Attached is a video demonstration on how to best plant pecan trees from Dr. Lenny Wells (UGA Pecan Specialist) and some key points to remember.
Key points to remember:
1-Prune lateral roots to 3-4″
2-Prune the taproot to 2 ft in length. The taproot on the tree in the video had been cut in the nursery at digging at about the right length. You can clip the tip of the root in this situation for a fresh cut. I did not in the video. Also, if the taproot is very short as sometimes happens when the tree has been growing in the nursery in soil with a high water table or hard pan, you can clip the tip of the taproot off. Pruning the taproot stimulates new root growth, which is what new pecan trees need before they can grow a top.
3-Its better to plant too shallow than too deep. The exposed root tissue will adapt and basically function as stem tissue as long as it has enough root in the ground to support tree growth and survival, but buried stem tissue will not grow new roots. As a result, trees planted too deep do not develop good brace roots and blow over easily in strong winds.
4-Dig the hole about 2 feet deep. This allows you to set the tree with the tap root at the bottom of the hole, which will help prevent settling. Holes should be at least 18″ in diameter.
5-Set the tree with the uppermost lateral root about even with the soil line or no more than 1″ below. This ensures good root aeration and allows proper development of brace roots to support the tree.
6-Fill hole 1/4 full with water and then back-fill with dirt. This removes air pockets, keeps soil moist for a while, and with no more than 1/4 of the hole filled with water you get less settling.
7-As you fill the hole it is ok, but not necessary to add P and Zn (no more than a handful) but do not add N, compost, or any other amendments.
8-CUT THE TOP OF THE TREE BACK BY AT LEAST 1/4 to 1/3!!!!!!!!
I know you just paid a lot of money for that 8-10′ tree but you will get much more vigorous growth if you cut it back. Very large trees will benefit from cutting even more. About chest high on a 6′ tall person would be about right for most trees. The larger the tree, the more you need to cut it back. They just lost a lot of roots in the digging and planting process and will recover from transplant shock faster when cut back.