Thursday, 15 February 2018

Try growing: happy, well-adjusted trees!

Until I actually started gardening my thinking about trees, and plants in general, while full of affection and interest, was that they were basically green furniture, things you planted in the appropriate spot and left to grow into whatever was shown on the label. 

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)?
Even a naturally tall and narrow tree will still grow broader in full sunlight than if it has only restricted access to sun. 
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.
A tree growing in a forest, surrounded on all sides with other trees, will grow taller and narrower than one in a clearing away from the shade of surrounding forest.
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.
Similar principles apply to trees that are grown in gardens where their access to sunlight is limited for part of the day. A special case is that of trees grown near tall fences or walls. These trees are in effect shaded on one side, and will grow sideways towards the light. They may end up leaning out from the wall towards the sun, and if their roots are prevented from spreading out to support the plant, they may eventually fall over.  
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 classic native trees that illustrate this behaviour are eucalypts, fast-growing species most of which need a lot of sunlight. In even part shade conditions they tend to leap up very quickly to form tall, narrow spar-like shapes, and only start thickening out once the crown has found its own place in the sun. Grown in gardens with shade for part of the day, most eucalypt species will develop a pronounced or even alarming lean towards the sun. 

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!   


No comments:

Post a Comment