Tim Johnson, Technical Officer Parks & Gardens, City of West Torrens

The City of West Torrens has been involved with TREENET street tree trials since 1998.  Council’s approach to establishing tree trials has progressed over time, with current methods based on experience gained over this four-year period.  The methods used in West Torrens will not be applicable to all situations though they may assist with development of techniques appropriate to other areas.

1.  Rationale

Many conferences, workshops and seminars have for decades detailed the need for increased street tree research and improved information standards.  A recent survey of local government (Lawry, D. & Gardner, J.  2001) confirmed these needs and revealed overwhelming support for the TREENET project.

The survey revealed 71% of respondents were dissatisfied with the current range of species used for street planting.  93% of respondents indicated they were interested in testing new varieties.  Factors influencing the success of planting programs were also detailed by respondents and given a rating from 5 (very influential) to 1 (not influential.)  The influencing factors and their ratings are summarised below:

Average Rating
1.  Availability / quality of information on suitable species 3.9
2.  Availability / quality of nursery stock 4.1
3.  Difficult site conditions 3.4
4.  Availability of funds 3.9
5.  Availability of human resources 3.8
6.  Access to specialist knowledge/skills 3.3
7.  Vandalism 3.7

The survey showed that along with staff and budgets, improved information and stock range are seen as highly relevant to the success of greening programs.  TREENET has the potential to guide, support and share the required street tree research and resulting knowledge between all stakeholders.

2.  Components of a Successful Street Tree Trial

It is useful to consider tree trials in terms of four fundamental components: staff, the site, trees and time.  While numerous factors affect each of these four components it is critical that a simple focus is maintained.  The primary objective of tree trials is simple, it is to establish trees in sites with differing conditions and to observe and record their performance over time.

2.1   Staff

West Torrens is fortunate to have an enthusiastic and knowledgeable outdoor workforce, the leaders of the landscaping and arboriculture teams being responsible for initiating and driving the street tree research program.  Maintaining the program has been relatively simple, supporting and working with colleagues who are constantly extending it.  The challenge is to keep pace with such staff.

2.2   Trial Sites

West Torrens is caretaker of approximately 700km of road verge, over 120 reserves, bikeways, linear parks and over $120 million in other properties.  The range of potential trial sites is enormous.

Site conditions

The physical, social and political characteristics of a site will determine its suitability for a tree trial, they include:

  • Soil
  • Average rainfall
  • Groundwater, irrigation water quality & availability
  • Aspect
  • Prevailing wind
  • Distance to coast (i.e. salt, weather effects)
  • Distance to school (i.e. vandalism)
  • Nature of pedestrian and/or vehicular traffic
  • Size of the nature strip or planting area
  • Local residents – individuals & groups, eg. ratepayers associations
  • Local politicians
  • Utilities & infrastructure

(examples only, list not intended to be exhaustive)

Some site characteristics (typically the physical ones) remain relatively fixed in our timeframes while others may vary.  TREENET aims to compare tree performance relative to fixed characteristics such as soil and average rainfall, which can be quantified and recorded.  Following the initial establishment period, tree performance will generally relate to the suitability of the tree to these physical site conditions.

The social and political characteristics of a site will often determine the type of trial that can be undertaken, so they should be determined through effective consultation prior to works beginning.  An effective consultation strategy will assist trial planning and will normally increase support for the research.

Site selection

Social, political, horticultural and budgetary considerations all impact trial site selection.

The trial must be large enough to give reliable data, but small enough to limit losses should the species/site combination prove unsuitable.

West Torrens a typical trial might involve five to ten trees, requiring five to ten nature strips.  Depending on the level of confidence in the site/species compatibility a larger or smaller trial may be established.

Any site where tree planting can be limited to about ten trees is a potential trial site.  Traffic islands, cul de sacs and short streets are all ideal.  Adelaide’s typical grid layout often uses short links between longer roads, these may be useful but caution is required when setbacks are reduced on corner properties.  A long street with few existing trees, trees that are deteriorating or have other problems may also provide an ideal trial site, allowing species to be exhibited as possible candidates for replacement of the entire avenue when the need arises.

2.3   Trial Trees

There are many factors to consider when selecting stock for a species trial, including:

  • species
  • subspecies
  • provenances
  • hybrids
  • selected forms
  • litter
  • infrastructure impacts
  • availability
  • adaptability to local conditions
  • hardiness
  • stock type
  • stock size
  • history / extent of cultivation
  • nursery (reputation/service standard/stock quality/delivery schedules)
  • client requirements
  • mature size
  • life expectancy
  • hazard development potential
  • disease resistance, etc.

The detailed information required to select a tree relative to the above criteria will in most cases be unavailable.  Systematic testing of trees under varying conditions will assist TREENET to provide this information but it will involve increased risk in the short term.   Replicating successfully established trials in neighbouring areas may be a safe place to begin tree trials, as would extending an existing local trial that shows promise into areas with slightly differing site conditions.

Selecting an unknown tree for trial is an obvious risk.  To minimise the risk, small trials are recommended.  Extensive research should be undertaken to gain an initial indication of the tree’s potential prior to planting.  Texts, arboreta, colleagues, the Internet and nurseries are all useful sources of information.  They should be used to guide species trials, but never to replace them.  The ultimate compatibility of the species/site combination will only be revealed by time.

2.4   Time

Social and political views, opinions and fashions vary with time.  As trials develop at any given site new cultivars will be released, tenancy of the properties will change and planning laws may be reviewed.  It must be within the trial site host’s ability to maintain the trees up to an age where their characteristics can be determined regardless of changing views or fashions.  An effective policy on long term street tree management is essential to the success of street tree trials.  The recent TREENET local government survey (Lawry, D., & Gardner, J. 2001) showed only 40% of respondents had a formal tree management policy.

Tree growth rates vary.  Trees that develop slowly may be less desirable to stakeholders in the short term.  Property values, amenity, shade and habitat may not improve as rapidly with slower species as they might have with faster growing trees.  If a species is known to grow slowly, it should be tested in a site where it will have minimal impact and where there will be no pressure to replace it.

3.  Site & Tree Combinations

3.1   Experience, Local Knowledge & Logic Vs Assumption & Hearsay

Whether learned through educational resources or personal endeavour, knowledge is based on experience!  Knowledge of trees is based on experience with them.  When planning a tree trial it is essential that limitations in knowledge be recognised.

Characteristics of seedling trees are often variable.  Logically, therefore, the suitability of a particular seedling or selection to a given site may vary from that of others within the species.  When experience is limited to a small number of seedlings of any species it will give only a limited knowledge of the species.  To assume that all examples of a species have the same characteristics as the limited examples on which one’s experience is based may lead to the rejection of the most suitable tree for a particular site.

Characteristics of genetically identical trees are generally more consistent throughout horticultural applications than seedling varieties.  Characteristics of cultivars from within a species tend to vary more between the different varieties than between seedlings of the species.  It should be recognised, therefore, that different varieties of a species might develop differently on a given site.

The simplicity of the above logic is often ignored or confused in practical horticulture.  Because a new selection has the same species name as others or seedlings planted in the past that failed, its use for a particular site may not be given serious consideration even though it might perfectly suit the requirements and conditions.  The characteristics of new selections should therefore be tested and determined under a varied range of conditions.

3.2   Which comes first, the site or the tree?

Street trees are typically selected to match site conditions, amenity considerations and other requirements, often in response to site determination based on decay of existing trees coupled with social or political influence.  These are exactly the conditions in which a TREENET trial should not be established.  The pressure to establish an effective avenue can be immense under these conditions.  A proven and reliable type should be used in these situations, established by usual means to give the highest chance of success.

Research should be separate from a normal greening program.  Neither the trial trees nor staff should be subject to external or unrealistic constraints or expectations when undertaking research planting.

Staff should focus primarily on tree selection, conduct an extensive literature search on the tree and then apply local knowledge in the search for a site in which it might be expected to grow well.  This is in complete contrast to the more common procedure of applying a limited knowledge of the available tree varieties to the task of making the best tree selection for a given site.

4.  Monitoring a Tree Trial Site

Developing a simple but adequate means of recording relevant details about tree trials has involved much discussion, debate and compromise.  Increasing the amount of standard data required will increase staff time requirements and so would ultimately reduce the number of sites that can be managed.  Reducing the standard data requirements may reduce the usefulness of the database.

The TREENET database is an evolving work-in-progress.  Feedback mechanisms have been built into the system to allow it to develop further, so standard requirements may vary with time.

4.1  Standard data entry

The following standard data requirements have been determined with the aim of optimising the usefulness of the database (aimed at local government and the nursery industry as “standard” users) while minimising the time commitment for data collection.  To improve efficiencies, data collection has been divided into two standard forms.  The forms will be available on-line, where data will be able to be entered directly by the participant.

Form 1.  Client Identification.      

Information required:

?    Organisation name ?    Telephone number
?    CEO’s name ?    Mobile telephone number
?    Contact name ?    Facsimile number
?    Address ?    Email address


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