Planting Trees? Dig Big!

Planting Trees? Dig Big!
by Michael Kuhns, Extension Forestry Specialist, for TreeUtah newsletter

Think that new tree needs a really deep hole to do well? Think again! Good tree planting requires the use of wide and shallow planting holes to promote root growth.

In the past a typical planting hole was vertical-sided, slightly wider than the root ball, and often deeper than the root ball is tall. The root ball then was set on packed backfill rather than undisturbed soil.Planting trees? Dig big!

Modern methods require that you dig a much wider hole - two to five times as wide as the root ball and even wider on sites with very poor, compacted soil. The hole sides should be slanted and the hole should be no deeper than the root ball is tall, so it can be placed directly on undisturbed soil. Tree roots typically grow more sideways than vertical and most roots stay fairly shallow, so a wide shallow hole fits the form of the root system. Also, the wide shallow area of worked-up backfill is less compacted and has more pore space than the undisturbed soil, so roots grow into it quickly.

Placing the root ball on undisturbed soil rather than packed backfill helps ensure that the tree is not planted too shallow. Heavy trees planted on packed backfill sink and end up planted too deep. Roots then do not get enough oxygen and will suffocate. A good rule of thumb is that a tree's root system should be planted at about the same depth as it grew in the nursery. Holes can be dug even shallower on compacted or waterlogged sites as long as the backfill soil is mounded up to the root collar, the point on the trunk even with the nursery soil level. With any hole I also recommend roughening the hole sides with the tip of the spade if they become glazed during digging - roots have a hard time penetrating such a surface.

Wide holes have a hidden benefit. The extra room around the root ball makes it much easier to remove wire baskets, burlap, and other root-ball packing materials once the tree is in the hole. Removal of such materials at planting time is important for the same reason I prefer wide holes - both ensure quick root growth out of the root ball and aid in tree establishment.

Here are several other planting tips:

  1. Do not fertilize or use other growth enhancers at planting time in most soils. Fertilizers have not been shown to increase root growth and high nitrogen may actually decrease root growth. Wait several years to fertilize until the tree's growth rate has recovered to pre-transplanting levels.
  2. Know the tree you are planting. Learn its requirements and preferences. If the site conditions are not right for your tree, change the site or plant a different tree.
  3. Mulch your tree with a 4 inch layer of coarse wood chips or bark spread out for at least several feet all around the tree. Wider mulch beds are better and keep deep mulch away from the base of the tree's trunk.