Q: What is the best way to plant a tree? My landscaper says that mixing organic matter in a tree planting hole ends up wasting time and money, because the roots of the tree will eventually grow out of the initial area.

A: Back in 1975, Dr Carl Whitcomb set the tree planting industry on its ear by releasing research that adding soil amendments to the hole around a tree did little good. In subsequent years, other researchers have found the same.

see
www.treelink.org/joa/2004/sep/gilman9-04.pdf

My recommendation is to loosen a large area 10 feet in diameter around the spot where the tree will go to allow roots to penetrate the native soil as fast as they can.

I highly recommend mulching that area and never letting grass grow under a young tree.