LOCATED IN SOUTHEAST ASIA, HONG KONG'S SUBTROPICAL CONDITIONS ARE IDEAL FOR FAST-GROWING TREES AND VEGETATION, such as Eucalyptus, Casuarina and various palms and vines. This fast growth puts overhead lines at high risk of exposure to vegetation contacts that are difficult to manage.

Prior to the year 2000, CLP Power Hong Kong Ltd. (CLPP) encountered significant unplanned tree-related electric service interruptions. Trees and vegetation in contact with overhead lines also posed safety hazards to company personnel and the public, as well as fire concerns. These contacts were also affecting our customers with a direct impact on the customer minutes lost (CML) statistics.

CLPP dedicated a vast amount of resources to conducting tree and vegetation management work in an attempt to enhance supply safety and reliability. At that time, trees were pruned when inspections showed that trees or tree parts were within the defined clearance zone and directly threatened line integrity. Yet, this practice has enormous room for improvement.

Because of inadequate arboricultural knowledge, improper cutting practices were being used. This resulted in the rapid resprouting of limbs, which resulted in the need to rework the tree within a short time frame. These fast-growing resprouts were also weakly attached, so they often broke off and fell into the lines.

Finally, the workforce did not possess the adequate arboricultural knowledge and skills required to understand how and where to prune properly, and how to remove trees and limbs overhanging energized lines safely and effectively. This required that the lines be taken out of service to perform tree and vegetation management work, which significantly increased tree-related service outages. With the introduction of a vegetation management program, the first of its kind in Southeast Asia, CLPP managed to turn around the situation and upgrade its vegetation handling to a world best practice.


To effectively address the vegetation challenge, CLPP management established a CML reduction program that included a vegetation management workgroup, consisting of professionals from different functions within the company. The workgroup was established to analyze and formulate strategies to tackle tree and vegetation conflicts that resulted in electric service interruptions and human safety hazards.

The workgroup conducted research on trees and vegetation along the overhead lines for six months, analyzing thousands of fault records. The records showed that trees and vegetation affected many overhead power lines. Outages usually occurred when trees and branches contacted energized lines and equipment during strong winds and bad weather. Many locations were observed where tree branches were overhanging lines, presenting a potential hazard to the lines. Research in the United States also showed that overhanging limbs are the largest cause of tree-related outages.

The workgroup determined that several root-cause factors contributed to these vegetation conflicts and their impact on the electric-delivery system. The primary factors included:

  • Use of improper cutting techniques, which resulted in rapid resprouting, growth and contact with energized power lines relatively soon after pruning. Improper cutting techniques also resulted in poor connections of resprouts to the main stem, which caused limbs to snap off and fall onto lines and equipment, resulting in electric service interruptions and damage to equipment. Rapid regrowth and poor stem connections required more manpower resources and increased the operating costs required to identify and manage these conditions. Figure 2 shows an 11-kV line before and after tree cutting in the same location.

  • Lack of a vegetation database and management system, which prevented managers from adequately understanding the location, size, condition and work requirements for trees and vegetation, threatening lines throughout the system. This inadequacy prevented timely, cost-effective planning, scheduling, and implementation of tree and vegetation management operations.

  • Lack of experienced, knowledgeable tree experts within the company with the ability to organize and administer a vegetation management program. This hindered CLPP's ability to fully recognize the components of the problem and satisfactorily identify, design and develop a program that would cost-effectively address these challenges.

  • Lack of skilled tree workers available and employed to provide tree and vegetation management services. In-house linemen and unskilled laborers who lacked professional arboriculture knowledge and understanding of tree biology to execute safe and effective tree pruning conducted the tree pruning. Further, to safely remove overhanging limbs, power to the lines was disconnected to allow the safe pruning of these trees to clear the lines, resulting in a large number of scheduled electric service outages.

Based on these findings, the workgroup concluded that CLPP had to develop and implement a state-of-the-art tree-pruning and replant program to avoid the high incidence rate of both unplanned and scheduled electric service outages. CLPP envisioned a program that would provide a structured, technically correct approach to tree and vegetation management with the objective of cost-effectively identifying, managing and preventing conflicts about people's perceptions against tree-pruning programs. Also, the program would require properly trained, knowledgeable and skilled tree workers capable of performing technically correct work to minimize the number of scheduled outages by permitting tree and vegetation management operations to be safely conducted while the lines remained energized.


Based on the recommendations of the workgroup, CLPP management decided to develop a world-class vegetation management program in Hong Kong. CLPP became the first company in Southeast Asia and the only organization in Hong Kong to put these environmentally sound methods fully into practice.

Since the level of professional arboricultural expertise was not available in Asia, CLPP outsourced overseas and contracted with HECO Integrated Vegetation Management Services (Honolulu, Hawaii, U.S.) to support and develop the program. An expert team led by Kevin K. Eckert (now Arbor Global, LLC; Kailua, Hawaii) worked closely with CLPP's workgroup and developed a vegetation management program that included policy, procedures, technical instructions and contract specifications.

In order to resolve the cultural problems and language barriers, a local workforce from CLPP Engineering was established to support and perform the operation. As there were no knowledgeable, skilled arborists in Hong Kong, the expert team conducted training to develop a competent Hong Kong-based workforce. This training included such skills as safe chainsaw operations, tree and limb rope rigging, pruning techniques, tree felling, safe maneuvering in the tree, basic climbing techniques and aerial rescue. The crews completed the training and assessment in 60 days. This training provided workers with the basic knowledge and understanding required to further develop their skills to be able to properly prune and safely remove trees, overhanging tree limbs and vegetation while the lines remain energized.


The workgroup's recommendations were used to establish a detailed and well-developed tree and vegetation management system. The program is defined and administered by a comprehensive, technically based vegetation management policy, procedures and specification document. A service contract has been established, and the contractor's performance is reviewed regularly. Tree workers have been trained and are competent to perform safe and effective tree work. Communication channels with various government departments have been established, and the vegetation management program has been well received by regulatory agencies and other stakeholders and interested parties.

Through the application of proper pruning techniques, the health and beauty of the trees has been maintained while satisfying CLPP's safety and electric service reliability needs. The use of directional and live-line tree-pruning techniques has significantly minimized the need for planned outages. Proper pruning techniques requiring trees to be pruned according to high arboricultural standards have significantly minimized the rapid and high-volume resprouting of limbs and branches in CLPP's service area. This has permitted the revisit cycle to be extended, and reduced current and future maintenance costs. Tree replant is also practiced if necessary.

Prior to the design and implementation of this vegetation management program, many tree-related trouble calls required high levels of emergency crew resources, especially during typhoons and inclement weather. Now, after implementation of the technically correct program, tree-related customer complaints and trouble calls have virtually disappeared, resulting in improved customer service and company image.

An interesting anecdote is that when the Dujuan typhoon hit south of Taiwan, Hong Kong and the neighboring Shenzhen of China in September 2003, it caused huge damage in these three areas, with 1 million customers in Shenzhen and 500,000 customers in Taiwan losing supply. However, only 300 customers were affected in Hong Kong, and CLPP believes its proactive vegetation management program contributed to this enhanced level of supply reliability.


Although the current tree and vegetation management program has been successful in many areas, it is regularly reviewed to identify opportunities for improvement. Based on the recent vegetation management program review, the company identified items that would further enhance the program.

  • Managing with herbicides and tree-growth retardant

    Subtropical weather conditions in Hong Kong greatly favor the growth of vines, and some vines can grow 8 inches (203 mm) per day, especially during the hot and humid summer. If they are not managed effectively, vines can reach the overhead line conductors at 10 m (34 ft) above ground level in 60 days or less. One of the solutions is to manage the vines; wild shrubs and fast-growing tree species with herbicides and tree-growth regulators.

    After a careful assessment and evaluation, CLPP has initiated the use of the herbicide Glyphosate to totally control the plant and prevent resprouting near overhead lines. The tree-growth regulator Paclobutrazol is now being applied to fast-growing trees that cannot practically be removed and controlled. Paclobutrazol should minimize regrowth and extend the time and cost required to prune these trees for line clearance. Since the use of herbicides and tree-growth regulators is new to CLPP, close monitoring will be conducted to determine the cost-effectiveness of these tools.

    All tree-growth retardants must first meet stringent environmental requirements for low toxicity to humans and wildlife, rapid bio-degradation in soils and low soil mobility.

  • Refresher training

    Like successful utilities and contractors throughout the world, CLPP recognizes that regular training on safety, technical knowledge, work quality and productivity are essential ingredients for program success. Therefore, contractors are required to organize regularly scheduled and professionally managed refresher training programs to maintain an effective level of knowledge and skills that will enable them to provide the quality and productive services demanded by CLPP.

  • Contract monitoring

    The contract system currently used was developed based on an established, not-to-exceed target cost. A ceiling target cost was established for each circuit. The contractor is required to perform vegetation management in compliance with contract specifications on all locations identified in the inventory according to the schedule. If the actual time and materials costs are lower than the established target cost for the circuit, the contractor is offered a bonus or a percentage of the cost savings. Conversely, if the total contractor costs exceed the target costs, the contractor only receives the target amount. This contract system has been monitored and reviewed, and has proven to be a good approach to contract management and, with minor fine-tuning, this system will continue.

    Due to the rapid growth of trees in Hong Kong's tropical climate and CLPP's desire to balance tree health and aesthetics with supply reliability and safety, trees and vegetation are managed on a 12-month interval throughout the system, regardless of voltage level. This frequency period is subject to review.

  • Community relationship building

    Positive interactive communications with community and government stakeholders is recognized as a critical component of a successful program.

CLPP regularly conducts outreach programs to share its program's goals, objectives and initiatives with various government departments and other stakeholders. Brochures have been printed to deliver CLPP's message of safety, service reliability, tree health and aesthetic value, and the benefits of the new professional vegetation management program. Strong proactive communications have been demonstrated to be quite valuable and productive in significantly reducing complaints and creating a positive working environment with regulatory agencies and the public. This program will be further developed.


  • Improvement of supply reliability and power quality

    Significant positive results have already been realized in spite of the fact that the program has only been fully implemented for a relatively short period of time. The number of vegetation-related electric service outages has been reduced by 80% on average, when compared with measurements in the same period prior to the year 2000. This reduction is especially evident during the high-outage periods in the summer and during the typhoon season. Supply reliability and power quality have unequivocally improved as a direct result of properly managing vegetation growth around electrical power lines and reducing the number of service interruptions caused by falling limbs and trees.

    The table presents the CML statistics for the past eight years. Note that the number of vegetation-related outages on networks from 33 kV to 400 kV is insignificant due to the higher ground clearance and the use of ring-connected networks.

  • Reduction of operational costs

    The cost-effective planning, timely scheduling and technically correct management practices implemented have reduced operating costs by 30% as well as the number of field operatives used on this activity. Trees grow continually, and in Hong Kong's tropical environment, they grow fast. By designing and implementing a technique-based vegetation management program, potential tree and vegetation conflicts have been cost-effectively identified and resolved.

  • In harmony with nature

    Since the initiation of the new program, no customer complaints have been received regarding improper pruning. The application of natural, directional arboricultural pruning practices and tree replant reduces power outages caused by trees, minimizes customer complaints and enhances customer satisfaction. Through the responses of regulatory agency staff, customers and the news media, it is apparent that the new program has been well received. CLPP now recognizes that trees and overhead lines can live in harmony with each other in the environment, and that a well-designed program will balance electric supply needs with the preservation of the natural environment.

Trees add beauty and value to homes and communities. However, trees planted under or near power lines can be a safety hazard, cause unnecessary and costly power outages, and require preventative maintenance. CLPP believes it has a responsibility to deliver power safely and reliably by removing any natural plant growth that could pose danger to the power lines and public safety. A well-designed, technique-based (or technically viable) and properly administered tree and vegetation management program can cost-effectively balance the protection and requirements of the electric system with the aesthetics and health of trees near overhead lines.

The introduction and development of CLPP's vegetation management program has been an outstanding success. CLPP continues to develop and enhance the program through research and participation in professional arboricultural associations such as the International Society of Arboriculture and the Utility Arborist Association.


The authors wish to acknowledge the expert advice and support given by Kevin K. Eckert, Arbor Global, LLC; Stephen R. Cieslewiczn and Robert Novembri, CN Utility Consulting; Davey Tree Surgery; Robert L. Tate Inc.; and Belt Collins Asia.

C.H. Cheung has been with China Light & Power Co. Ltd. in various roles since 1980. In 2001, Cheung was appointed regional manager/north overseeing an overhead-line intensive region in Hong Kong and Guangdong, China, until August 2005, when he was appointed regional manager/west. Previously, he worked in Hong Kong Electric Co. Ltd. and for the Hong Kong government. Cheung holds a bachelor's degree in electrical engineering from the University of Hong Kong, as well as an MBA and a master's degree in engineering.

John K.C. Ho is the vegetation manager of CLP Power Hong Kong Ltd. For the past five years, he has focused on implementing and managing a comprehensive vegetation management program for the company. In addition, Ho now manages a mechanical pole-testing project on 60,000 poles in the company to evaluate the remaining life of the poles, and assess and formulate a strategy on pole life extension. He holds a bachelor's degree in electrical engineering from The Hong Kong Polytechnic University.

Year Data 400 kV 132 kV 33 kV 11 kV
1998 CML due to vegetation 0.0000 0.0000 0.0000 2.2039
No. of outages due to vegetation 0.0 0.0 0.0 180.0
1999 CML due to vegetation 0.0000 0.0000 0.0000 3.2194
No. of outages due to vegetation 0.0 0.0 0.0 138.0
2000 CML due to vegetation 0.0000 0.0000 0.0000 1.2317
No. of outages due to vegetation 0.0 0.0 0.0 104.0
2001 CML due to vegetation 0.0000 0.0000 0.0000 0.6591
No. of outages due to vegetation 0.0 0.0 0.0 100.0
2002 CML due to vegetation 0.0000 0.0000 0.0000 0.3744
No. of outages due to vegetation 0.0 0.0 6.0 87.0
2003 CML due to vegetation 0.0000 0.0000 0.0000 0.3623
No. of outages due to vegetation 5.0 0.0 2.0 59.0
2004 CML due to vegetation 0.0000 0.0000 0.0000 0.0990
No. of outages due to vegetation 0 0.0 3.0 36.0
2005 CML due to vegetation 0.0000 0.0000 0.0000 0.11910
No. of outages due to vegetation 0 0.0 3.0 42.0
Overhead line circuit length 400 kV 132 kV 33 kV 11 kV
Year 2005 503 km
(313 miles)
206 km
(128 miles)
115 km
(71 miles)
574 km
(357 miles)

The CML statistics for the past eight years for CLPP's 11-kV to 400-kV systems.