The London Plane

Natalie Secen, The Halifax Tree Project

2020-09-29

The London plane (Platanus x acerifolia) is a hybrid between the American sycamore (Platanus occidentalis) and the Oriental plane (Platanus orientalis), and is speculated to have been discovered in Europe in the mid 17th century (whereabouts exactly is unclear). From around this time, it thrived as a boulevard tree in Paris which inspired its installation along London streets during the industrial revolution - As you would expect, the air quality was poor and heavily polluted. One could say that the London plane’s natural habitat was “urban” from the get-go, and it’s still an extremely popular street tree all throughout Europe. If you’ve walked Las Ramblas in Barcelona, you have witnessed a fine example of this. The species has since made its way to North America, and is now a very popular urban tree worldwide.

But why is this 17th century discovery such an urban champion? For one thing, the bark isn’t beautiful by accident. It’s speculated that as the bark breaks away in large flakes, exposing the camouflage pattern, it rids itself of pollutants. It also requires relatively small amounts of soil volume to survive and can grow in a variety of soil types. In urban areas where visibility, size, and leaf litter are an issue, the London plane is a good option because of its strong candidacy for pollarding. This is a high maintenance pruning technique where the main branch system of a tree is heavily reduced to short stubs (see photo, captured by Peter Duinker in Bordeaux). While it doesn’t contribute shade like a natural growing tree, a pollarded tree is certainly better than no tree at all!

The unique, camouflaged bark of a London plane tree makes it easy to identify from afar in all seasons. Even for those without an interest in the urban canopy, this one is hard to miss. From closer up, with three to five lobes the leaves look similar to that of a maple (hence the “acerifolia” in its latin name), but all maples have opposite branch structure whereas a London plane’s branches are alternately structured. To me, it is most easily confused with one of its hybrid components, the American sycamore. While the bark of a London plane is slightly duller than that of a sycamore, there are a couple of other things you can turn to for differentiation. Firstly, the central lobe of a London plane leaf is as long or longer than it is wide (whereas on a sycamore, the central lobe is wider than it is long, and the notches are shallower). Secondly, the fruit of an American sycamore is aggregated into a single round ball. But the moment you spot two or three aggregates hanging off a single stem, you know it’s a London plane (note that sometimes, there’s only a small handful of double aggregates on the whole tree)! Overall, if you see camouflage bark in an urban environment, odds are good it’s a London plane. Sycamores prefer deep, rich, moist soil, and tend to stick to more naturalized habitats. Similarly, you’ll rarely find a London plane that’s escaped its urban environment. Here in Halifax, we certainly have more London planes than sycamores in the urban streets, but the sycamores are still out there so be sure to double check using your new ID tricks!