…because it's only protecting the trunk, not the root zone.
|Click photo for a closer look|
The Australian Standard says the radius of a TPZ should be twelve times the Diameter of the trunk at Breast Height (DBH). This tree** is about 200mm at breast height. Twelve times 200mm equals 2.4m.
That means this TPZ should be 4.8m... four times the size it actually is!
If you can't be bothered with all that, just think "TPZ should extend beyond canopy".
It's a pretty good rule of thumb.
But wait, there's more.
This TPZ is also inadequate because it doesn't provide any temporary irrigation to get the tree through the construction phase. I talk more about that here.
If we don't do these things we're just pretending. We have to ask ourselves: do we actually want to preserve trees or do we just want to be seen to have tried?
* Apart from anything else, star pickets and orange mesh doesn't cut it. If you're going to be serious it has to be proper site fencing.
** In this case I must admit I'm not too fazed. It's a Bottlebrush on a wide verge with no overhead powerlines and y'all know what I think about THAT :)