American Sycamore (Platanus occidentalis), while not the tallest tree, is considered the most massive tree as defined by its circumference in the entire eastern half of the United States, where it is native including all of Ohio.
This species is easily identified by its height, its spreading canopy with several massive branches, and its white bark in winter. The paths of creeks and rivers can be easily seen from a distance in winter by following the white bark of barren Sycamore canopies. While many Ohio landscapes contain its European cousin, London planetree, which is more resistant to anthracnose, this tree towers above most others in native environments. Also known as sycamore or American planetree, this tree easily reaches heights of 80 feet tall and 60 feet wide but can grow much larger.
As a member of the plane tree family, it is closely related to other planetrees, and distantly related to sweetgum. American Sycamore prefers deep, moist, rich soils of variable pH that are either well-drained or moderately drained, as it is often found naturally along the edges of bodies of water, and in wet areas of fields or woodlands. It tolerates much drier and poorer soils but does not like the city pollution that London planetree survives. It is found in zones 4 to 9, in full sun to partial sun.
American sycamore is very prone to annual infections of anthracnose, a fungus that destroys the new growth in spring. This causes dieback of the emergent leaves and stem, and subsequently the lateral buds break (either at the base of the new growth, or from previous year’s branchlets) and form a whorled pattern of new stems, which resemble witches’ broom growth. This secondary growth occurs in late spring and usually becomes the growth of the season, as drier weather does not encourage new fungal growth. Other diseases and pests may occur on American sycamore, but the most serious problem after anthracnose is usually hollow trunks, which eventually make the tree subject to storm damage, and of course getting too big for its space in urban areas. In both cases, the tree may need to be removed.
American sycamore has leaves of variable shape. Some have leaf bases that are cordate, hanging well below the point of attachment to the petiole, while others have leaf bases that are truncate. Leaves are medium green, fuzzy in spring when they emerge, and have three to five lobes that vary greatly in the number and size of incisions (large teeth) on the lobes, ranging from many to few. Anthracnose, the major disease of wet springs for this species, causes the terminal stem and leaves to die back, forcing a second round of shoot emergence in late spring. American sycamore is monoecious, with male and female flowers on the same tree but hanging from different stalks in mid-spring. American sycamore differs from the closely related London planetree in that the native tree usually has only one fruit hanging from a long stalk, whereas the non-native tree usually has two or three fruits hanging from a long stalk. Twigs of American sycamore are zig-zag with prominent winter buds. Branches ascend in the upper canopy with a rapid growth rate but are pendulous in the lower reaches. Branches regularly peel off large sections of gray-brown bark in mid-summer, revealing a smooth, white interior bark that then becomes the exterior bark. This coloration is retained in winter, when the low angle of the sun reflects brightly from the white branches. With maturity, the lower trunk retains more of its plates each year, creating a mosaic of gray, green, and brown patches that contrast with the decreasing amount of white inner bark.