Pollarding
If you scan the street trees of San Francisco, you’ll see some odd-looking trees, primarily Platanus × hispanica. They’re short and squat with bulbous branches sticking out the top, often surrounded by a mass of twigs. They look nothing like the stately giants you’d expect from the age of the tree.
What’s going on?
It turns out to be due to pollarding - a fascinating arboreal technique to shape the development of trees.
It’s relatively basic - on a schedule, depending on the tree, you pick a set of points - for Platanus × hispanica, typically on 3–5 branches up from the trunk - and you cut the tree back to these points. The tree starts to grow a profusion of new branches out of the bark in the area, which gradually thicken and produce distinctive heads. If you repeatedly prune back to these heads, you’ll get consistent shape and growth out of them.
This is in contrast to “coppicing”, which cuts a tree down to near ground level and lets it regrow from there. This is the root of the term copse, from dense stands of trees that were coppiced to encourage specific growth.
In antiquity, pollarding could be used to make trees a more easily renewable resource - encourage the growth of firewood or a profusion of nuts, at a height safely above livestock (cutting lower might make the shoots coming out of the stump a tempting treat). The technique was mentioned by the ancient Romans and probably known even before that.
Today, it seems to be more commonly used for space management - a pollarded tree has a much more constrained height and width (one that can be repeatedly maintained). It’s also used for aesthetics, to create a particular shape or size of tree.
It’s a bit of a one-way gate - once you pollard, you should continue to do so, given the changes in the load-bearing structure of the plant.
Pollarded trees stay in a more juvenile state and are less susceptible to wind (being less big and branchy, to be scientific), and so tend to actually live longer.
For arborary.world, an interesting side effect is that our age inference based on crown size and height is wildly inaccurate for pollarded trees! We’ve had to explicitly start annotating these - and as with other trees that are substantially human-shaped - deriving facts from the raw shape is much more difficult.