How 'Not' to Plant a Tree or Shurb

More trees and shrubs fail because of improper planting than any other single reason. Digging holes too large, setting plants too deep, and adding amendments to the backfill soil all contribute to plant failure.

When planting trees or shrubs, do not dig an oversized hole! Many people dig the planting hole deeper, often much deeper, than the height of the root ball. This is often done with the mistaken belief that digging a deep hole will encourage deep rooting. It doesn't! Most roots emerge from the top half of the container root ball. As they grow outward, they also gradually grow downward. Eventually two to three feet deep, but rarely deeper. Roots of container grown plants do not grow directly down from the bottom of the pot and they do not develop tap roots. So digging deeper than just deep enough to set the plant into the hole is a waste of time and effort. Worse yet, the loosened soil in the bottom of the hole can cause the plant to sink in the hole. As it does, soil washes over the tree trunk, or main stem in the case of shrubs, causing it to slowly rot.

Always plant so that the top layer of roots are level, or slightly above the surrounding soil. Dig the hole only as deep as the root ball is high. If you dig the hole a little too deep, add some soil to the bottom and compact it until you raise the bottom to the right level. The hole should, however, be dug wider than the root ball. This will provide a loosening of the soil and ease the growth of new roots. It is advised to dig the hole twice as wide as the root ball.

Do not mix organic amendments into the soil used to backfill the planting hole. Peat, compost, manure or topsoil mixed with or used as backfill for the hole will inhibit the development of new roots. Worse yet, the hole filled with improved soil acts as a pot in the ground. As roots grow to the edge of this "in-ground" pot, they stop growing out and begin circling around the inside of the hole. With such a small area to grow roots, trees planted in this fashion often blow over in a strong wind. If they don't blow over, they begin a slow decline as roots grow in on themselves and the trunk.

The best soil backfill for the planting hole is that which was dug from the hole. Of course, large rocks should be removed. The soil, and any caliche present should be broken up with a shovel or garden rake to eliminate any large soil clods. The loosened soil can then be filled back in around the roots of the newly planted tree or shrub. With no artificial soil mix present, the roots immediately grow out into the native soil. From there on, they just keep growing, forming an expansive, healthy root system.

A troublesome aspect to planting can be caliche. This hard, nearly concrete-like substance underlies the soil in many locations in and around Tucson. The thickness of the caliche and it's proximity to the soil surface varies greatly. Sometimes it is just a few inches below the surface. Other times it may be several feet down, or even non-existent. If present, it can be a solid layer, but more often it is fractured with cracks and crevices that allow water to drain through.

Rarely is it necessary to dig out the caliche when planting. Unless the caliche is at or just under the soil surface, trees and shrubs can be planted on top of it. By selecting plants in small containers, five or fifteen gallon sizes, the depth of the planting hole can be kept at a minimum. If after digging down to the caliche, not enough room is available to set the root ball all the way in, simply mound up soil around the exposed portion of the roots. Tapering the soil out gradually from the root ball will prevent soil from washing away during heavy summer rains. As the roots grow out they will first grow over the caliche. Then as they find cracks in the caliche, will grow through it.

Take special care to avoid the "do-nots" of digging deep holes, setting the plant too deep or adding organic amendments. This applies to non-desert plants like citrus and oleander just as much as it does to desert palo verdes and Texas rangers. All require the same planting methods for success

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on April 16, 2000
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