Formative Pruning: How to Set Trees Up for the Long Term

Gardeners and landscapers with a passion for long term sustainability will be interested in formative pruning, which is all about the sustainability of urban trees for those who play the long game. In Australia, the term ‘formative pruning’ is used to describe the practice of correcting defects in young trees, but in other parts of the world formative pruning is part of the more general practice of structural pruning that corrects defects in trees of all ages.

Formative pruning can be defined as the selective removal of stems and branches early in a tree’s life to create a safer, stronger and more aesthetic structure. Many of the problems and defects of mature trees can be traced back to faults early in life (Moore 2012). Formative pruning aims to correct any and all defects in a young tree, up to about 6m tall. It is done from the ground using secateurs or pole saws and so is safe and cost effective. However, it requires genuine arboricultural expertise and knowledge of the young tree’s growth habit.

Formative Pruning has Many Benefits

The advantages of formative pruning are many, including:

improving the structure and health of young trees
improving their appearance
reducing urban tree maintenance costs
lowering the risks associated with large trees as they mature
improving worker safety, and of course
improving the sustainability of urban trees.

It is an impressive list of advantages, expanded on in the table below, so it is surprising that so few gardeners, even professionals, practice formative pruning as a matter of routine.

Table 1. Some of the many advantages of Formative Pruning (Modified from Moore 2012).

Benefits of Formative Pruning	Advantages for the Gardener
Simple Equipment	Low cost, low energy use and low embedded tools
Greater Sustainability	Initially sustainable and increased sustainability as trees age
Small Wounds	Rapid recovery and growth, reduced risk of disease and decay
Earlier Intervention	Healthier and longer lived trees
Light Weights	Easier to prune and much safer for gardener
Quick and Easy	Quick operation with less effort for small branches and trunks
Saves Money	Cheap and low cost gardening
Better looking trees	Trees that have higher aesthetic and economic value
Safer trees	Trees with fewer structural defects are safer
Longer lived trees	Formatively pruned trees with fewer defects live longer

What Can Formative Pruning Achieve?

There are many aspects of young tree structure that can be corrected by formative pruning (see table 2 below), but some are more common and more significant than others. Crossing and rubbing branches are obvious matters for correction, but perhaps the most significant structural matter as the tree matures and for its sustainability is the elimination of codominant stems, especially if bark is already being included.

A photo of a tree with crossing branches that would benefit from formative pruning — Crossing and rubbing branches on a young tree that can be easily and quickly corrected by formative pruning.

A photo of a young eucalyptus tree with codominant stems. — A young eucalypt with a tendency for codominant stem unions. Early formative and later structural pruning would provide a better specimen for a local park.

These codominant stems can lead to trouble later on if bark is included and the two or more stems begin to split. This is not only problematic for the tree but presents an increased risk of failure in a mature tree. Codominant stems with included bark do not always fail, but if the opportunity for formative pruning exists early in the tree’s life to reduce the risk, it would seem prudent to do so.

Table 2. Common faults corrected by Formative Pruning (Modified from Moore 2012).

Fault to be Corrected	Objectives of Formative Pruning
Co-dominant Stems/Trunks	Removes multiple leaders that may be prone to splitting
Included bark & Co-dominants	Removes twin or multiple leaders that have bark trapped or retained between the leaders which may increase risk of splitting
Rubbing Branches	Removes rubbing branches which can be weakened and provide entry for disease and decay
Crossing Branches	Removes branches that have the potential to become rubbing branches as the tree ages
Dead Wood	Removing dead wood removes potential risks, improves the appearance of the tree and promotes a healthier growth
Broken Stems/Trunks	Removes damaged plant tissue and promotes a healthier plant
Broken Branches	Removes damaged plant tissue and promotes a healthier plant
Epicormic Shoots	Removes epicormic shoots that may be weakly attached, unsightly or growing in the wrong place and could become co-dominant stems of rubbing or crossing branches
Low Branches	Removes low growing branches that cause obstruction or pose a hazard to pedestrians or property

Who Should do the Work?

There is a tendency by some to think that formative pruning can be undertaken by lesser trained and inexperienced horticulturists or arborists, but this is folly. Working with young trees requires knowledge of young trees, but also of the mature specimen and the possible future risks.

Some local councils have formed formative pruning crews for their street tree plantings in particular, and the results are clearly evident within a few years; trees with a better structure and a better, longer, more sustainable future. The efficiency and speed of the work undertaken explains some, but not all, of the economic benefits of formative pruning.

Long-Term Savings on Tools and Equipment

Clearly, formative pruning requires equipment that comes at a lower economic and carbon cost than complex and reactive remedial work that might be required on a mature tree using trucks, chainsaws and elevated work platforms. Movement between trees, especially in larger street tree plantings, is usually by foot and so again there is a low carbon footprint. Working with young trees is much quicker for the same corrective procedure than work undertaken 10 or 20 years later on the tree when it has grown and matured.

For most formative pruning work, there will be no traffic management plan required, even for street trees, as work can be undertaken from the footpath or nature strip. The economic and sustainability benefits are substantial and continue to accrue over time. Formatively pruned trees will require less maintenance and fewer emergency arboricultural interventions during future storm events. These reactive unplanned arboricultural works are notoriously costly when compared with planned routine works of structural pruning.

Which Tree Species Benefit Most from Formative Pruning?

Not all species have the same requirements for formative pruning. In research undertaken in Melbourne, plane trees (Platanus x acerfolia) required most work, followed by Chinese elm (Ulmus parvifolia), callery pear (Pyrus calleryana), pin oak (Quercus palustris) with lemon scented gum (Corymbia citriodora) requiring least work. The cost of the work undertaken on the plane and pear in this study was greater than 3-4 times that for the lemon scented gum. Even the most expensive work undertaken was less than 10% of the same work required on a 20 year old tree. The economic case for formative pruning speaks for itself (Ryder and Moore, 2013).

Formative Pruning to Extend the Lifespan and Carbon Benefit of Street Trees

It is known that the closer you get to a central business district or major shopping strip or hub, the shorter the life span of the street trees that grow there. This is a world-wide phenomenon; not just something that occurs in Australia. There are two mortality peaks for urban street trees. One is in the first 2-4 years after planting, and a second peak occurs when trees are about 30 years old. This second peak again is a world-wide occurrence, but has significant impact on urban forest sustainability. While trees are great at sequestering carbon, street trees can take 25-30 years before they become carbon neutral and longer before they are positively sequestering. Think of the carbon and embedded energy in transporting and planting advanced street tree stock.

So if planted trees do not survive the first 2-4 years, and there is a second rise in tree deaths and removals around 30 years, many urban trees never reach carbon neutrality. This situation is unsustainable environmentally and ludicrous economically, but seems to go unnoticed. We accrue all of the costs of growing and planting these trees but recoup none of their benefits. However, both formative and structural pruning can reduce the carbon footprint, prolong the useful life and improve the sustainability of an urban tree. Formative pruning is one of those practices that make so much obvious sense and yet is so rarely implemented.

Dr Greg Moore is a Senior Research Associate , School of Agriculture, Food and Ecosystem Sciences, University of Melbourne, Burnley, and a member of SGA’s Board.