The keys are timing, spacing, improved root systems, proper nutrition, shifting plants into larger containers at the proper time and leaving the side branches on as long as practical. These six factors are critical or you get to sake and stake and tie and tie and wonder why your trees have poor stem diameter/caliper and roots. You must pay attention to timing relative to the tree's growth. Once trees in containers, any containers, reach the sidewalls and space for further root development becomes limiting, the tree's growth pattern becomes like squeezing tooth paste out of the tube, the growth is weak and straight up, and there is no way to get it back into the tube. ALWAYS leave the lower limbs on the young tree trunks as long as practical. Research has shown that leaves on the lower trunk contribute most to the development of trunk diameter and strength and good root growth. Air Root Pruning containers are the very best containers for developing a strong trunk and a good root system. It is a matter of root tips and their function. One root tip can absorb "X" amount of water and nutrients in a day, week or month. If, by container design and simply using air to dehydrate the root tips, thereby stimulating one root tip to branch and become 5 and 5 to become 25 and so on, the absorptive capacity of the root system increases dramatically and in turn the leaves are supplied more efficiently with water and nutrients and manufacture more sugars to run the system and positive results occur. As soon the seeds have germinated and are in the propagation containers, they are placed outside in full sun and with full wind. You want the trunks to bend in the wind as the more the trunks flex the larger in diameter they become (assuming you do not have some other limiting factor) this has been confirmed by some excellent research. It is the resistance against the wind that builds strong straight trunks. Full sunlight, movement of the young trunks by wind, and proper nutrition are essential, then add an air root pruning container to build a superior root system and you are off to a great start. - Millet