Street trees need water AND air (and all the rest of it)!
Peter and Lesley Brenner
All the good planning, traffic calming, pedestrianisation etc are for nothing if street
trees are not planted in a professional way as recognised in the rest of the world.
A detail important to successful street tree planting was brought to our attention
again during a recent meeting with the Head of "Grün Stadt Zürich": street trees in
fact not only need water but - surprise – an assured air supply around their roots.
The latter is just as important as the former. That's why a special mix of substrate
has been devised in Zurich (and elsewhere). It is: 50% humus, 25% river gravel,
25% aerated fired clay pellets (to maintain the capacity of storing air for all times)
known here as “Perlite”.
Where necessary, the Zurich planters insert (sometimes retrofit) air pipes into the
ground to supply the tree roots with air.
As a rule planting holes are 6 square meters (4 m2 is the absolute minimum). If
traffic has to move closer than that, the planting hole is covered by a grid or even a
suspended concrete slab with inserted air vents.
Trees are planted with an absolute minimal clean trunk height of 2.5 meters, but
usually with 3.5m in pedestrian areas and 4.5m in (truck) traffic areas.
They are protected from vandalism by three solid, over 2 meters high timber stakes
close to the trees. These tall wooden stakes (to about 20 cm under the nursery-
established crown and leader structure) are NOT intended to support the tree. The
tree is loosely tied to the stakes so that it can sway just enough to transmit the
movements to the roots that will settle in the ground more firmly as a result.
The tall stakes for the “clean trunk, CPTED sightlines compliant tree” can therefore
stay in place for a good while in heavily used areas such as streets, parks and
parking areas. The use of cost effective timber stakes is regarded as part of a
pivotal social strategy that enables the population (pedestrians and motorists) to
acclimatise to the role and benefits of dappled shade street trees.
The new tree trunks are also wrapped in hessian for sun protection in their earlier
years. Over time the hessian disintegrates and the trunks are left bare.
Unfortunately the technical part of tree planting in Hobart is totally underdeveloped
and the "best practice standards" they apply here are not backed anywhere in the
world where trees are used as an important infrastructure investment. It is
embarrassing that this contemporary wisdom and practice is so defiantly rejected in
a Capital city such a Hobart.
Instead, low height stakes and low branch methods are vehemently and publicly
defended by HCC officers as being the only relevant guidelines for planting trees in
HCC trees get vandalised because they are not planted as serious assets. This
results in the use of costly on-going maintenance of ripped off vandalised branches,
which are obviously intentionally accessible to the walking public. Wilful vandalism
will always be around, but officially creating a tree stock of damaged trees, failing
all liveability functions from the outset, cannot seriously be seen as good practice.
All this is otherwise known as a waste of money and an obstacle to liveability
What really needs to be achieved is that the Gregory St (and other) project(s) could
at last become an example for proper street tree planting in Hobart.
There has been a lot of professional response out there since we won the two PIA
awards in Community Based Planning (in 2004 – 05) with our Liveability Slide Show
CD. However, the Council tree planting practitioners are fixed on antisocial and
In Hobart well over a hundred undersized trees have recently gone into footpaths,
squashing pedestrians out and inviting vandals in. The exact opposite of what street
trees are meant for. Since the Council staff and Councillors are not the patient and
knowledgeable educators of the public, nurseries should become the strong lobby
group for up-to-date greening expertise.
HOWEVER, before engaging in honing a sales pitch it is crucial, that the subject
matter is understood 100%, based on the best available successful models. And
this means getting to know all the social, economic, public health, tourism, traffic
calming etc reasons and techniques behind it. It also means that the Tasmanian
inner resistance to looking at Europe has to be overcome. Vancouver, Seattle, San
Francisco, China, Hong Kong etc, etc have now come around to taking the
European experience on board to correct earlier failed planting ventures. The UK is
still behind the rest of Europe.
And worry not, there is no language barrier. Most important documents and
research is available in English.
Street trees are not optional embellishments. They are powerful agents of social,
economic and environmental regeneration, and vital contributors to the health and
wellbeing of cities and its citizens. They are as essential as a roof on a house.
Right in spirit, wrong in execution Dismal lack of street trees in Hobar,t
(not enough clean trunk height) even though there is plenty of room.
Liveability Initiative, email@example.com Last update May 07