Saturday, December 19, 2015

How to Choose A Street Tree - Part 2

Deciduous or Evergreen?

This decision needs to be made early in the process and once made will eliminate many species from further consideration. Deciduous trees are often preferred in urban situations as they allow plenty of sunlight into the street in winter. Streets with large evergreen trees can become dark, and the constant dripping of water off the leaves long after a shower has passed can be very annoying for pedestrians. There can also be a problem with slippery algae / moss growth on pavements beneath evergreen trees in winter due to the lack of sunlight reaching the ground.

Proven track record

Important places like city streets are not the place to experiment with untested species. This is best done in small pockets of the city where, if they do not succeed, they can be replaced at little social, environmental and economic cost.

Speed of growth

Extremely slow growing species will generally not be acceptable. However, species which are very fast growing are also often short-lived. A compromise usually needs to be made.

Life span

Species that will be relatively short-lived in an urban setting (less than 25 years) will not usually be acceptable. Many Australian species fall into this category when used as urban street trees.

Damage to infrastructure

The roots of any reasonably large tree in an urban environment will almost inevitably cause some damage to infrastructure. It's a question of minimising the damage where possible. This can be done to some extent by installing root control systems at the time of planting but ultimately it comes down to species selection.

Pest and disease tolerance

Only trees with few pest and disease problems will be acceptable as street trees.

Structural integrity

Trees with excessive limb-shedding characteristics or very brittle wood are usually not suitable for urban street tree applications.

Shedding characteristics

All trees shed leaves and fruits - it's part of their normal growth cycle. However, this can be more problematic with some species than others. For example, the fruits (nuts) of some species can cause a serious pedestrian hazard. Some species may also cause unacceptable staining of pavements (eg some Eucalyptus species).

More to come...

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