Myth No. 1: The root system of a tree is a mirror image of it's above ground portion.

Since studies began on the structure of tree roots back in the 1930's, researchers across the country have found a consistent pattern to tree root development. This root pattern, regardless of species or location, is shallow and wide. Findings indicate lateral roots of trees extend 1.5 to four times the width of the canopy and are usually within the top two feet of soil. As soil depth increases, root growth diminishes, due primarily to a decrease in the levels of oxygen and moisture. Occasionally roots can grow deeper, but only if soil conditions are just right. Here in Arizona, a mesquite tree was found to have a one fourth to one half inch diameter roots extending down more than 100 feet in a gravel vein of a pit mine. This is the rare exception.

Myth No. 2: Large planting holes, both deeper and wider than the root ball, are best for tree establishment.

A common practice here in the Southwest, as in other part of the country is to dig a large hole to accommodate root development after planting. These holes can be of cavernous proportion, as wide and as deep as 5 feet. Yet when we look at the natural, shallow-wide configuration of root systems, common sense brings into question the validity of this approach.

Studies looking at the effects of these large holes have consistently shown they are a detriment to tree establishment. This is primarily due to subsidence of the root ball. Soil dug and disturbed below the root ball will settle after planting. As the root ball sinks, soil covers the roots and trunk base. As little as half an inch of excess soil over the root ball can inhibit or prevent water from entering the root ball. Slower to develop, but just as serious, are problems related to contact of soil with root collar tissue. The root collar is the junction of the roots and trunk, typified by the flare of major lateral roots. The root collar is a part of the trunk and , as such, is not specialized to resist constant soil moisture. Gas exchange between the atmosphere and phloem tissue of the root collar is restricted. This causes a gradual death of phloem tissue and an increased susceptibility to pathogens such as Phytophthora root and crown rots.

Planting holes should be dug no deeper than the height of the root ball. The point is to set the root ball on undisturbed soil to prevent settling. Because of the wide-spread development of lateral roots, the soil hole should be dug three to five times wider than the diameter of the root ball. Roots will grow more quickly into loosened soil, thus speeding up the tree's establishment period.

Myth No. 3: Amending the backfill of planting holes with organic amendments is beneficial in promoting root growth.

Over 30 studies have been conducted over the past 15 years and only one has shown a consistent advantage to using organic amendments such as peat, compost, or bark. In most cases organic amendments inhibited top and root growth. When combined with a deep planting hole, organic amendments increase subsidence of the root ball. As the organic matter decomposes it shrinks significantly. This results in even greater settling of the root ball.

Much has also been learned about the detrimental effects of soil interfaces on root growth. These interfaces are created when divergent soil types come in contact - interface each other. They are unavoidably created when trees are planted. Soil of the root ball, whether field or container grown are almost always different from the soil at the planting site. Roots have difficulty passing through this interface. When backfill soil is amended with organic matter, an additional interface is added. The first interface is between the root ball and the amended backfill, the second is between the amended backfill and the existing soil. Failure to penetrate these interfaces is a common cause of circling roots and unsuccessful plant establishment.

Planting holes should be backfilled with the same soil taken from the hole. No amendments should be incorporated into the backfill. Even if soil conditions on the planting site are poor, trees must be able to establish roots in the conditions that exist, for long term survival. Loosening or tilling the soil, to the depth of the root-ball, in a circular area three to five times the diameter of the root ball will be most effective in speeding root establishment.

Myth No. 4: Pruning at the time of transplanting aids plant establishment by balancing top growth with existing roots, thus reducing transplant shock and easing establishment.

In most plants, buds and young leaves produce compounds which stimulate root initiation and growth. Numerous studies on the effects of transplant pruning have shown that removing buds and young leaves reduces new root initiation and growth. Although pruning stimulates growth of individual branches, it generally has a dwarfing effect on overall size. Any removal of top growth also reduces foliage, and with it the ability of the tree to manufacture sugars and carbohydrates necessary for new root development. Pruning at transplanting has been shown not to enhance survival or establishment of container-grown or bare-root plants.

Myth No. 5: Pruning paint promotes rapid closure of the wound, repels water, and protects the wound from decay and disease organisms.

The practice of applying pruning paints or wound dressings to pruning cuts continues. This despite the fact that research has shown no value in the use of such products. Wound dressings do not prevent cracks, mushrooms or wood rot. Rather than sealing off a wound and protecting it from rot and disease organisms, just the opposite is true. When exposed to the sun, the protective coating often cracks, allowing moisture to enter and accumulate in pockets between the wood and the wound dressings. If pruning shears or saw are contaminated with a disease organism, such as sooty mold or slime flux, the wound dressing can seal the organism against the wound. With the accumulation of moisture, a perfect environment is created for the disease to develop. In addition, asphalt-based wound dressings can be phytotoxic.

Much has also been made regarding the size of the cut in relationship to pruning paint. Many believe that pruning paint does not need to be used on small cut, but any wound larger than 1 inch in diameter should be treated. Regardless of the size of the cut, pruning paints or wound dressings should not be used. Properly made pruning cuts will successfully callus over much more rapidly if left unsealed.

Myth No. 6: Branches should be cut off flush with the trunk to hasten closing of the wound and prevent decay. To this day, much of the pruning that is done does not take into account the importance of the branch collar. The collar is the place at which the branch joins the trunk. Usually it appears as distinct swelling at the base of the branch. In some species, this swelling is not as noticeable, but the collar does exist on all tree species.

Numerous studies have shown the important role the branch collar plays in resisting the spread of disease organisms and decay. Chemicals within the collar provide the protection. When flush cuts are made, these chemicals are removed. Even worse, the cut is made into trunk tissue, making callusing-over of the wound difficult, if not impossible.

Cuts should never be made flush with the trunk. Cuts should always be made just to the outside edge of the swollen bark collar region. In this way the collar remains in-tact and only branch tissue is cut, with no damage to the trunk.

Myth No. 7 Caliche and other hardpan layers in the soil must be removed at the time of planting to accommodate the roots ball and future root development.

In the desert Southwest, a common obstruction to planting is a layer called "Caliche". It is a hardpan that formed by the cementing together of soil particles with lime (calcium carbonate). It can be as hard as concrete and anywhere from a few inches to a few feet thick. Sometimes a caliche layer can be found at or within a few inches of the soil surface. Other times it may be several feet below the surface.

Many types of hardpans, such as caliche, exist in other regions of the country. In Arizona, the standard practice has been to remove it with pick axe, crowbar, or even jack hammer. Much to much effort has been put into removing caliche, especially layers that are down several feet deep. Even relatively shallow layers of caliche can be left in place. Generally these layers are not solid or continuous, but rather fractured allowing water to drain through. In the desert native Palo Verde, mesquite, and ironwood grow successfully over caliche layers very close to the soil surface. Excavations of mesquite trees growing on caliche show the trees ability to root over and even into and through this hardpan. As long as several inches of soil overlie the caliche hardpan, trees can be planted without removing the caliche. The same type of mounding technique employed on poorly drained soils or those with high water tables can be used for planting over hardpans. Smaller root balls must be planted where hardpans are close to the soil surface.

As with standard planting recommendations, the root ball should be set on undisturbed soil, in this case the hardpan itself. Soil is then bermed up to cover the portion of the root ball that extends up above ground. If the hardpan is solid and will not drain, then a drainage column can be dug through the hardpan, not under, but to the side of the root ball. This will funnel water away from the root ball base and reduce the danger of roots becoming waterlogged.

By no means are these all the prevailing myths of horticulture. They are, however, some of the most common and the ones that have been addressed with much scientific research. It behooves all those working in the horticultural profession to get the word out, and help put these myths to rest once and for all.