Tree Planting Fundamentals

Advantages

The primary advantage of using trees produced in containers is that 100% of the root system is moved with the transplant. The plant therefore undergoes no transplant shock if given adequate post-transplant care. This is a real advantage if the tree is to be planted during the growing season, when removal of part of the root system predisposes the rapidly transpiring tree to shock. The season of planting is therefore much extended.

Container-grown trees are generally much lighter than B&B trees because the perched water table created by the container requires the use of a well drained, and therefore lighter, potting mix. This facilitates easy handling and shipping. A 2 in (5 cm) caliper tree produced in a container can be lifted by two people, but a 2 in caliper tree B&B would require a tractor. Container-grown trees can be loaded and unloaded much more quickly than B&B trees.

Disadvantages

Perhaps the principal disadvantage of container-grown trees is the possibility of deformed root systems. Roots do not continue encircling the root-ball when planted but will grow straight into the backfill soil if reasonable tilth is present (14). Problems arise, however, when the trees have been held for too long a time in the container. This ‘pot-bound’ condition reduces the vigor of the plant in the nursery and has a dwarfing effect on the tree. Dwarfed container trees are no bargain. The entangled mass of roots is a physical barrier to root regeneration, and the tree may develop girdling roots.

Container-grown trees are often more expensive than B&B trees because of increased production costs. Container production requires that most capital expenses be invested before harvest, whereas B&B production requires much of the expense at harvest after the plant is already sold. For this reason, larger sizes are often unavailable in container-grown trees. Since roots are less cold hardy than shoots, container-grown trees may need winter protection in the north, further increasing their cost.

Irrigation is required more frequently when planting container-grown trees. When a container of porous media is planted into a finer textured soil, thereby removing the perched water table, water drains from the potting mix into the surrounding soil. A container-grown tree may therefore need more irrigation after planting than it did before (34). Container-grown trees also have high post-transplant transpiration rates. Since 100% of the root system is planted, the tree transpires at pre-transplant levels, using up the available water quickly. In other types of transplants, stomatal response to root severance conserves moisture by reducing transpiration thereby depleting the available soil water reservoir less quickly.

Fabric containers

The use of fabrics to control root growth was introduced by van der Werken (44) and refined by Reiger and Whitcomb (38). The fabric container is placed in the ground, backfilled with native soil, and the tree liner is planted in the container. The fabric container is removed at transplanting. Roots can penetrate the container, but radial expansion is limited, thereby producing a girdling effect. The girdling effect, in theory, causes a concomitant increase in root branching inside of the container and a general increase in the fibrous nature of the transplanted root-ball.

Advantages

Moving B&B trees is the traditional method of transplanting. Workers are used to handling B&B trees, and consumers are used to seeing them planted. In addition, B&B trees are readily available. Large sizes are available, and are limited only by the equipment available to lift and ship. A major advantage to planting B&Btrees as opposed to trees produced by other production methods is that soil types can be matched, thereby reducing any interface problems that might inhibit water flow between the surrounding soil and the root-ball.

Disadvantages

The main disadvantage of field-grown trees moved B&B is that over 95% of the roots are left behind when dug (15,47). This may be overcome somewhat by cultural methods such as root pruning (48) or by buying relatively small trees which were lined out from containers. Harris and Gilman (18) harvested 51 % of total root length of East Palatka holly transplanted B&B 17 months after lining out from 1 gallon containers in Florida. Trees moved B&B are subject to seasonal restraints, although many operators use special care and move trees year round. The most favorable seasons are when transpiration demand is low and root regeneration potential is high. These do not usually coincide, since the most favorable time for root regeneration is probably after the first flush of growth has hardened (31). With the much reduced root system, water relations following transplanting are most important (27). B&B trees are heavy. Moving landscapesized B&B trees requires equipment and skilled personnel to operate it.

Advantages

Advantages to planting trees bare-root are primarily financial. Bare-root trees are much cheaper than trees produced by other production methods because of ease of digging, storing and shipping. Many species respond well to moving bare-root. Longer root lengths are possible since weight is of little concern. Bare-root trees can potentially retain a greater proportion of the original root system. Inspection of the entire root system is possible, and inferior root systems or defects, such as girdling roots, can be detected.

Disadvantages

The range of sizes obtainable from other production methods are not available for bare-root transplants, since larger sizes usually do not transplant very well. Many species of trees cannot be moved bare-root (20). This is primarily due to a lack of desiccation tolerance (5). Careful attention to handling is required because the exposed root system must be protected from drying influences.

Transplanting bare-root trees is more affected by seasonal restraints than other production methods. Trees should be dormant. Short windows of opportunity in northern latitudes can be a major restraint when summer comes ‘quickly’ or winter comes ‘early’. Larger bare-root trees usually have to be staked since a leafy crown without a secure root-ball is more subject to windthrow in the spring.

The planting hole

The addition of soil amendments to the backfill is a time honored tradition, and use is still recommended by some contemporary horticulturists (12,26). Most research, however, reveals that the amendments offer no consistent advantage (7,24) or even may prove harmful (4,35,49). Backfill should, in most cases, be the soil removed from the planting hole. If the backfill is of extremely poor quality, good topsoil is the best alternative (51), or better yet, don’t plant a tree in that spot. An exception is the case where entire beds can be amended, such as to leave room for several years root growth. The most important contribution to the backfill is increased aeration (2). This is best accomplished by digging a hole that is no deeper than the rootball and at least three times as wide (7). Digging a deeper hole creates an opportunity for settling of the root-ball and an increased chance of root suffocation. Backfill should be lightly tamped periodically as it is returned to the planting hole to eliminate air pockets. Air pockets create a problem with water movement and will decrease the volume of soil available for root regeneration.

Root-ball coverings

Coverings to retain intact root-balls are necessary and increase in number and type with the size of the root-ball. Burlap should be pulled down from the top of the ball at planting, and twine should be removed from around the trunk. Burlap exposed at the surface can act as a wick to dry out the root zone. Synthetic burlap does not degenerate in the soil and will girdle roots. Large root-balls are often covered with wire fencing or specially designed baskets which hold the balls together during transit. The effect of wire baskets on future health of trees is a subject of concern and debate. Recent research in Canada (17,33) alleviates some concerns for trees already in place, but the question of removal before planting has not been answered with certainty. Tree response to root girdling by wire baskets is likely species dependent, and further information is needed. Until the question has been answered more definitively, it is best to remove wire baskets before planting or to place the tree in the planting hole with the wire intact and remove or fold down the upper half of the basket. This will allow the majority of roots to escape girdling.

Wrapping, staking and guying

Trunk wrapping is another tradition whose effectiveness has recently been questioned. Trunk wraps, in theory, help prevent sun scald and frost cracks, particularly on trees with thin bark such as birch and maple. They also help deter damage from rodents and offer some protection from mechanical damage. Litzowand Pellet (32) tested the effectiveness of paper trunk wrap in buffering trunk temperatures and reported that temperature fluctuations were actually greater under wrapped trees than where no wrap was used. The most effective material was a greenhouse insulation material. The effectiveness of other materials are currently being studied by researchers in Virginia (1). A survey of ISA members reports that many wrap trees primarily to give physical protection from vandals and equipment.

Staking and guying newly transplanted trees is often unnecessary. Prolonged staking and guying not only reduces normal taper of the trunk (20) but is Habile to cause accidents, particularly if guy wires are not clearly marked. Guys that are not removed after one growing season can quickly disfigure and girdle the tree. There are situations when staking is advisable, however. Trees that were staked in the production nursery will often require continued staking in the landscape. Trees in very open and windy sites, particularly in wet conditions, will also require staking. The decision to wrap, guy and stake should be made on an individual tree-by-tree basis and should not be required for all plantings.

Pruning

A standard recommendation to landscape contractors has been to prune back 15-40% of the top prior to transplanting (10,26,27). The premise is that pruning the top will reduce the amount of new growth during the spring flush, and that transpiration demands and the resulting water deficits will be reduced. Shoup et al. (39) reported that degree of pruning had no effect on survival, but that pruning more than 15% of the tops before transplanting reduced the visual quality of all species. Ranney et al. (37) reported a decrease in transpiration on cherry trees pruned 50% before planting, but a reduction in growth rate of roots as well as shoots occurred compared to unpruned controls. The benefits of indiscriminate dormant pruning of deciduous trees at planting are doubtful. Pruning should be restricted to corrective pruning to improve form only.

Fertilization

Recommendations for posttransplant fertilization vary considerably. Some researchers report no effect of fertilization at plantings (39,40,43) while others report significant response (22). No advantage has been demonstrated to any form of application except broadcasting evenly upon the soil surface, but a slow release type fertilizer may be mixed with the backfill if desired.