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Vegetation Management

Image: a Hydro One bucket truck assisting forestry workers in a forest
Tree contacts account for more than 30 per cent of power outages in Ontario and can also cause fires and property damage. That's why Hydro One takes tree clearance requirements for power lines seriously and invests time and money to make sure that our more than 100,000 circuit-kilometres of lines are kept clear and safe.

Our Urban Forestry program

Cities are the largest consumers of electricity. Factories, homes, high rise buildings, malls, office towers and arenas all consume large amounts of electricity. Electricity is supplied to cities by high voltage transmission lines and lower voltage distribution lines. The transmission lines bring large amounts of electricity to a transformer station and then lower voltage distribution lines deliver this electricity to individual homes and businesses. Hydro One owns about 29,000 kilometres of transmission line and 122,000 kilometres of distribution line across the province. Like other North American utilities, we're obligated to maintain our rights-of-way to comply with North American Electricity Reliability Corporation (NERC) standards, as well as standards established by the company. Prudent maintenance of our assets, including managing vegetation on our rights-of-way, is a vital part of providing Ontario with the electricity we all depend on.

Compatible vegetation

Compatible vegetation are shrubs and other low growing plants that, at maturity, will never grow to a height that will interfere with the safe operation of the power line. The variety of compatible species will vary based on the individual line voltage and the area of the province. Most Hydro One rights-of-way support a natural ground cover of grasses, shrubs and ferns. Hydro One’s brush control programs are designed to control the regeneration and re-establishment of compatible vegetation amongst the ground cover. By selectively removing trees we are encouraging a more robust and species diverse ground cover that will never threaten the overhead lines and will reduce future vegetation maintenance activities. Hydro One will also consider including the control of non-native invasive plant species as part of the brush control program. This effort is usually done in cooperation with local conservation authorities and municipalities.

You can help 

Before you plant trees or shrubs on your property, locate overhead and underground power lines, check the maximum growing height of the species you want to plant and plan the proper planting location. Together we can work toward reliable electricity service that co-exists with the beauty and value of trees.

Download ourRight Tree, Right Placebrochure.
Download ourRight Tree, Right Placeposter (4.3 MB).

Incompatible vegetation

Incompatible vegetation includes any plant or tree species that, at maturity, could grow within the clearance area required by NERC standards. Hydro One removes incompatible vegetation from its rights-of-way with period maintenance programs using a general target of six to eight-year cycles depending on the location within the province and local conditions. This involves using an integrated approach to remove incompatible vegetation from the right-of-way.

Using an integrated approach

Hydro uses an integrated approach to control the incompatible vegetation on its rights-of-way and to promote the growth of compatible vegetation. The integrated approach could involve the following:

  • Selectively removing trees and brush identified as incompatible from the right-of-way.
  • Selectively applying federally and provincially approved low-volume herbicide, if agreed to by the landowner, to prevent re-growth.
  • To provide clearance to trees growing to the side of the line, branches may be trimmed or pruned back to meet NERC standards. Proper arboricultural techniques are used to provide the required clearance while ensuring the branches are supported by the remaining foliage of the tree.
  • Removing vegetation that could interfere with accessing the right-of-way in emergencies.
  • Hydro One may remove trees growing to the side of the line if the tree could fall and damage the transmission line or if the health of the tree due to location, weakness or leans could interfere with the safe delivery of electricity.
  • In areas where vegetation has become too dense, mechanical removal may be necessary. In these circumstances, Hydro One will clear the right-of-way and re-seed the area with a compatible ground cover mixture.

Image: Transmission corridor, or Right-of-Way TREES CAUSED THE AUGUST 14, 2003 BLACKOUT: A tree on a right-of-way in Ohio came into contact with a power line and triggered the blackout of August 2003. The North American Electricity Reliability Corporation (NERC) investigation* following the blackout found that if the company that owned the line had maintained its right-of-way, the blackout would have been prevented. As a result of the blackout investigation, compliance to international standards is now a mandatory requirement for all North American utilities. Available from the North American Electricity Reliability Corporation’s website at www.nerc.com.

Selective use of herbicide

Cutting many species of plants can have the adverse effect of actually encouraging the plant to grow from the well established root system. Cutting a single stem of poplar or birch brush can cause many stems to sprout up from the established root system and actually increase the brush density over time.

To break this re-growth cycle, Hydro One may selectively treat the cut stems of plants with a lowvolume direct application herbicide that is federally and provincially approved. Hydro One forestry professionals are well trained and IPM certified to selectively apply herbicide.

Image: disposal of debris using a wood chipper Disposal of debris

Hydro One will clean up branches and leaves debris in most areas. In areas of limited access due to rough terrain or in less populated or rural areas, all debris will be left on site to promote re-forestation and soil health and provide habitat for animals. Any wood generated from tree removal is left onsite unless in an area regulated by the Canadian Food Inspection Agency due to insect infestation. In these areas, Hydro One will dispose of the wood to prevent the spread of the infestation.


What are minimum clearances?

Hydro One requires a minimum clearance between  the power line and any vegetation. The minimum clearance is regulated by the Occupational Health and Safety Act (OHSA), and the Electrical Utility Safety Authority (EUSA). There are a number of very important factors used to determine proper clearances to sustain the power line between maintenance programs. The minimum clearance must be taken from the point of maximum sag and needs to allow for plant growth over the general target of six to eight-year cycles depending on the location within the province and local conditions. Maximum sag is the lowest possible distance the power line can drop. On a warm summer day, the line will sag more than on a cooler winter day, making the centre of the line much closer to the ground. The line will also sag more as the electricity load increases. A highly loaded transmission line in the summer can sag many metres more than the same line carrying the same amount of electricity in winter.

Diagram: Even at maximum sag, a 230 kV line must be 4.5 metres from the nearest vegetation.
  Maximum sag is measured at maximum load on a very  hot day. Even at maximum sag, a 230 kV line
must be 4.5 metres from the nearest vegetation.

Notifying affected landowners

Before any work occurs on a section of transmission line, a Hydro One forestry technician will assess the area and determine the work that needs to be done. The technician will notify any affected landowner that will require tree trimming, tree removals, brush removal, herbicide application or access on their property. In cases with absentee landowners, every effort is made to contact the owners or their representatives. If you have any questions about which forestry practices will be implemented on or near your property, or why this important vegetation management work needs to occur, please contact your local forestry technician.

Learn more

Watch the video below (YouTube) to learn more about Hydro One's Forestry department and vegetation management practices.

Click to watch our video "Forestry" (YouTube)

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