Urban Tree Selection

Selecting the right tree for the right place will save time, money, and later disappointments. As with any major investment, it is crucial to take the time to investigate and invest wisely.

(Gilman E.F. 1997). Further, Neely (1988) comments on the public awareness of the benefits of trees in the landscape. He cites a US survey of public attitudes that found 99% believed trees were an asset.

Most people enjoy trees for the sheer delight of having them in their every-day surroundings as they stimulate the senses and give peace to the weary mind. To mention a few; street trees purify the air, provide shade from the sun, reduce traffic noise and attract wild life. (Capon B. 1990).

There are many factors that need to be considered to make a successful selection for trees to be planted in the urban environment:
Initial species selection
Site conditions
- Planting requirements

Initial Species Selection
Selecting trees that are adapted to their intended site and will fulfill their landscape function. For example providing shade, screening areas and softening the harsh appearance of our suburban landscape. A person cannot over look the importance of matching the inherent qualities of a plant with the characteristics of the site.

Often due to the limitations of altering existing site conditions, the selection of a species adapted to those conditions, is the critical step to ensure the future growth and vigor of the plant. (Harris Clarke and Matheny 1999)

One of the best ways to evaluate how a tree will perform, is to observe mature specimens in surrounding streets or parks. There is no perfect tree. Selection is a comprise among the proposed function of the plant, its adaptation to the site, and the amount of care it will require. (Harris Clarke and Matheny 1999)

When making a species selection there are many things one needs to consider. For example:
Growth habit
Plant form
Is the requirement for a spreading tree or is upright growth required?
The rate of growth, will it out grow its environment to quickly and cause a maintenance problem?
Is it long lived?
Will environmental factors effect how long it will live?
Does it get diseases or pests that will cause an on going maintenance cost?
Will it produce flowers fruits or seeds that will cause a problem for pedestrians, vehicles and or may be toxic to humans and animals?
Will leaves break down quickly or does it have large leaves that may cause problems with drains?
Does the tree produce thorns or prickles that could cause problems?

When people consider their species selection it is all very well to consider the points mentioned above, but the most crucial factor to consider is; has it got the ability to grow at the selected site? A specific planting site must be evaluated to determine the cultural and physical attributes required of trees at the site.

Site Conditions
Tree selection does not end with choosing the appropriate species or cultivars for the planting site. Quality is the key, if you start with a quality product, it makes growing of the tree much simpler. This helps to minimize the maintenance program that is required to establish the tree, in relation to after care, for example the amount of formative pruning that is required or how often a tree will require water after planting. (No data viewed but an observation made by myself over many years) Nursery stock must be inspected carefully to the pick the healthy(the health of a tree is characterized by its vigor and freedom from injury and pests) (Harris, Clark and Matheny 1999) quality stock.

Points to consider when purchasing a tree are:
- Tree to be straight and have uniformity in the canopy as possible, with trunk calliper decreasing in size form the bottom to the top in a uniform manner. (This is called trunk taper which is the decrease in trunk caliper with increasing height)

- In the case of ornamentals the bud or graft union should be strong and healthy looking with wound closure or callusing accruing.

- The tree should be self supporting in the container the need for a stake to the hold the tree upright not required.

- Presence of included bark, trunk form and branch arrangement and attachment should be checked.

- Old pruning cuts to be checked for healthy wound closure.

- Presence of pests, disease and any injury to the bark caused by stakes, plant labels and or damaged caused in handling as this can cause a slower growth rate or greater transplant shock.

- Leaf colour and the presence of any die back in the canopy and branches.

- Check the root system for any deformities for example “J” roots, root circling, kinked roots and extremely pot bound trees. Root and shoot quality can determine not only performance, but also even survival. (Harris, Clark and Matheny 1999)

Root ball defects can occur on all trees, regardless of the method of production. Once formed, severe defects on the main roots close to the trunk are difficult to correct, and they can have a significant impact on the ability of trees to survive and grow. (Harris 1992)

Cultural Preference
A number of studies have shown that people’s preferences for trees vary. Many municipalities have large numbers of diverse cultures and nationalities, Hume City Council fits into this category. Unfortunately they have their share of wanton vandalism of street trees throughout the municipality of its street trees.

Campbellfield, one of Hume’s suburbs, had a large number of newly planted street trees being pulled out and disappearing for no apparent reason. It was noticed that many residents planted their own street trees with a strong preference to the genus Olea europaea (European Olive). These trees were doing extremely well because the residents were nurturing and watering them. As a result, Hume’s Parks Dept trialed a street tree theme Olea europaea ‘Tolleys Upright). This smaller variety produces little or no fruit. So far the decision has worked well, all trees are still intact and the Parks Dept has not received any complaints.

Planting Theme

Many commercial landscape designs call for a number of uniform trees of the same species planted in rows, or in other formal arrangements. Modern trends are leaning toward cultivated varieties (cultivars) of popular species for street tree planting for their uniform size and canopy habit, and of late, their availability (Gilman E.F. 1997). Flannery (2002) argues that in the past, exotics were often chosen because their biology and aesthetic performance in urban environments were well understood. However, in good horticulture and landscape design, there should be no place for simplistic dogmatism.

Hume City Council has adopted both of these principles in it’s new street tree policy, it quotes using single species plantings in streets to provide unity in the landscape, highlighting also to explore the use of both exotic and Australian species to emphasise the existing landscape. (Hume City Council. Street Tree Policy, Draft,2004)

Climatic factors include temperature, radiation, light, rainfall, humidity and wind. Given a particular site, climate is a key factor in deciding which plants are best adapted. In fact Harris goes to the extent of stating in his book that climate is more important in determining the growth and well being of plants than soil. However I could argue this point that it is easier to overcome lack of rainfall than improving the health of a tree planted into severely compacted soils or poorly drained soils.

With urban development comes a modification in weather patterns due to radiation from buildings, footpaths and roads. Less effective rainfall due to drains paths and roadways. Increase in wind velocity bought about by wind tunnel effects from the buildings.

When selecting a tree for the urban environment not only must you consider what are its normal tolerances to climatic conditions but also can the tree cope with the extremes in weather it may have to endure in the urban environment.

Service & Utilities

Little trees do grow to become big trees. Planting large species under power lines are major starting points for later mutilation. To avoid later problems if space is limited, dwarf or smaller species must be planted, (Shigo, A.L.1991) requiring little or no pruning, (except for crown lifting), this saves time and money. Power line management strategies for existing problem trees under power lines should include removal and replacement programmes with a better suited species, alternative conductors such as (aerial bundled cable), relocation of lines (including underground), or the use of taller poles. (Fakes, J. (2000).

The Victorian Government Code of Practice Act (1999) states the tree clearance distances from all live voltage conductors, all local government Councils and arborists are, or should be aware of this, planting the correct species will save time, money and above all put tree workers under less danger of electrocution.

Under ground utilities such as electricity, water, gas, and sewerage pipes are also an important consideration when planting new trees. Tree roots and under ground services often co-exist without problems if research is achieved before planting, visual identification of gas and water meters on houses can be achieved when marking positions for new street trees. The biggest danger to under ground services occurs during planting; never assume the utilities are buried deeper than you plan to dig.

Nature Strip Width

Tree related damage to street infrastructure, particularly concrete footpaths and curbing, is common with many councils. Yau and Krause (1996) state that many of Melbourne’s local councils spend hundreds of thousands of dollars each year on tree root cutting, footpath repair and the installation of root barriers.

Removal of the tree has been the approached by many municipalities because, in most cases the tree roots are to blame for the cracking and lifting of footpaths and curbs. It is equally valid to point out that the structures have not been engineered properly to withstand growing trees and their root systems. (Frank. F. 2003 Urban Tree Planting Seminar. 2003).

Alex L Shigo (1991) states; giving trees at least 2.46 metres between sidewalk and street will result in very few sidewalk cracking problems. If space is limited choose smaller species and or low invasive root systems.

Soil Conditions

Soil provides plants with water, nutrients, and root anchorage. Plants growing in soils that allow adequate root development and provide optimum moisture, aeration and nutrients have the potential to grow vigorously and maintain good health. (Harris, Clark and Matheny 1999)

The performance of plants in the landscape depends on how well the species can adapt to the environment in which they grow. Apart from the quality of the planting stock, planting methods and on going maintenance after planting, soils and soil conditions have a large impact on how a tree will survive in the landscape.

Soil compaction reduces the amount water that will penetrate into the root ball, this can affect drainage and aeration of the soil. Has an effect on root penetration which effects root development of a newly planted tree.

Points to consider when looking at the planting site in relation to the soil:
- Is the area compacted?
- Will it have to be improved to allow root penetration?
- Drainage, is it good does it need improving?
- Water holding capacity of the soil
- Nutrient holding capacity is it deficient in any areas
- What is the pH level?
- Is salinity a problem?
- Is there a high water table?

Each of the above soil conditions can cause problem and has a different solution, as most are best solved before planting it is important to conduct a thorough site analysis to identify problems and fix them before planting.


Leaves are the part of the plant most likely to show symptoms of air pollution injury first, before any other parts of the plant. For example bark damage, or a general decline in overall health of the tree is noticed.

Symptoms of acute injury can be highly variable, depending on the plant species and stage of growth, the type and concentration of pollutants, the length of exposure, the amount of moisture in the leaves, humidity, light, temperature, wind and other factors. (Scott, 1973)

Just as tree health can be affected by air pollution, trees can also improve air quality by absorbing gaseous pollution through stomata. Plants can further enhance air quality by having the effect of lowering air temperature in inner city areas.

Even though carbon dioxide, is absorbed by plants from the air, and oxygen is released. There is a view that plants however have little effect on the carbon dioxide and oxygen levels within a city area. (Weidensaul 1973) States that it is not accurate to say that land plants really play a significant role in maintaining the concentration of oxygen and carbon dioxide in the atmosphere.

In Australian capital cities air pollution is not the problem it is in many American and European cities, and the effects of air pollution on trees is not a big consideration when making a species selection.

Planting Requirements


It is evident that simply making a species selection, and then planting that selection into the urban environment, is not the best arboricultural practice. To achieve the best results in relation to enhancing the landscape, providing a tree with good health and vigor for years to come, many criteria have to be considered. After all not only do trees contribute monetarily value to our landscape and property, they also contribute to a higher quality of life and benefit society.

Evaluate the site, take into account all the environmental conditions & consider what can be improved. For example:

- Soil conditions and drainage.

- Take into account services and utilities, which may limit how large the species can grow or how deep or wide the planting hole can be dug. This may determine the size of the tree planted.

- Look at the current theme in the landscape and consult with residents, advise them on the reason for a curtain species selection. By getting them involved may help with the on going maintenance and help prevent vandalism.

When a thorough evaluation has been carried out this can be related back to which species are best suited for the conditions and only then can an informed decision be made on a species selection. Once the decision has been made the next most important decision must be made and that is to make sure the correct planting method is carried out. Remember it is better to put a $100 tree in a $200 hole than a $200 tree into a $100 hole.