By Jack Radecki


Nursery trade lists now contain a diverse selection of trees. Other useful information contained in these lists include hardiness zones, ultimate height and/or spread and plant descriptions. It is usually from these lists that the industry makes its decisions on tree selection. This is certainly a good start, but in order to know more about the trees we are planting, there is a need to learn more about each species and the environment in which they are being placed. This information can be attained only through education and experience. The more we learn about the trees we plant and the more we watch them grow, monitoring their response to the environment, the more successful our plantings will be.

     The two most common mistakes are in the design of plantings and the use of monocultures. Too often, our landscape designs are developed for the present, with little regard for the future. We should not be planting large-growing deciduous trees at six-metre centres if their potential growth is 20 metres spread or height. Monoculture plantings have proven to be detrimental to some species of trees as insect and disease problems can become uncontrollable because the rate of spread is facilitated by the close proximity of the same species.

     Many nurseries and consumer groups are advocating the use of native trees. This is an excellent concept. However, if we are to be successful, we must also provide an appropriate site for these trees. Areas altered by clearing, grading and filling may not support some native trees even if they were there in the past. We must be careful to provide the same environment as required.

     Some introduced species such as Norway maple and Siberian elm can be considered invasive and should not be used near natural areas. Native understory trees are being out-competed at an alarming rate, especially in municipal woodlot areas.

     In order to understand site and species matching, a list of criteria for tree selection is required. Even more important than growth and aesthetic considerations are the environmental factors that can stress the tree.


Climate and temperature
The cold hardiness zone is not only determined by the average low temperature, but also by the amount of snow, rainfall and wind for that geographic area. Microclimates are areas protected from drying winds and fluctuating temperatures. Planting in areas established by trees and structures can modify the climate to allow for the usage of less hardy trees.

     Some tree species and especially thin-barked trees are more susceptible to frost and winter sun damage. Evergreens are more susceptible to desiccation of foliage from wind or sun as they transpire in the winter.


Moisture
The water table of the planting site must be deter­mined. Too much or too little water are both detrimental to tree health. Some trees, such as red maple (Acer rubrum) and pin oak (Quercus palustris) tolerate excessive moisture better than trees such as sugar maple (Acer saccharum) and red oak (Quecrus rubra). Drought resistance is more evident in those trees with coarse or tap roots. Shallow-rooted trees, such as maples and elms do poorly in drought conditions.


Air
It is important to remember that tree roots respire and therefore require air. However, air in the form of wind can desiccate and cause damage to trees in storms.


Light
All trees require some light to grow (photosynthesize). Some trees require as little as two hours of light each day, while others have a six-hour minimum requirement. Most of our planted trees are shaded by other trees or buildings. Most nursery trade lists will categorize trees by their light requirements of full sun or partial shade tolerances.

     When pioneers cut the large, shade-intolerant white pine (Pinus strobus) to build their homes, the understory of sugar maple (Acer saccaharum), Beech (Fagus grandifolia), yellow birch (Betula alleghaniensis) and Hemlock (Tsuga canadensis) took over to form the Great Lakes-St. Lawrence forest.


Soil
The texture of soils is very important. Trees prefer a combination of sand, silt and clay (e.g. 40-40-20). A silt loam is best. Five per cent of the soil should contain organic matter. The poorest site for trees is in developed areas where the topsoil has been removed. Trees planted in construction fill and compacted soils must grow in this poor media and should have at least a 50 per cent mixture of amended soil. (This subject will be discussed in more depth later in this series.)

     Soil tests may be required to determine availability of macro and micronutrients. It is especially important to know the cation exchange capacity and pH of the soil. Most species prefer a slightly acidic soil. Some trees, such as oak and red maples perform poorly on alkaline soils. As tree roots require air, many compacted soils require aeration.


Size
Always take into account the ultimate height of spread of the tree to be planted. Tree size can vary from five to 30 m.


Form
With the introduction of many new cultivars, we now have available globe, pyramidal, columnar and weeping tree forms. Most trees, if given the space, will grow as wide as they are tall. Some trees, such as ash (Fraxinus) and hickory (Carya) grow taller rather than wide.

     The texture or thickness of branches as well as their arrangement (opposite vs. alternate) should also be considered.


Growth habits
The rate of growth or annual increment should be considered. Other growth habits, which should influence your tree selection include invasive roots, prolific seeding and poor branching structure. Trees with double leaders and/or V-crotches are very susceptible to failure from snow and ice loading.


Competition
There can be both root and canopy competition among trees. Roots of trees will intertwine while searching for air, water and nutrients. Some trees, such as black walnut (Juglans nigra) release hormones from their roots, preventing other woody plant species from growing within that root zone. This is known as allelopathy.


Location and space
Most large trees require they be planted on centres of 20 m or more. With the introduction of new varieties such as Ivory Silk tree lilac (Syringa reticulata 'Ivory Silk') and the reintroduction of older varieties such as Callery pear and crabapple varieties, it is possible to plant at six-metre spacings. If a mass or forest effect is desired, the spacing for larger trees can be reduced. Evergreens are also effectively planted in masses or groupings. Three-metre spacings are common, but five-metre spacings are preferable.

     When planting in the city, consider the distance from buildings, wires, sidewalks and sewers. Tree roots will grow under asphalt as it is a surface with some porosity. Most tree root problems occur in sewers, weeping tiles, and foundations.


Aesthetic value
Trees are not only valued for their size and shape, but flowers, seeds, fruits, bark, fall colour and even spring and summer leaf colour can also be determining factors. Some trees are chosen because they attract birds and wildlife. Some grow to be outstanding specimens, while others have historical value.


Age
A peach tree usually lives no longer than five years. The oldest white oaks in Oakville, Ontario are over 300 years old.


Landscape practices
Trees are used as windbreaks, boundary plantings, and street or boulevard trees. They are used in groupings or as single specimens. When we in­cor­porate a tree into a flowerbed, we can injure the roots of the tree when we dig and roto-till these beds.


Transplantability
Nurseries rarely grow species such as hickory because the long tap root makes it very difficult to transplant successfully. Still, other species have proven to transplant successfully in the spring season only.


Maintenance requirements
Some species may have higher maintenance requirements than others due to their susceptibility to insect and disease problems. Poor branch formation, brittle wood, annual pruning requirements, nutrient and pH requirements and drought resistance will all add to a tree's overall maintenance requirements.


Environmental problems
In the city, trees must be able to contend with pollution, salt spray1 and acid rain. Ground level ozone, along with compaction, has been killing the sugar maple (Acer saccharum) population. Al­though attracting birds and animals is usually seen as a desirable characteristic, porcupines, bears, wood­peckers and sapsuckers can cause damage to trees.


Species
A decision must be made whether to use native species, introduced species or a combination of the two. There is also a choice between evergreen and deciduous. Each has its own size, form, texture and rate of growth. Caution must be exercised when planting species collected or grown in other geographical areas. A red maple (Acer rubrum) originally grown in Tennessee will not perform as well in Ontario as a locally grown selection.


Tree root misconceptions
For thousands of years, diggers of holes for fence posts and ditches have had to contend with the rope-like roots at or near the surface of the soil. However, mature trees are huge - larger than the biggest whale. Individual leaves and roots are extremely small in relation to the whole tree. Very few people have ever had the privilege of actually seeing even a comprehensible fraction of the root system of an entire tree.

     Illustrations in textbooks, natural history books and even manuals of landscape architecture and tree care, usually creations of artistic imagination, are usually terribly incorrect as depicted in a typical diagram at left. Inaccurate illustrations such as this one have led to misconceptions and therefore harmful practices in the management of trees in both forest and urban situations.


From: Tree Roots, by Dr. Roger C. Funk, Davey Tree Inc.


Jack Radecki is the Supervisor of Arboriculture Services for The Mount Pleasant Group, where he has been employed for the past 22 years. Experienced in all maintenance aspects of trees and with extensive knowledge of nurseries, landscape design and planting, Mr. Radecki is an expert in tree identification and selection. He shares his knowledge with the trade and public through seminars, contin­uing education courses at community colleges and tree study tours, as well as on a consulting basis in landscape appraisals, hazard tree evaluation, diagnostics and species/site matchings. A Graduate Forester from the University of Toronto (B.S.C.F.), Mr. Radecki is registered Arboricultural Consultant with the American Society of Consulting Arborists, director and past president of the Ontario Shade Tree Council, and member of the International Society of Arbiculture, the Seneca College Advisory Committee Environmental Landscape Management and the Sir Sandford Fleming College Advisory Committee, Urban Tree Maintenance Program.


1 A list of salt-tolerant species is available in the Landscape Ontario Guide to Tree Selection. The $10 cost of this publication covers shipping, handling and taxes. Call 905-875-1805 to order a copy.