success with 3 gal RB2s than 1 gal but I did have some success with 1 gal.
Apple junkie, you may be doing more harm than good babying your bare root trees in smooth nursery containers. Yes, they get better care at home, but that is offset by the potential for circling and j-hooking roots and the transplant shock. If you plant a bare root tree in the field, the saying goes, the first year is sleep, the second creep and finally in the 3rd year they leap. This is because the root system has major disturbance and is often pruned as the tree is extracted for shipping. It takes time for that root system to reestablish itself before the tree can start growing at full rate again. With those smooth plastic nursery containers, when a root hits the side of the container or bottom, it will either reverse course or, more likely, circle following around the container. While smooth container trees can look great above ground and even do well after transplant for the first few years, if those circling/j-hooking roots constrict themselves over time, you can have problems with the tree. So, in order to avoid this, you really need to remove the tree from the containers and remove enough media to ensure there are no circling roots. If there are, you can manually prune them. The down side is that once you have disturbed the root system to do this, the tree suffers transplant shock all over again when you plant it in the field.
You get the biggest benefit from root pruning containers if you start trees from nuts/seed and use a system from the beginning. The small containers (18s) are shaped to direct the tap root to the holes in the bottom. The containers are hung from a rack so there is air flow below those holes. When the tap root hits the air it naturally desiccates and seals off. This cause the tree to react by significantly increasing the amount of upstream branching. Research shows that the majority of the upstream branching occurs in the last 4" of root before the air prune. The secondary and tertiary roots produced upstream are also redirect to the holes and they too are pruned and the process repeats until the tree runs out of room in the 18 cell. In theory the next sized root pruning container would allow for 4" of expansion on all sides of the root ball. This doesn't need to be exact and practical consideration regarding container size and construction factor in. Once again the larger size containers are designed to route roots to holes where they are pruned.
Root pruning containers trade off a long tap root needed to ensure access to water when a young trees is establishing in nature for a denser root system with more root tips for more efficient uptake of water and nutrients. This means you need to ensure the root ball has sufficient access to water until it is established in the field and is large enough to get what it needs. How much of this is needed depends on your climate. In my area, we get enough spring and fall/winter rain that I don't provide any supplemental water in the field once I plant my trees. In arid regions you may need to provide supplemental water for a year or two.
The thing about these containers is that they are designed so that the root ball is completely undisturbed during transplant. The entire root ball from one sized container is easily extracted and placed in the larger container if the container system is well designed. For example the rootbuilder II containers unwrap from around the root ball. So, when you are planting these in the field there is very little transplant shock to the root system. There is some adaptation required to the new microclimate and soils but this is minor. Unlike bare root trees which must be transplanted when dormant, you can plant these trees in the middle of summer when they are actively growing provided you provide the supplemental water needed.
Bare Root Trees:
You can put bare root trees in root pruning containers and get some benefit. Keep in mind that since most upstream branching occurs in the last 4", you won't get the same benefit as you do using an entire system from see/nut to tree. If you decide it is worth keeping trees at home where you can baby them root pruning containers are much better than smooth nursery containers. They won't cause circling and you won't have that second transplant shock because the entire root ball will stay intact at planting time.
The size container you need really depends on the size of the root ball of the bare root tree. You don't want to circle the roots from the bare root tree to fit into the container. This defeats the purpose. You want the roots to fit into the container. It is probably better to prune roots by hand to fit them in the container rather than wrap them. It is best if you have a container large enough so no manual pruning is required. I doubt many bare root trees you buy would be small enough to fit in 1 gal RB2s. Except for M111 I've always used 3 gal for small bare root trees. When you get larger than 3 gal, things get pretty heavy for transport and planting and such, but there is no reason you can't use larger containers if your trees are larger and you don't mind dealing with the size.
Hope this helps,
Jack
