«Amenity Tree Pruning Version 3 April 2011 Page 1 of 16 Contents 1. Scope 2. Reasons for pruning 3. Pruning Procedures 4. Natural Target Pruning Cut ...»
NEW ZEALAND ARBORICULTURAL ASSOCIATION
Best Practice Guideline
Amenity Tree Pruning
Version 3 April 2011 Page 1 of 16
2. Reasons for pruning
3. Pruning Procedures
4. Natural Target Pruning
Cut to Branch Unions
Branch Bark Ridge Pruning
Removal of Limbs with Included Bark
5. Pruning Types and Classes
5.1 Crown Maintenance Dead Wooding Formative Pruning Thinning
5.2 Crown Modification Crown Reduction Crown Lifting Crown Renewal Crown Retrenchment Pollarding Juvenile Tree Pruning Palm Pruning
6. Tree Clearances Minimum Approach Distances
8. Acknowledgements Version 3 April 2011 Page 2 of 16
1. Scope These pruning guidelines set out the requirements for the maintenance of amenity trees and gives an indication of currently accepted correct arboricultural pruning practices. The intention of this document is to encourage pruning practices and procedures that reduce the risk of hazard development, branch failure, pathogen infection or premature tree death. It is intended for use on amenity trees, including palms, and does not include practices which are specific to timber, foliage, fruit and flower production, root pruning, chemical pruning or sculptural forms such as topiary, espalier, and pleaching.
These Amenity tree pruning guidelines are intended for use by tree workers wishing to comply with NZAA promoted pruning practices and should be applied in conjunction with the ANSI A300 Standard promoted by the ISA. The guidelines will also serve to assist property owners, contractors, and those undertaking contractual arrangements which specify arboricultural pruning procedures and practices.
2. Reasons for Pruning The first consideration when pruning, is whether cutting off and removing living branches will actually benefit the tree. It is important that the arborist undertaking the pruning has an understanding of the biology of trees and how they respond to pruning wounds and the reduction in stored energy reserves and photosynthetic ability that pruning entails.
Prior to pruning being undertaken, an inspection of the tree should be carried out to assess the tree’s health, growth habit, structure, stability and soil environment. The inspection should also consider species, age, wind exposure, foliage distribution and any potential hazards. The potential impacts of the proposed pruning on the tree’s biological processes, longer term health and structure, plus wound size and response should also be considered.
Objectives The objective of pruning is to promote strong, healthy and attractive amenity trees, which enhance the environment and do not present a hazard to public safety. No tree should be pruned without first establishing clearly defined pruning objectives, which may include the following;
• Pruning for safety – Where branches:
Are extended with heavy end weight and poor taper.
Obstruct lighting, lines of sight on streets or driveways.
Present a hazard likely to cause injury or property damage.
Are growing into utility service lines.
Exhibit weak attachments.
• Pruning for tree health Removal of dead, diseased and unthrifty branches.
Removal of crossing and rubbing secondary branches.
Removal of broken and storm damaged branches.
Removal of stumps and stubs to encourage wound closure.
3. Pruning Procedures - General All tree work should be in accordance with specifications set out in following
Codes of Practice, and their subsequent revisions:
• The Approved Code of Practice for Safety and Health in Tree Work (1994) Part 1: Arboriculture.
• The Approved Code of Practice for Safety and Health in Tree Work (1996) Part 2: Maintenance of Trees around Powerlines.
• The Approved Code of Practice for Safety and Health in Tree Work Part 3: Tree-work in Rivers, Streams and Waterways.
• The Approved Code of Practice in Forest Operations (April 1999) Sections - Cable logging, Rules for Helicopters, Use of wire ropes.
• The Approved Code of Practice for Elevated Work Platforms (EWP)
• The Approved Code of Practice for Cranes (March 2001) Part 1 – General Rules Part 2 – Duties of Controllers Part 8 – Mobile cranes Part 11 –Vehicle mounted cranes All pruning work should employ natural target pruning in accordance with the principles of Modern Arboriculture. Pruning work shall endeavour to maintain the natural form, structural safety and visual appearance of the tree. Where necessary, ropes should be used to lower large branches and stems. When removing branches, cut sections should be dropped or lowered in a way that will avoid damage to the bark or cambium on the remaining parts of the tree. All branches that are too large to be hand held when removing must be under cut to prevent bark tearing and damage to branch collars. All branches removed must be extracted from the canopy.
Care should be taken to avoid excessive pruning. At least one half of the foliage distribution in younger trees should be on branches that arise in the lower two-thirds of the trunk. In general pruning should employ thinning cuts. Care should be taken not to expose the inner canopy, (shade leaves) to intense sunlight by over-thinning or excessive cleaning out of the entire inner canopy (lions-tailing). All saw and pruning cuts should be made back to either collars or branch nodal junctions. Techniques other than those applied in Natural Target Pruning should only be used in specific circumstances such as Pollarding or Retrenchment.
When shortening, branches should be pruned back to a lateral which large enough to assume the terminal role. Spikes/spurs or other equipment that may damage bark on the tree must not be used unless the entire tree is being removed or an emergency situation arises.
Apical dominance Some species with strong apical dominance with an excurrent form retain a single central leader through to maturity. Such trees should not be reduced in height unless this is necessary to address an actual hazard situation.
Version 3 April 2011 Page 4 of 16 Epicormic shoots The contractor should determine the cause of epicormic growth before removing it. Situations such as plant stress, previous incorrect pruning and sudden light exposure should be considered. Where restorative pruning is undertaken, selected “elite” shoots should be retained and “multiple” epicormic shoots thinned or removed.
4. Natural Target Pruning
Before the selection of a pruning class, the reason for pruning should be carefully considered. With the exception of pollarding and retrenchment pruning, all classes are based on the principles of natural target pruning. Pruning cuts should be made back to either collars or branch nodal junctions.
Diagram 1: Target Cuts at Branch Unions
Pruning cuts should be at branch unions [arrows in Diagram 1]. Do not make inter-nodal cuts between arrows. To avoid splitting or tearing of the branch collar or trunk, branches should be pre-cut or undercut. The remaining stub is then removed with a final cut made as close as possible to the branch collar without cutting into the branch collar or leaving a protruding stub. (See Diagram 2)
Diagram 2: Collar Pruning
Version 3 April 2011 Page 5 of 16 The final cut should be made as close as possible to the branch collar. Do not cut into the living tissue of the collar. If a collar is not clearly visible use the branch bark ridge as a guide.
In the absence of a visible collar, the branch bark ridge should be used to determine the angle of the cut when removing a branch. The aim of the cut is to prevent damage to trunk tissue and the branch bark ridge. (See Diagram 3)
Diagram 3: Branch Bark Ridge Pruning
If living branches do not have a definite collar, the branch bark ridge can be used as a guide for the final cut. Line A-X is a line parallel to the stem or trunk occurring just outside the branch bark ridge. Line A-C indicates the angle of the branch bark ridge and line A-B represents the angle and location of the final cut. Angle `a’ should be equal to angle `b’.
When removing a codominant stem, it may not be possible to determine a distinct branch trunk collar. Co-dominant stems may have stem bark ridges which are long or may be difficult to discern. In the absence of a clear collar, the stem bark ridge can be used to indicate the angle of the cut when removing a co-dominant stem. In removing one stem, the adjacent stem should not be damaged.
In Diagram 4, line A-B represents the angle and location of the final cut, point C is the bottom of the stem bark ridge. The final cut should be made without damaging the remaining limb.
Reduction pruning of the upper and outer crown area should be accomplished using dropcrotching techniques, where the final cuts in reducing branch extremities are targeted at internal secondary laterals, branch-stem junctions, or positions of previous reduction pruning.
5. Pruning Types and Classes The selection of the type and class of pruning will depend on the tree’s location, form and the desired outcome.
5.1 Crown Maintenance Crown maintenance is pruning according to the growth habit of the tree. It may include dead-wooding, crown thinning, and formative pruning. Crown maintenance pruning does not significantly reduce the area of the crown and retains the structure and size of the tree.
Crown Cleaning and Dead Wooding Crown cleaning may be used where a tree is being maintained as a specimen within a public Reserve, street or an ornamental garden. The removal of dead, dying, diseased, detached or broken branches is undertaken to improve crown appearance and the overall tree aesthetics.
Where the deadwood has remained in the tree for a long period the collar may have extended itself along the dead branch. This collar should be left intact when deadwood is removed from the tree. It should be noted that dead wood is an essential habitat for a large number of organisms in the ecosystem in which a tree lives. The formation of dead wood within the crown of a tree is part of the natural system of tree life and removal of deadwood should not always be considered as essential to the maintenance or promotion of healthy tree growth.
Formative pruning can be completed on semi-mature trees, but should be avoided on mature specimens. The main management objectives of this type of pruning are to encourage a crown form which needs less pruning when mature, and where ever possible limits the development of weak structural features which may fail in later life. The aims of formative pruning are to enhance form and improve structure, to directionally shape the young tree, and to reduce the development of structural weaknesses. (See Digram 6)
The removal of dead and diseased limbs, crossing or rubbing branches. This type of pruning may required on developing trees to improve structure and form but should not be applied to mature trees.
Thinning Crown thinning, is the selective removal of small, live branches throughout the entire crown, with the aim of reducing the density of the tree leaf area. There should be no external alteration to the tree’s size or shape and the majority of branches should be removed from the outside third of the tree crown. The maintenance of an inner crown leaf area is essential to many tree species. Crown thinning reduces canopy density through the removal of secondary branches whilst retaining the main structural branches and established tree form.
The percentage of a crown that can be removed without having a detrimental effect on tree health and vigour is species and age specific Thinning can be used to reduce wind resistance, reduce the weight of limbs, increase light penetration and promote air movement through the tree crown. Thinning should avoid cleaning out the entire tree interior, and avoid the visual effect of “lion tailing.”
Crown modification is pruning that changes the structural appearance, size, form and habit of the tree. It includes reduction pruning, crown lifting, pollarding, and can also be used to reduce limb weight.
Crown Reduction (Drop Crotching) Reduction pruning reduces the height and/or spread of the crown of a tree, by removal of branch extremities back to internal laterals or branch-stem junctions. Reduction should be accomplished using drop crotching or removal cuts not heading cuts. The use of heading cuts may spoil tree architecture and can significantly increase maintenance requirements.
When a mature tree is reduced, no more than one fifth (20%) of its foliage should be removed, whereas up to one third can be removed from younger more vigorous trees without significantly reducing tree vitality.
Reduction cuts should be targeted at remaining lateral branches of sufficient diameter to assume the terminal role. At such a size, the lateral branch should be able to produce enough energy to keep the parent branch alive, and there should be enough growth regulators present to suppress excessive epicormic sprouts. This will vary with tree species, age and condition. Old, stressed or late mature trees could decline or become more stressed if too much foliage is removed. Excurrent species should not be reduced unless to address an actual hazard situation.
Crown Lifting The selective shortening and removal of lower lateral branches to affect a vertical lift of the crown, allowing space under the tree for light, people, vehicles or buildings, while maintaining as many low branches as possible to sustain good trunk growth and taper. When pruning is complete 50% of the trees total foliage should be on branches arising from the lower two thirds of the trees height.