Ok, here goes nothing. Field growing has a wide range of applications and uses depending on the tree being collected. I am by no means an expert, but I have employed these techniques with some success over the last few years. My approach to field growing is pretty straightforward and I break it down into 3 steps.
1. After I've identified and chopped a tree I like, I leave it in the ground for at least one growing season untouched. This allows the tree's existing and wide root system to redirect established and unharmed sap flow to new buds and to quickly thicken and stabilize new branches. As others have said, field growing is an accelerator, so it will not create a highly refined tree.
2. After at least one growing season, I'll return to the tree and dig a trench around the the existing roots but leave the tap root in the ground for stability. The trench should be a few inches wide and being slightly inside of the size of the grow box you want to place the collected tree in. Once the roots are severed, I fill in the trench with my collection mix, which is about 20/20/60 compost, sphagnum, and 1/4" pumice, and I'll topdress it with a mix of compost and topsoil.
By digging a wide trench, you're letting the tree push more fine roots inside of the container space before its in a container. This obviously also depends on your local soil type. If the tree has deeper roots to the water table, for example, you'll likely end up cutting them when you lift the tree from the ground, so they can stay in to continue to provide energy for the tree through established sap lines.
3. After at least one season like this, depending on the tree, its ready to be lifted from the ground. You should have a strong leader along with a few branches, and enough fine roots for the tree to continue to survive in a grow box. If you don't want to collect it you can continue to field grow and refine the trunk. If you choose to keep the tree in the ground, you can lift it and sever the tap root and plant it back in the same hole with more soil amendments to encourage even more fine root growth, or you can leave it as is and continue to let the trunk feed off the new small roots and existing deeper roots.
Another benefit of leaving the tree in the ground is that severe cuts will heal faster, so you can make somewhat drastic trunk chops as the new leaders grow.
I have a few trees in development using this technique at the moment and I'll post pictures as I make new steps here. They are a pretty large yew (canadensis), a winged elm, two willow oaks, and a really thick buckeye with a huge lateral root just for fun.
