ISA HK/China --- Principles of ISA Tree Selection Method

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Dear Station Members,

Some prominent arborist said in the past that 'if the right trees are planted at the right places, arborists may be out of a job.' He can not be more correct. It is all too often that when the wrong trees were selected for the wrong places, coupling with poor nursery stocks & improper planting practice, that the trees would exhibit a variety of symptoms like poor growth, reduced flowering, general decline & eventual failure. At that time when the arborists is called in, there is usually very little he can do besides to advise to change the trees. When trees are not properly matched to the landscape site, they usually become more of a liability rather than asset. Just look at the thousands of tree failures after the mini typhoons in Aug 2006 in HK.

There are a few ISA publications on Tree Selection for technical viewing:

1. Arborists' Certification Study Guide --- ISA
2. Arboriculture --- Harris, Clark & Matheny
3. Tree Section & Planting (A Collection of CEU Articles) --- ISA
4. Introduction to Arboriculture (Tree ID & Selection) CD --- ISA

All above can be obtained on-line at ISA website.

Basically, ISA Tree Selection revolves around several major principles:

1. Matching Tree & Site
2. Site Considerations
3. Tree Considerations
4. Selecting Quality Nursery Stocks

Before all these would commence, it may be worthwhile to throw some light into the benefits of trees as a reason for tree planting:

"Trees are the tallest & longest living creatures on earth. They provide economic values throughout our civilization & they are the most prominent feature in a landscape. They provide clean air, flowers & fruits for our admiration. Trees also cool our summer heat & block unwanted views. Tree roots grasp soil from erosion & their canopies reduce rain run-off by capturing & holding water above. Recent research also found trees can have positive psychological effects such as reducing stress, reducing crime rate & reducing recovery time for hospital patients. Trees are part of our life & most human do not enjoy living in a desert."

After knowing the general benefits of trees, now we want to install trees into our urban landscape to dilute the concrete engulfment. This is how we can do it:

1. Matching Tree & Site

If the Tree Selection is only for short term objective, e.g. an exhibition, then there is no worry to use anything in anyway as long as the selection would survive the show. Some clients & designers use this principle for their project work of which the Defects Liability Period is only 12 months. Such thinking has installed a lot of wrong trees in the wrong places in the past in many locations. On the other hand, if the Tree Selection is for permanent use like roadside trees, park trees, hospital trees, etc., then the right trees must be matched for the right places for function & stability if these trees are to offer us their lifelong service.

All too often when the wrong trees are picked, heavy maintenance will follow & future performance of the trees becomes a liability, such as wind failure. In a place like HK where typhoons strike every summer, wind resistance should become a major concern over flower display which seems to be the over ridding priority in selecting a species. In addition, over fertilization & planting under shade seem to be the limiting factors locally in inhibiting flower display for many of our landscaping trees.

Matching tree & site is like putting on the right clothes for the event. If jacket & trousers are dressed for farming, like those seen up in China, these expensive clothings may not last for too long for the laborious work.

2. Site Considerations

It is recommended that a Site Analysis is to be carried out before selection of the species would begin.

In simple terms, site factors to be considered for the Tree Selection may include sunlight intensity, temperature range, moisture variation, irrigation, drainage, growing space above & below ground, soil condition, pest occurrence, prevailing wind, maintenance difficulties, etc. All these factors are essentials to ensure the long term performance & function of the selected trees. If one determining factor or more is/are to be ignored, like wind &/or drainage, the selected trees may go into a slow decline to eventually become a hazard to fail in the slightest wind in future.

Once the Site Analysis has been carried out, the next step would be to consider the Design Criteria. Design Criteria are based on the functions that the tree is expected to serve. Landscape functions include engineering & architectural considerations such as controlling pedestrian traffic, hiding unsightly building features, climatic control, pollution buffer, human comfort & aesthetic appearance. Among all these factors, the adequate growing space above & below ground is usually the limiting factor in HK because our city is already overcrowded with structures above & below ground for most municipal design.

3. Tree considerations

Factors to be considered here for the selected tree would include the cold hardiness, heat tolerance, growth rate, size at maturity, mature growth form, pest resistance & maintenance requirements. If the tree is to be selected for a difficult location such as hill top where irrigation is only from rain, or coastal front where salty wind prevails, species adaptation for the location is important.

In our city area where growing space above & below ground is usually a limiting factor, the mature tree form & size would be a priority consideration in the species selection. Some species can be kept small by constant structural pruning but many a times in HK, pruning is confused with topping which would cause sideway expansion of the crown which is the opposite desired effect. Introduced species with heavy flowering habit in its place of origin may not flower at all if the growing condition can not be matched locally. Certain species with invasive root development such as Ficus & Camphor, should be carefully screened off for planting near thinly covered pavement & bituminous surfaces. Species requiring a lot of water uptake such as Khaya should be placed away from where subsidence can be a problem like retaining wall. Species giving heavy shade may kill the grass below in courtyard & golf courses.

For roadside tree planting, the common practice in a lot of western countries would be using Excurrent Trees (tree with a central leader) for their predictable growth form in the design. Excurrent trees are also easier for Tree Stability Assessment calculations due to its uniform structure. Decurrent Trees (tree without a central leader) would likely be used for shade & flowering in parks & open spaces where sideway development of canopy is not a problem. Trees with non-aggressive root system like Lagerstroemia can be used near sewage pipes where root invasion is unlikely by such species.

4. Selecting Quality Nursery Stocks

Vigorous quality nursery trees will establish faster & develop stronger structure against wind sooner. Inferior stocks, like most from China nurseries at present, will drag their feet to take longer to establish & will invite heavier maintenance, & will perform poorly in wind. In a place like HK where typhoon is a yearly threat, wind resistance should be given precedent to flowering because if there is no tree left standing, there will be no flower to look at after wind.

ISA recommends the ANSI z60.1 2004 as the standard for nursery stocks selection. Previous Station Mail on Specification of Nursery Trees at ETF has taken essence from this international standard with local adaptation. If HK landscapers insist on using quality stocks from now, the China suppliers would have no choice but change their attitude in production if they want our business. The quality landscape to appear in HK in future will also influence our brothers & sisters up north who have been ferociously imitating western landscape design already.


ISA Tree Selection principles do not come by accident. It conglomerated 83 years of experience from commercial, research & utilities arborists all around the world who have learned through trail & error. Mistakes in the past become experience in the future. And that is the spirit of ISA arboriculture, the forever learning bunch of international arborists who always strife for the best...

best regards,

Sammy Au
Station Manager