Utility performs field trials of surface materials, new methods and equipment to kill weeds, but concludes no viable alternative to herbicides.
Opposition to the use of pesticides in British Columbia, Canada, has been growing steadily in recent years. Environmental groups and the general public have increasingly expressed concerns about the use of herbicides to control weeds. In fact, the province of British Columbia introduced the Integrated Pest Management Act, which requires a decision-making process be followed to ensure herbicides are the best control option and alternatives to herbicide use have been considered.
To address these issues and determine whether herbicides are a necessary part of weed management around electrical substations, BC Hydro established a research and development program to test a variety of non-herbicide products and techniques.
Weed Threshold Level
BC Hydro's first step was to determine when it is essential to control weeds within substation boundaries. A small number of weeds can be tolerated as long as the individual plants are not too large. The vegetation management team worked closely with substation engineers to develop a threshold level for weeds. They determined weed populations should not exceed 5% coverage in the electrical compound area and weed control should be initiated to ensure the weed level never exceeds this density.
It is essential to control weeds in and around electrical facilities because they can compromise the safety of workers. A grid of bare wires buried underneath the station provides a common grounding system for electrical and metallic structures. This grid protects staff and the public from electrocution in case of a system fault, equipment failure or lightning strike by limiting the electrical potentials to safe levels. A surface of clean, crushed rock — similar to gravel — is laid over the electrical ground grid to provide an insulating layer between the grid and the surface. Weeds in the crushed rock interfere with the ground grid, reducing its ability to function as an insulating layer. This poses a hazard to electrical workers on site.
Furthermore, the presence of weeds poses an additional safety hazard to workers by increasing the risk of tripping and slipping, and can cause startle shocks when crews brush against plants. Weeds can cause power outages when they grow into electrical components and increase the rate of corrosion of steel equipment. They also can reduce the ability of workers to access equipment for maintenance and safety inspections.
In addition, the risk of unauthorized entry and theft may be increased if fences are covered or hidden. Weeds can serve as food and shelter for rodents, ants, termites and other pests, which can infest control buildings and other structures within the facility. Finally, weeds are a fire hazard because they can act as a fuel source.
Alternative Surface Materials
The layer of clean, crushed rock covering the surface at the facilities functions as a preventive barrier to the establishment of weeds. To be effective, the layer of crushed rock must be maintained to engineering standards within and around a facility. This surfacing layer must cover the site completely, because any areas of exposed mineral soil allow weeds to become established.
The rocks must be about 0.75 inches (19 mm) or smaller in size, evenly graded and layered to a depth of 6 inches (152 mm); the depth of the rock is the most important factor for weed control. In addition, the rock must be fractured on at least two faces because round rock causes tires to spin, which disturbs the crushed rock layer. Most importantly, the crushed rock must be free from sand, silt, clay and organic matter. If the grounding grid extends outside the facility, the crushed rock surfaces also should extend outside the fence line.
In its search for alternatives to crushed rock that would reduce the problem of weed growth around substations, BC Hydro designed a research program to determine the most effective surface material. Five different materials were tested at three substations: compacted limestone, organic stabilizer, recycled asphalt, grass and geotextile.
The utility expected limestone to be effective because its high pH can exclude the growth of some weeds. However, results showed limestone does not reduce weeds better than crushed rock. Furthermore, it produces a fine dust that covers electrical components and causes other problems. An organic stabilizer product that acts to bind crushed materials into a cohesive surface was used with crushed glass, limestone and 0.25-inch (6-mm) aggregate, but this product did not provide better weed control than the standard crushed rock.
A layer of geotextile, which is porous sheeting, was laid underneath the crushed rock and tests showed it could improve the effectiveness of crushed rock. This product also can be staked directly to the soil surface instead of crushed rock, but it is not practical to use over larger tracts of land. Geotextile allows drainage but prevents the growth of weeds. It works particularly well for annuals but less so for longer-lived perennials such as trees and shrubs. However, it can only be used in areas where there is limited vehicle traffic because it can be easily damaged or torn. Geotextile should not be used near oil-filled equipment because it will cause the oil to spread more readily during a spill.
Asphalt proved to be highly weed-resistant and makes an excellent driving surface. The use of asphalt is generally limited to access roads and storage areas inside facilities, or for new facilities specially designed to use asphalt. However, this surface cannot be used around oil-filled equipment because it will cause oil to spread in the event of a spill and it will burn at high temperatures.
Grass has been used successfully at some facilities to cover large undeveloped or undisturbed areas within and around the facility. Grass, which should be mowed periodically, can be effective in reducing the establishment of broadleaf weeds, many of which have rapidly spreading airborne seeds.
Of the various materials tested, crushed rock, which is the current standard, was found to be as or more effective than any of the other materials tested at preventing the establishment of weeds, provided it is free of organic matter. Crushed rock has the additional advantage of being readily available. Organic debris and weed seeds blow into or drop onto the site over time. Furthermore, the crushed rock layer becomes contaminated with debris because of vehicle traffic or construction activity.
Organic Debris Removal
To remove organic debris, BC Hydro developed and tested a tractor-pulled device called a spreader grader that tumbles crushed rock and other aggregates. The device works by inverting the top layer of crushed rock, thus shaking the organic matter underneath. Similar to how a soil tiller works, this process brings the existing rock to the surface and the organic matter, soil, seeds, leaves, weeds and twigs to the bottom.
A clean, crushed rock layer results in improved weed control over the long term. The spreader grader is effective on large to medium flat areas accessible by tractor. However, it has some limitations when under electrical equipment with height restrictions or in areas where the surface has been compacted.
The effectiveness of any weed control program is increased if the surrounding areas also are managed to prevent weeds from entering the facility. Trees and other tall-growing vegetation nearby should be removed as they may interfere with site security, act as a fuel source for fires and grow within electrical clearances, causing power outages. Hazard trees can fall on the site, damaging equipment. Trees, especially with branches hanging over the site, drop debris onto the crushed rock, which increases the amount of organic matter and encourages the establishment of weeds.
In urban areas where the facilities are landscaped, plants that do not spread rapidly are recommended. Plants spread by suckering or underground rhizomes, and vines and plants with airborne seeds should not be used. Landscaped areas should be kept free of weeds. Outside the fence line, rapidly spreading species such as blackberry, horsetail, broom and groundsel must be removed. Finally, physical barriers such as solid fences are sometimes used to reduce wind-borne seeds from entering the site.
BC Hydro agreed to eliminate the use of herbicides at a few facilities because of community concerns. However, after five years of trying hand pulling and brushing, the crushed rock surface was completely covered with weeds and the entire site had to be resurfaced with new clean, crushed rock. After subsequent negotiations with the concerned parties, BC Hydro was successful in reintroducing herbicides to control weeds at these facilities.
BC Hydro is often asked why hand pulling alone cannot control weeds. Hand pulling of weeds inside facilities degrades the crushed rock surface. Excessive hand pulling increases organic matter in the crushed rock because soil is pulled up through the rock with the weed roots. Root extraction also cultivates the soil, stimulating dormant seeds to germinate.
Hand pulling is not viable as a control measure for many deep-rooted plants because the plant can snap off at ground line. It can then regenerate from the root left in the soil. This method is only effective for larger, established weeds that can be easily uprooted. Weed seedlings and grass species are too small and numerous to remove by hand. If roots are deep enough to be in contact with the ground grid, workers pulling weeds can be at risk for electrocution. However, hand pulling is still used if there are few weeds on the site, 100 or less. Weeds also still need to be removed by hand in areas where herbicides cannot or should not be used, or for reducing bulk vegetation to a manageable level.
Justified Use of Herbicides
BC Hydro's research and development program also tested several alternatives to herbicides for the control of established weeds within electrical facilities. At several sites, equipment to burn weeds with torches and other devices was tested. Burning was effective on shallow-rooted species or small seedlings, but this method did not kill weeds with deep root systems. In addition, the risk of fire was too high for use around electrical and oil-filled equipment.
Several manufacturers approached the utility with requests to test machines that control weeds with steam or hot-water sprays. Again, this equipment was only effective on weeds with shallow root systems; weeds with deeper roots were able to survive and regrow. An additional problem was the machines required substantial amounts of water, which was not always available.
After numerous field trials of surface materials and the introduction of new methods and equipment to kill weeds, BC Hydro concluded there is currently no viable alternative to the use of herbicides for managing weeds in all situations. There are several non-herbicide methods and practices that can be used to reduce the use or reliance on herbicides, but, at this time, herbicides are an integral and necessary part of weed control programs in electrical facilities and cannot be eliminated entirely. Mechanical and physical methods can be used in conjunction with herbicides, but they are not effective by themselves in achieving an acceptable level of weed control.
In spite of all the preventive measures described in this article, it is inevitable some weeds will become established over time because organic matter builds up in the crushed rock surface. Currently, it is not possible for BC Hydro to maintain electrical facilities below the acceptable threshold for weeds without the use of herbicides.
Herbicides have been used widely in the electrical utility industry as well as in many other industries, including manufacturing and refining sites, railways, roadways and pipelines, for more than half a century. The information in this article should be useful to utilities when justifying the use of herbicides in their programs.
Gwen Shrimpton (firstname.lastname@example.org) is a British Columbia-based consultant, providing services for land management of linear corridors, specializing in environmental issues, vegetation management and First Nations negotiation. She is a registered professional forester and a registered professional biologist, and she holds a master's degree in pest management. Shrimpton worked for BC Hydro for 20 years developing vegetation management programs for the transmission system. She is the president of the Integrated Vegetation Management Association of BC and a member of the vegetation standards drafting team for NERC.