Sycamore (Acer pseudoplatanus) and Norway maple (Acer platanoides) are by far the commonest maples in Dulwich, as elsewhere in the country, but they can be difficult to tell apart. This article suggests a way of doing so, using the “rule of three” (that an ID can only confidently be given if a species can be distinguished from others in at least three different ways). In this case the three features I suggest are: differences in bark, in flower and fruit, and in leaves.
Both trees have smooth grey bark when young, but they develop very differently. The bark of Norway maple remains grey throughout its life, developing closely corrugated ridges in mid- and later age. A mature sycamore develops a much browner trunk, with the bark flaking into small square plates, revealing a pinkish or orange under-bark. This can give the trunk a beautiful polychrome appearance after rain, somewhat similar to the trunk of a plane tree.
Flower and fruit
Norway maple flowers relatively early, usually a full month before sycamore. Its flowers are held above the leaf and have a characteristic acid-yellow colour. Sycamore flowers are yellow-green and held on hanging 6-12 cm tails below the leaf. Similarly, the fruits of sycamore hang below the leaves whereas those of Norway maple are usually held above. Another difference between them can most clearly be seen when their winged seed-cases fall: the two wings of a Norway maple seed-case are held much further apart than those of a sycamore.
Sycamore leaves are darker green, have shallower lobes and are more coarsely toothed than those of a Norway maple, which have a much spikier outline. The autumn colour of a Norway maple is yellow-green to bright yellow, whereas that of a sycamore is brown-green to brown. In winter, the buds of sycamore are green whereas those of Norway maple are dark red-purple, occasionally blackish. Tree books usually mention that the leafstalk of a Norway maple exudes a milky latex when broken, unlike a sycamore’s. That is true, but the latex is sparse when compared with other latex-producing plants, such as spurge, so that is not always an easy distinction to make in practice.
Norway maple and sycamore both naturalize abundantly, some would say far too abundantly in the case of the latter. The nursery trade has produced many varieties of both species but they all display the characteristic differences set out above. As sycamore is an unpopular tree, there are few fully mature examples planted in public places in Dulwich. Perhaps the best local example is in Brockwell Park, to the south-east of the tennis courts. A good place to see Norway maple, a common tree in Dulwich, is the island between the junction of Alleyn Park and Hunts Slip Road. There are also plenty of naturalized saplings in the cuttings by the nearby railway track, which regenerate successfully after each “leaves on the line” clearance.