Let's discuss alternatives, or a second best solution, on planting a tree in training into the ground for thickening the trunk. Let's say you need to move your trees inside for the winter or you don't have a backyard to plant the trees.
There are popular methods like using big wooden boxes or big plastic pots, but I see more and more people using net pots, mesh baskets or plastic crates. The big benefit of using those kind of well aerated containers is that the roots get air pruned and you get better ramification. But is this method good for thickening a trunk, as the upper part of the tree (branches and foliage) is a reflection of the lower part (root mass)?
How about growing trees on the ground, so that the roots can grow from the container into the ground. Are there any benefits on doing this, rather than planting straight into the ground? We are talking about a period of 5-15 years. And I'm pretty sure cutting the roots every year before winter will be too stressful for the tree.