Part 2 of 3:
The ABCs of successful tree planting:
Prepare the site before planting
The ABCs of successful tree planting:
Prepare the site before planting
By Jack Radecki
Whether on municipal road right of ways, commercial or even residential properties, most trees are planted on altered sites. Parent topsoils have been removed and the remaining subsoils are graded and compacted. A six-inch layer of topsoil may be added to support turf. Unfortunately, this type of land development provides very poor growing conditions for newly planted young trees. It is no wonder that of the thousands of young trees planted on urban sites, few do well. Tree roots grow indiscriminately in the soil, searching out water, oxygen and nutrients. On typical urban sites as described above, the majority of tree roots can be found within the first 12 inches of soil. Compare this to a normal depth ranging from four feet for trees with fibrous root systems to seven feet or more for species with tap roots if these trees are growing in parent soils.
The depth of tree roots is determined primarily by the amount of oxygen in the soil. Soils containing less than three per cent oxygen will result in root, and consequently tree, mortality. Soils containing from five to 10 per cent oxygen will allow for root elongation, whereas levels of 12 to 18 per cent oxygen are required for root initiation.
An ideal soil for most tree types is a sandy loam soil (40 per cent sand, 40 per cent silt and 20 per cent clay) containing a minimum of five per cent organic matter. Typically, this soil mix has a 50 per cent particle and 50 per cent pore space composition. As this ideal soil rarely exists, we must develop strategies to increase tree transplanting success.
The diagram of correct tree root structure as indicated in Part I of this article (see May 2000, Landscape Trades) shows roots at or near the dripline of a typical tree. Under favourable growing conditions, tree roots will easily grow to two or even three times larger than the crown's circumference. The majority of feeder or absorbing roots are contained within the outer perimeter of the root system. What this means is, when a tree is dug from even the most reputable wholesale nursery, as much as 90 per cent of the absorbing root system is removed. A typical 50 to 60 mm caliper deciduous tree generally requires a minimum of three years to rebuild its root system.
Continuous soil trenches
The most important criteria to consider when planting trees in altered areas is the soil volume available within the rooting area.
One planting method to consider, especially where the adjacent backfill is compacted subsoil, is continuous soil trenching. A continuous furrow, connecting the soil from tree to tree, is an ideal architectural specification for sidewalk or plaza planting options. Structural planting soils, which can be compacted to meet normal engineering compaction requirements and still support healthy root growth below the pavement, are now available and should be used in place of subgrade materials. These structural soils are comprised of approximately 80 per cent crushed stone (not limestone), 20 per cent clay loam and a small amount of hydrogel stabilizing agent. Even when this gravel mix is compacted, the amount of soil in the mix is not sufficient to fill all of the space between the gravel particles, allowing for better penetration of air and water. Hydrogel copolymers are used to bind the mixture. Studies of this type of structural soil have been conducted at Cornell University and have proven successful in field trials in the U.S.
Beware of typical tree planting specifications
Tree planting diagrams, usually prepared by landscape architects and used by landscape contractors, private institutions and municipalities, require amendment. The typical tree planting cross section is correct in its depth specification but rarely do they allow for a sufficiently wide planting pit. When planting on an undisturbed parent soil or, within a continuous soil trench as described above, these specifications are adequate. However, on altered and compacted sites, planting holes should be as much as five times the width of the root ball. If the quality of the fill is sufficiently poor, it should be completely removed.
Other specifications, including the Landscape Ontario Reference Guide for Developing Planting Details,(1) show the outside is to be scarified five times the width of the root ball to a depth of 12 inches. This method is acceptable only if it is practical. Most roto-tillers will not penetrate hard-packed clay soils. Mechanical vibrating sub-soilers are a good alternative if they are available.
The use of root path trenches is also acceptable. Drainage materials can be placed in radial trenches forming a "spoke" pattern from the typical tree planting pit. This method allows for better flow of air and water, as well as better root penetration.
Avoid using tree stakes if at all possible. Trees have a natural elasticity in the stem, which is compromised by staking. For bare root trees and trees in open, exposed areas, staking is justified. In this case, place a flexible guying material (not wire encased in rubber hose) as close to the base of the stem as possible. This will hold the tree straight until establishment.
DO NOT USE tree wrap. Bark on young trees photosynthesizes, as the cortex tissue contains chlorophyll. The use of tree wrap can girdle young trees and be a home for insects such as earwigs. If the use of tree wrap is required to prevent sun scalding of newly planted trees, use only during the initial winter months and be sure to remove wrapping material at the beginning of the spring season. Tree wrap will not protect young trees against frost cracks. Frost cracks are almost always initiated by an injury to the stem, usually during the handling process.
DO NOT FERTILIZE young trees at the time of planting. Spring fertilizer applications for newly planted trees deplete the carbohydrate reserves necessary for apical shoot initiation. The root system of the tree must be established, and the root/shoot ratio must be in balance before fertilizer can be applied. Fertilization in the fall after leaf abscission is recommended. At this time, fertilizer is used to rebuild the root system. Please follow instructions on the label of any commercial fertilizer.
Pruning at planting time
Only corrective pruning is required at planting times. Do not compensate root loss by pruning the crown. Even though most trees experience crown dieback after planting, it is impossible to know which branches will die or if we will see reduced shoot growth. Corrective pruning includes the removal of broken and interfering branches and double leaders, if evident.
Some nurseries place a small, painted dot on their trees to show the orientation of the stems to the sun. Sun scalding can be prevented if young trees are planted in the same directional orientation as in the nursery.
Water is the most important ingredient for the successful establishment of newly planted trees. There has been some research conducted on the use of drip vs. rain jet or oscillating watering systems. When watering at night using conventional water systems, trees become susceptible to fungal attacks. Research on drip irrigation systems has shown excellent production of fibrous root systems.
Too much and too little water are equally detrimental to tree roots. When trees and lawns are planted in close proximity, avoid over watering the lawns. The same excess water, which leaches past the shallow rooted turf can instead end up in the planting pit. Especially if the tree pit is located in compacted soils, water will be retained, resulting in tree failure.
In periods of drought, trees on dry soils require an inch of irrigation once or twice a week. For newly planted caliper trees, usually two five-gallon pails of water will suffice. Some mature trees with tap roots, such as oak, have a high initial water uptake during the spring. Others with fibrous roots such as maple and elm require water throughout the growing season.
It has been scientifically proven by Dr. Glen Lumis of the University of Guelph that wire baskets do not girdle trees roots. This author still questions the long-term effects of the galvanized wires on roots as trees mature. I, therefore, recommend removing the top part of the basket and any ropes used for tying the basket to the root collar. An ordinary bolt cutter can be used and specially designed cutters are available for those occasions when it is appropriate to plant trees in a narrow pit.
The debate over whether or not to remove fibre pots is always ongoing. It is my personal experience that the paper mache used in fibre pots does not break down readily once planted in the ground. For plants that have been grown in pots for a year or more and have therefore re-established a denser root system, remove the pots carefully by tapping the sides and by using a knife or a blade. For recently potted plants with a less stable root ball, remove the bottom of the pot before placing it in the hole and then slit the sides of the pot after placement into the hole while slowly backfilling.
Before planting caliper trees, check the root collar. Many trees in the nursery have had excess soil around the root collar from cultivation practices. When untying the top of the root ball, any excess soil should be removed. The planting height for trees is most critical. A root collar below existing grade impedes proper root growth and can cause girdling of the roots.
Buy from a reputable supplier. Check trees for handling damage. Make sure trees are properly stored between the time of digging and planting. Look for structural problems. Make sure species and varieties are true to name.
Hand- vs. machine-dug vs. bare root trees
The advent of the mechanical tree spade has made the hand-dug string ball almost non-existent. Tree spades arbitrarily cut a root ball to a pre-determined size. Unfortunately, the root system is severed in the same manner. Root systems of trees vary according to environmental conditions and competition. Root systems are also not necessarily symmetrical. The old-fashioned hand digging method was an art and experienced diggers followed the roots, ensuring large roots were not severed in the wrong location. Because of this, hand-dug balls were not always round, but were sometimes oval to egg-shaped, depending on the root system. When appropriate, consider the use of bare root trees. They are not only less expensive, but the bare root harvesting equipment generally allows for a much larger root system than is provided with a mechanical tree spade.
There are many factors to consider in choosing between spring and fall planting. Some tree species, such as magnolias, oaks, birch, willow, and red maple prefer spring only planting. Some evergreens, including white cedar and Austrian pine, are best planted in September. Fall planting is recommended for many species because it allows for root development prior to shoot growth.
With the availability of container grown trees, planting can now be accomplished in the summer months. It is important to remember, however, that any transplanted tree will experience some shock, and must be nurtured until it is reestablished.
Following proper planting procedures is more important than worrying about the time of the year. However, it has been my personal experience that a tree planted immediately after the frost has left the ground has the best chance for survival.
Staking: An idea from Europe
This method of large tree staking is commonly used throughout Holland and Germany. Staking is critical, as the tree's large crown will sway in the wind, resulting in a certain amount of shifting of the root ball until such time as the roots are more firmly established. The staking is low enough that it will not compromise the trunk's elasticity. A secondary benefit is the additional protection that is afforded the tree against mechanical damage from mowers or trimmers as well as vandalism.
Quick reference: Proper tree planting details
- On poor sites, dig a hole that is five times the width of the root ball.
- Where the tree is to be placed, dig the hole the same depth as the root ball and one and one-half times as deep outside the rootball zone.
- Use parent soil amended with organic matter (minimum of five per cent) as a planting mix. A mix of 50 per cent parent and 50 per cent amended soil is acceptable.
- Add two to four inches of amended soil to the bottom of the hole so that the root collar is elevated. This is particularly important in poorly drained soils.
- Remove the top two-thirds of the wire basket if applicable.
- Backfill the hole with a well-granulated planting mix and tamp well.
- Form a raised ridge or dike around the outside edge of the planting pit to collect rainwater and facilitate watering.
- Water thoroughly (until water percolates from the top of the planting pit).
- Mulch with a wood chip to a depth of two to four inches. Be sure that there is less than one inch of chips at or near the base of the stem.
Jack Radecki is the Supervisor of Arboriculture Services for The Mount Pleasant Group, where he has been employed for the past 22 years. A Graduate Forester from the University of Toronto (B.S.C.F.), Mr. Radecki is a registered Arboricultural Consultant with the American Society of Consulting Arborists.
(1) The Landscape Ontario Reference Guide for Developing Planting Details is available for $10 from the Landscape Ontario office, by calling 905-875-1805.