Tree and forest health

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Tree pruning

The City owns over 330,000 street and park trees and works hard to keep them healthy.

Why does the City prune trees?

  • to promote good health
  • insects and disease control  
  • remove potential safety hazards
  • for vehicle and pedestrian clearances
  • to reduce storm damage from high winds, snow, and freezing rain
  • for streetlights, buildings, and utilities

When and how are trees pruned?

The City prunes trees routinely according to species, age and, in some cases, location and uses different types of pruning:

  • Crown cleaning is the removal of dead, dying, diseased, crowded, weakly attached and low-vigour branches from the crown of a tree.
  • Crown thinning is the selective removal of branches to increase light penetration and air movement through the crown. Thinning opens the foliage of a tree, reduces weight on heavy limbs, reduces water intake, and helps retain the tree's natural shape.
  • Crown raising removes the lower branches from a tree in order to provide clearance for buildings, vehicles, pedestrians and sight lines
  • Crown reduction reduces the size of a tree, to make room for utility lines. Reducing the height or spread of a tree is best accomplished by pruning back the leaders and branch terminals to lateral branches that are large enough to assume the terminal roles. This method maintains the form and structural integrity of the tree.
  • Crown restoration removes damaged limbs to restore an appropriate stable form to the tree. This is often necessary following storm damage.

Why does the tree look so different after pruning?

After being pruned, a tree might look rather bare. It will begin to look normal during the next growing season, with a healthier and more attractive form and structure.

Will the City prune my trees?

The City will only prune trees on City property, such as the City owned portion of the roadway, in front and at the side of residential lots and City parks.

How you can help:

  • Water the tree during dry spells (see Watering Your Tree)
  • Monitor the tree's condition and reporting to the City any disease, damage or infestations
  • Limit construction near the roots of the tree – at least 10 centimetres away from the trunk for every centimetre of trunk diameter. (see the City of Ottawa's Tree Protection Guidelines)
  • Many herbicides or weed killers that are used on grass can cause severe damage to trees when misapplied. This can occur on windy days, causing the drift to fall on non-target plants, or on hot days when the herbicide may vaporize and diffuse into the air. While most herbicides do not kill tree roots, some chemicals, such as soil sterilants, will have a detrimental effect on growth. Herbicides that can cause tree damage should have statements on their labels warning against using the product near trees.
  • Keep in mind, the property values of landscaped homes are 5 to 20 per cent higher than those without plants.

Should you require additional information on tree pruning, please call 3-1-1.

Watering your tree

Newly planted trees

Newly planted trees require a regular supply of water to survive since they do not have a network of roots and are less able to absorb water. Always water the hole before planting and water regularly.

During dry periods, use a soaker or drip hose. The best way to ensure that the water reaches the roots is to maintain the earth ring or saucer around your tree. Place the soaker hose in the earth and water for two hours twice a week. If it rains for two or more days, watering is unnecessary.

Watering should be done in the morning before the heat of the day. If the water is pooling or running off to surrounding areas, either the flow rate is too high or the ground is saturated and you should stop watering. Use a hose water timer to ensure that you don't over water.

Mature trees

A tree that has been planted for 15 years or more may appear to be able to survive without your help. Its root system can reach water and nutrients even when surface conditions appear very dry. Trees of all ages suffer when there is a drought. Water does not reach trees planted in areas such as sidewalks, patios, or raised lawns where water naturally drains away. Be sure to pay extra attention to these trees. Extra water to all trees during a drought can prevent pests or disease.

Trees near foundations

During very dry weather, soil particles will “lock up” water molecules at a threshold level. This is especially true of clay soil particles, which have very strong electrical charges and hold water very tightly. This is the beginning of a battle between soil particles and roots for water uptake. Clay soils will actually shrink in volume due to water loss, which reduces the soil’s capacity to support adjacent structures such as foundations. Foundation damage from unstable clay soils may be avoided by ensuring that trees close to foundations are always well irrigated. Again, the overnight trickle from a garden hose will help maintain the water balance necessary to keep clay based soils stable. During drought conditions, water overnight weekly. Should you require additional information, please call 3-1-1.

Protecting trees

The primary purpose of the City of Ottawa’s Tree Protection By-law is to ensure that trees are protected from injury or destruction. The by-law identifies guidelines to follow when working around trees since trees can be seriously injured if their roots are compacted, cut or damaged.

The Tree Protection By-law requires that anyone working near protected trees must, unless otherwise authorized by the City:

  • Erect a 1.2 m high fence around the outer edge of the critical root zone (CRZ) of trees prior to beginning other site work, and maintain the fence until the work is complete (see the City’s Tree Protection Specification [ PDF 377KB ] for more information)
  • Not place any material or equipment within the CRZ of the tree
  • Not raise or lower the existing grade within the CRZ of a tree
  • Not extend any hard surface or significantly change landscaping within the CRZ of a tree
  • Not attach any signs, notices or posters to any tree, except as required by this by-law for trees to be removed
  • Not damage the root system, trunk or branches of any tree
  • Ensure that exhaust fumes from equipment are not directed towards any tree's canopy

The critical root zone (CRZ) is established as being 10 centimetres from the trunk of a tree for every centimetre of trunk diameter. The trunk diameter is measured at a height of 1.3 metres for trees of 15 centimetres diameter and greater and at a height of 0.3 metres for trees of less than 15 centimetres diameter.

It is an offence under the Tree Protection By-law to fail to adequately protect a tree that has not been approved for removal.