posted 4 months ago
Just like the top of most trees are radially oriented, the roots are mostly radially oriented also. The soil that the hole is backfilled with is light and fluffy compared to the hard compacted soil outside the hole. Easy for new roots to penetrate through. But, as the roots continue to grow and spread out radially, they all will hit hard compacted soil all at the same time.
Because the young tree is used to uninhibited root-growth for some time it will have luxurous top growth that is being fed by all the new roots. Suddenly all the roots hit the undisturbed hard, compacted soil at about the same time, which suddenly puts the brakes on growth. The young trees suddenly go into shock because the lush top growth can no longer be supported.
By putting the new tree in a square hole, the radial growth of the roots does not hit the compacted wall all at the same time, so the tree has time to adapt to the changing soil conditions. And yes, the right angles tends to direct the growing roots downwards.
I've planted ~120 seedlings in the ground now, following the square-hole for all of them. The only dead trees I've encountered were from animal damage.