Pruning is the most common tree maintenance procedure. Although forest trees grow well with only nature's pruning, landscape trees in urban settings require a higher level of care to maintain their health and to ensure public safety. In cities, a well-structured tree trained by regular pruning generally resists damage from storms far better than one left to grow freely. In addition, structural problems are more easily prevented and cost less to manage for trees that are trained while young. Public trees are not pruned solely for aesthetic reasons.
Trees, as living organisms, continuously experience new growth while other branches die off. Regular and systemic pruning, as a good preventive measure, enables the elimination of weak and dead branches before they break. By improving public safety and reducing potential claims against the City for damage to private assets, consistent maintenance becomes a municipal responsibility carried out on behalf of citizens. If the City were only to act on requests or complaints, some serious structural and safety problems, often imperceptible, risk being left undetected for too long.
An interval of three years between prunings appears to be a cost-effective compromise between safety criteria and maintenance needs. Extending this period even to five years could have disastrous consequences; it is generally observed that any dead branch 10-15 cm in diameter or more within the crown of a tree will fall to the ground by itself after a maximum of 5 years.
The Public Works Department
maintains an up-to-date database of all City-owned trees. The City is divided into three large sectors and all public trees are inspected on a three-year rotation. City personnel note all security or structural concerns for a given tree and refer the work to a licensed arborist to be completed. In addition, where residents express specific concerns, the City representative will make an assessment and send an arborist to complete the work.
To help young trees develop properly long-term, pruning should take place two or three times during the first eight years after planting. Pruning young trees will:
• encourage the development of a single straight trunk;
• favour a strong leader (dominant upright stem);
• favour growth at the top of the tree’s crown;
• eliminate weak, interfering and dead branches;
• space and spread the future structural branches of the crown;
• progressively clear the trunk of lower branches
A tree is called ‘mature’ when it is between 20 to 60 years old. Pruning mature trees consists of partially or completely trimming secondary branches and removing dead, weak or interfering branches while maintaining the fundamental shape of the tree.
The pruning of mature trees is also done when necessary to respect a minimum clearance for private and public infrastructures. This includes the clearing of public rights-of-way essential for the visibility of street lamps, traffic lights, stop signs and other signage, as well as trimming branches next to buildings and other infrastructures as needed (e.g., walls, roofs, utility wires, etc.). The generally-accepted standards for clearance tolerances for tree branches are:
• buildings (including roofs) 3 metres
• clearance above streets 4.25 metres
• clearance above sidewalks 3 metres
Excessive thinning of a tree’s crown (i.e., the removal of more than a third of its branch volume) and topping (i.e., removing large branches from the top of the canopy) are not accepted practices in arboriculture and are not performed by the City, as they negatively affect the shape and/or natural appearance of the crown.
Westmount’s approach to tree pruning – timely, gentle interventions limited to essential work – encourages the growth of well-structured trunks and branches, prolongs the life of the trees and keeps maintenance costs low. Training a young tree is far more cost-effective than repairing a mature tree or any tree damaged by a critical weather event.
Trees more than 60 years old are called ‘historic’ or very mature. The pruning of these trees is strictly limited to safety pruning – the removal of branches that are dead, sick, dangerous, weak or interfering.
Because older trees are particularly vulnerable to damage from extreme weather events, assessments are carried out as needed following exceptional storms or other unusual weather conditions.
The felling of a public tree, whether on a street or public green space, is not carried out in Westmount unless that tree poses a threat to public safety. A tree will be deemed dangerous only when the use of alternative arboricultural interventions, such as pruning the problem area or cabling branches, fails to secure the safety of the tree.
The use of measures other than pruning can ensure the good health and preservation of trees. The most common type of arboricultural work involves the cabling and/or bracing to compensate for structural defects (e.g. repairing the main fork of a weak or open branch), or even fertilization to improve the tree’s health and increase its resistance to threats such as parasites.
For trees with structural concerns, a forestry engineer is hired to perform an in-depth inspection, including core sampling, to ensure that the structural stability of the tree is not compromised. The engineer then sends the City an extensive report, complete with the core analysis and recommendations to improve the safety and prolong the life of a given tree.
In cases where the tree has deep decay or cannot otherwise be repaired, the resident will be contacted prior to the tree’s removal and a new tree will be planted in its place the following spring.
In cases where the core of the tree is stable but where branches need extra support, a flexible cable is installed, thus redistributing the weight and extending the life of the tree.
Many trees in the City of Westmount have a rich and extensive history. These trees are part of our heritage and treasures to preserve, especially in an urban environment. All measures are taken to protect and care for these historic trees for as long as it is safely possible. With this in mind, City trees are pruned for health and safety purposes only.
If a tree on your property requires maintenance, you should hire a private arborist to perform the necessary work. It is strongly recommended that you consider hiring a recognized member of the International Society of Arboriculture (in Québec: la Société internationale d’arboriculture du Québec; www.siaq.org).
If the tree is City-owned and you have concerns, please call Public Works to ask that the tree be assessed. If the tree is deemed dangerous, a City-hired arborist will perform the necessary work.
No private contractor or resident is permitted to perform any type of work on a City-owned tree. This is to assure that the work history is documented properly in our records as well as to assure the quality of the work performed conforms to the International Society of Arboriculture (I.S.A.) standards.
If you are not sure whether a given tree is private or public, please contact Public Works at 514 989-5213.
Friday, March 11, 2011