The pruning that happens after planting is different in nature from nursery pruning. It is essential to get it right if your trees are not to suffer problems in future.
I would like to start this article with a quote, which encapsulates much of my argument.
‘Pruning is the selective removal of plant parts to meet specific goals and objectives. One of the most compelling goals for trees, planted or naturalised, is a long life span made possible by optimum trunk and branch structure’
Dr Ed Gilman. University of Florida
Too many people who are planting young trees in the landscape regard what emerges from the tree nursery as a finished product. They consider that the crown of the tree is formed and can then be allowed to develop without further management. While this may be a perfectly sound approach for field-grown trees which have ample space to develop fully, it is almost certain to be unsatisfactory within the constraints on the urban environment.
Here factors such as human safety, proximity to buildings, sight lines, necessary crown height along transport corridors and many others all impact on what is an acceptable branch structure in any given situation. It is a constant truth that many of the problems and potential hazards associated with tree canopy in the urban environment can be prevented when the tree is young. The secateurs and handsaw can pre-empt the use of the chain saw.
The branching system forming the young tree crown is, when it is exported from the nursery and delivered to the planting site, the branching system that will remain with the tree throughout its life in the landscape unless pruning is undertaken. Branches will thicken and lengthen but their height and position on the tree remain unaltered. A branch emerging from the main trunk at three metres on a 10 year-old will grow considerably larger, with variations for different species, but will still be at three metres 20 years later. Mechanical defects and other potential problems of the future will already be apparent and will only become exacerbated over time as the trees grow.
At the nursery much of the pruning work is formative, concerned with the production of a strong dominant leader and a lateral branching system which will not compete with that leader but is subordinated to it by judicious pruning. The prime role of the nursery is to retain the strength of the upright leader while retaining enough photosynthetic integrity to allow the tree to grow. Trees up to 140–160 mm in girth in the nursery will be produced with a clear stem of between 1750mm to 2000mm, yet often the demands of the landscape require stem clearance of anything up to and beyond 3500mm. This means that branches which may be valuable on the nursery are redundant, extraneous and can be considered temporary. However, stem clearance has to be created gradually. It is amusing that often, as a nursery, we are asked by landscape architects in particular for 200mm girth trees with a clear stem of 3500mm. Such a tree would look like a totem pole with a terminal bud at the end. It is only when the final clear stem height required has been created that the permanent branching system, anything above the final clear stem required, can be developed and the pruning becomes structural rather than formative.
Yet such pruning is rarely carried out in the landscape. Branches that are too low on the main stem are often allowed to develop and expand only to become problematic as the tree grows.
The importance of the leader or central stem in the nursery has already been mentioned, but the retention of this leader remains important as the tree develops in the landscape and the structural branching system is developed. While all tree species have clearly identifiable, individual growth characteristics, all broad-leaved deciduous and evergreen trees can be placed into two growth categories. Excurrent trees have a naturally occurring straight leader which remains prominent throughout the life of the tree. An example of this type of growth habit is seen in sweetgum (Liquidambar styriciflua). Decurrent trees, in contrast, lose their leader dominance as they develop. An example of this type of growth habit is seen in sycamore (Acer pseudoplatanus). Research has suggested that, irrespective of the natural growth habit of the tree, a strong leader should be retained until the tree has reached at least two thirds of its mature height.
In the landscape young trees often develop competing leaders. This is where lateral branches below the leading shoot develop in competition with that leader. Each branch on a tree is a semi-autonomous unit and will compete with its neighbour for resources. To retain the dominance of the leader, these competing lateral shoots need to subordinated by pruning. This exercise will have been practised in the nursery but needs to be continued into the landscape once the young tree has been planted.
The subordination of laterals also impacts on the diameter of these lateral branches in relation to the diameter of the central stem/trunk stem which is carrying them. Research has indicated that for full structural integrity it is wise that the diameter of the lateral branch is never more than 50% of the diameter of the main stem/trunk at the branch union. This is the point at which the lateral branch meets the main stem/trunk. This proportional relationship, known also as the aspect ratio, becomes even more important when included bark is present. Included bark occurs where the union between branch and main trunk /stem is incomplete. This fault is often apparent in its early stages on the nursery but does not have serious implications until the weight of the lateral branch increases with branch growth. Pruning at or shortly after planting is easy and requires minimal effort but allowing this fault to remain will often become difficult and expensive to correct as the tree matures. Failure to remove can and does lead to branch failure on the mature tree with obvious implications.
It is impossible to fully describe all aspects of structural pruning in an article of this length. There are many subtleties and variations. These include branch arrangement, spacing between individual branches, crossing branches, branch whorls on the main stem, deformed or duplicating branches and many others. It is also advisable to prune from the outside of the tree rather than pruning branches out of the middle. I have tried to cover a few of the basic principles and differentiate between nursery formative pruning and landscape structural pruning and emphasise the necessity of landscape structural pruning being programmed into any management plan that is produced for young trees planted from the nursery.
The main points are as follows:
The growth and development of young trees is not complete when the tree leaves the nursery. Many of the branches present at the nursery may be only temporary in the landscape.
The branch system that is apparent in the nursery will be retained through to maturity if there is no intervention.
Although there are many similarities, structural and formative pruning differ and those differences need to be understood and implemented in the landscape.
Leader retention and the subordination of co-dominant laterals is an essential part of the structural pruning exercise.
The early removal or subordination of branches with poor branch unions can save money and reduce risk.
You must appreciate that crown development is sequential and gradual rather than a one-off exercise that will last forever.
It is also fair to stress that the above cannot be universally applied to either multi-stemmed trees or to conifers. For those trees, different management and pruning regimes will probably be more appropriate.
It is also true that structural pruning is an adaptive process which becomes a necessity where people and the urban environment with all its constraints meet with the need and desire for trees.
has more than 20 years experience in local government as nursery, parks and operations manager. He spent eleven years with Notcutt’s Nurseries with responsibility for tree sales to local authorities and other trade outlets. Currently he is sales director of Barcham Trees.
Specification Manual: A Guide to specifying young trees from the nursery
by Keith Sacre Barcham Trees. (Free of charge, on request, from Barcham Trees, [email protected]
An Illustrated Guide to Pruning, Third Edition by Ed Gilman
The Pruning of Trees Shrubs and Conifers, Second Edition by George E Brown revised and expanded by Tony Kirkham.
BS 8545 Trees: From Nursery to Independence in the Landscape. Published February 2014. British Standards Institute
BS 3998 Tree Work Recommendations 2010. British Standards Institute