If you are trying to do root pruning, you want a "chunky" mix. When a root tip is trapped by the material inside the bag, it can't circle or j-hook. This stimulates the root to branch upstream. Most of this branching occurs in the 1st 4" from where the root is trapped. That is why root pruning requires a series of containers. When working with apples, we are generally using clonal root stock so there is no tap root. This let's us skip the first stage of 18s which are intended to prune the tap root. So, you are looking for root trapper bags that allow roughly 4" of growth in all directions on the roots. This is sort of guess work when dealing with clonal rootstock but you get the concept.
Roottrapper bags (or root pruning containers in general) allow for a much larger tree above ground than you would expect for the container size. This is because of the dense fibrous efficient root ball they create. Here is where the mix comes in. Some mixes are very dense and don't offer much area for the roots to fill. A "chunky" mix creates a lot of voids in the mix. These voids create space for the root system to develop. One example of this is using a different kind of root pruning container like a Root builder II that uses air pruning instead of trapping. When I first transplant into one of these and water it, the water runs very quickly through the well drained mix and it doesn't take long to see it coming out the lower holes. After the tree has been growing in one of these containers for quite a while sand is near ready for transplant, it becomes very difficult to top water. You put water in the top and have to wait for it to soak in. You then add more water and wait again. After repeating this process over and over, eventually you see water running out the lower holes. Why? because all of those voids were filled by the dense root ball.
To get more to a direct answer. For very small root pruning containers like 18s, I use straight promix. The containers are so small that adding chunky material beyond the normal perlite and vermiculite in the mix can be a problem. Once I get to 1 gal containers or larger, I use promix as my base. Lately, I've been using about 1/3 promix. To make the mix more chunky I'll add about 1/3 wood chips. I've used mini pine bark chips from lowes, but I recently had some trees taken down and chipped so I've used those. The last third I use a mix of perlite and compost from my yard.
However you get there, you want some chunkiness to the mix so there are plenty of voids for the roots to fill. One note of caution: The chunkier the mix, the more frequently I need to water. The more well drained the mix, the faster it dries out. My trees love this frequent drenching and drying out, especially chestnuts that don't like wet feet.
This is not to say other finer mixes won't work. They just won't maximize root development like a chunky mix.
This light mix can create an issue in some soils like mine, heavy clay. You can actually create a pool by planting this kind of amendment into heavy clay soil. The roots can drown in wet weather and dry out in dry weather. I've developed a good planting technique for planting rootmaker trees in heavy clay. I've written about it in a number of threads. If you need it and can't find it, let me know and I'll try to search for it.
Thanks,
Jack
