The pruning of landscape trees

“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 lifespan made possible by optimum trunk and branch structure.”

– Dr Ed Gilman, University of Florida.

FOR many planting young trees in the landscape, what emerges from the tree nursery is viewed as a finished product. The crown of the tree is seen as formed and then allowed to develop without further management. While this may be a perfectly sound approach for the field-grown tree with 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 removed when the tree is young. The secateurs and handsaw can pre-empt the use of the chainsaw.

The branching system forming the young tree crown, when exported from the nursery and delivered to the planting site, is the branching system which 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 with time as the trees grow.

Nursery-pruned trees are not the finished product.

At the nursery, much of the pruning work carried out 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 girth in the nursery will be produced with a clear stem of between 1750 mm to 2000 mm, yet often the demands of the landscape require stem clearance of anything up to and beyond 3500 mm, which means branches which may be valuable in the nursery are redundant 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 200 mm girth trees with a clear stem of 3500 mm. Such a tree would have the appearance of 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 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 on the nursery has already been mentioned, but the retention of this leader remains important as the tree grows 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 seen in sweetgum (Liquidambar styraciflua). Decurrent trees lose their leader dominance as they develop. An example of this type of growth habit is seen in sycamore (Acer psuedoplatanus). 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.

Lower branches left in place will cause future problems on this young Gingko. (Image: EA)

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 be subordinated by pruning. This exercise will have been practised at 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 per cent 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 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 at 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 be 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 size. 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 more. 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 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 apparent on the nursery is retained through to maturity without intervention.

While 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 sub-ordination 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.

An appreciation that crown development is sequential and gradual rather than a one-off exercise which lasts forever.

It is also fair to stress that the above cannot be universally applied to either multi-stemmed trees or conifers where different management and pruning regimes are probably going to 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.