A few failed gardening experiments later and I've been much chastened by trees running rampant, falling over, dying and generally not behaving as expected. Gardening books probably explain this stuff much better than I can, and I should have engaged with them much earlier, but the main lesson I've learned is that plants are living creatures with their own interests, preferences and (in)tolerances. These need to be understood and respected in any garden or landscaping context.
One key factor is trees' response to light. The amount and direction of light a tree receives have a profound effect on its height, shape and health. Leon Fuller, author of Wollongong's Native Trees, recently did some line drawings showing trees' behaviours with different amounts of sunlight, which I've reproduced here. (I am guilty of turning them into these dreadful digital things!)
In general, a tree with unrestricted light from the sun will grow into a naturally broad and spreading shape.
|A happy and well-adjusted tree growing in full sunlight - perhaps a Native |
Celtis(Celtis paniculata) or Native Quince (Alectryon subcinereus)?
|Our friend on the left has full sunlight, while the chap|
on the right has (you need to imagine) been growing
surrounded by other trees that limit its access to sunlight.
|With a fair amount of sunlight, this small tree will grow|
relatively short and bushy.
|Closely hemmed in by tall surrounding trees, this|
one has grown tall and slender as it reaches towards
the limited sunlight.
|It is pretty tempting to plant trees and shrubs right|
up against a wall, but it does compromise their
growth and make it very one-sided.
|Eventually such plants may lean way over and become|
vulnerable to collapse during windy or rainy periods,
or simply under their own weight.
The role of soil, nutrients, rainfall and other variables are also essential, but much harder to illustrate with my basic skills, so they'll have to wait for another time.
Try growing a tree or two, and see if they conform to what's on the label!