Dear Station Members,
HK has practised for years to carry out tree planting on rooftop including the common Podium Planting. Its main objective is to provide greenery for aesthetic purpose & environmental benefits. It is usually done due to lack of ground space for planting after structure development. There are also late recommendation for rooftop planting to cool our city, although this Station has provided comments on such practice some months ago.
For any tree planting on roof, one major factor that the Arborist would need to consider is wind loading. HK has typhoons & it is known that the higher the ground, the stronger would be the wind there. If tall species are chosen for planting on rooftop, the risk of it being blown over can be significant, contour depending. Common staking with guy wires would not hold a tree in typhoon due to the small soil friction offered by such devices. On the other hand, if the guys are stronger than the pull, the trunk or branches would be snapped off in strong wind, completely destroying the tree.
The other factor that the Arborist would need to consider would be the root temperature while planting on a roof. If the roof where the planting area is situated is not adequately insulated against temperature fluctuation, the temperature of the planting medium can become similar to the ambient air temperature. That may mean in winter time when the air is below 5 Celsius on a very cold day, the roots of the tree would also be at around 5 Celsius which would be unfavourable when compared to the same tree planted in the ground. Ground would offer a better buffer in temperature changes than on the roof.
On the other hand, on a very hot summer day when the air temperature is over 33 Celsius, the planter wall or growing medium may absorb heat to further warm up the roots inside to a temperature maybe higher than 40 Celsius, at least partially or on a gradient. Again, would tree roots enjoy growing in such exceeding temperature? Probably not for most species.
Another factor affecting the success of tree planting on roof would be the competition for water & nutrients between tree roots & roots of other smaller plants planted around them. A typical design in HK would be stuffing shrubs & groundcovers next to trees in podium planting, disregarding the competition between the roots of the various species. Success sometimes would base on trial & error. If the tree wins, that's would be good luck for it & bad luck for the smaller plants underneath. However, the tree does not always win.
It can now be seen that any successful tree planting on rooftop depends on a varieties of criteria. It is always Mother Nature Makes the Rules, not us for any successful planting. If a tree is not designed, planted, maintained, inspected & risk managed properly, it is likely to become a liability rather than an asset, no matter where it is planted.
Then would we call the tree planting on rooftop as shown in the attachment seen commonly in HK any success? If even for now, then would be for how long??
The ISA Mission - Through research, technology, and education, promote the professional practice of arboriculture and foster a greater public awareness of the benefits of trees. (http://www.isa-arbor.com/)