While safety, reliability and legal compliance are at the heart of utility vegetation management, they are not the only consideration of a utility that must maintain vegetation. Part of our business and culture is good public and customer relations, and environmental stewardship. Of course, since utility vegetation management is a $3 billion per year undertaking, companies are concerned about costs, which affect their customers’ energy bills.
This complex set of concerns means no utility can have a simple vegetation management program. Sometimes, tree pruning is required. Sometimes it is total tree removal. On the very high voltage power lines and pipelines, the brush maintenance or the control of young vegetation is best. Each situation needs to be evaluated and a specific strategy implemented.
Tree Trimming and Pruning
Utilities that employ properly trained, professional utility vegetation managers try hard not to trim trees. Trimming implies indiscriminate location of cuts. The arboriculture industry has long understood that proper pruning is best for the tree’s health and best accomplishes the utilities goals of providing for long-term service reliability – in most cases. Most utilities adhere to the ANSI 300 national pruning standards or other standards in non-North American countries, and the ISA – International Society of Arboriculture Best Management Practices for Tree Pruning.
Frequency of pruning
Most deciduous trees (non-evergreen) quickly regrow after they have been pruned. This means utilities must prune the trees over and over again, as they re-encroach upon the lines. The length of time depends on the amount of clearance obtained at the time of tree work, the rate of tree growth and the amount of tree-line clearance to be maintained at all times. This in turn depends on tree species, tree size, local site conditions and the ever changing weather from year-to-year.
Extent of pruning
A very important factor determining the frequency of tree pruning is the amount of clearance that can be obtained when the work is done – the less clearance obtained the more frequently the tree must be pruned. It may seem that obtaining less clearance more often would be healthier for the tree but that is not necessarily the case. It is a tradeoff between more, smaller cuts more frequently or fewer, larger cuts less often. There is not a universal answer and each utility should set the line clearance or tree maintenance cycle based on local conditions.
Since safety, reliability and legal compliance are the essential part of the equation, leaving a nice looking tree is often irreconcilable with quality of service. In this case, complete removal of the tree is often advised. Many utilities will apply a registered product to the cut stump to prevent it from sprouting and again becoming a maintenance problem. Many utility companies have a tree replacement program that provides vouchers that can be used to purchase proper replacement trees from local nurseries to encourage replanting.
In many circumstances, tree pruning is simply inappropriate. This is particularly the case on the very high voltage transmission lines where electricity can arc from the electric conductors to taller vegetation. Much of the vegetation on electric transmission and pipeline corridors is best maintained by converting the vegetation community that is dominated by tall growing trees to lower growing vegetation communities. This conversion and subsequent maintenance is best accomplished using an integrated vegetation management (IVM) strategy. The national standard (ANSI A300 Part 7) defines IVM as a system of managing plant communities in which managers set objectives; identify compatible and incompatible vegetation; consider action thresholds; and evaluate, select, and implement the most appropriate control method or methods to achieve those objectives. The ISA has published a Best Management Practices for IVM as a practical guide to help utility vegetation managers implement the ANSI standards in the field.
The UAA encourages utilities to consider the Wire Zone-Border Zone (WZ-BZ) strategy in its implementation of IVM. The WZ-BZ strategy recognizes that plant species that are not compatible on some portions of the right-of-way may be compatible on others. This practice creates a more diverse plant community a better habitat for many wildlife species.
Wire Zone-Border Zone (WZ-BZ). Courtesy of Environmental Consultants, Inc.