Trees of Washington Park Arboretum: The Wheel Tree

By: John A. Wott

John A. Wott, Ph.D. was a founding faculty member of the Center for Urban Horticulture, University of Washington, and is Director…

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Trochodendron aralioides. Author’s photographs

Trochodendron aralioides. Author’s photographs

Quite aptly named, wheel tree (Trochodendron aralioides) is one of the most unusual trees in Washington Park Arboretum. Its generic name is taken from the Greek words trochos meaning wheel and dendron meaning tree. When seen in flower, it is clear that its name alludes to the numerous stamens that spread out like spokes of a wheel to form the unusual flower. The specific epithet suggests a resemblance to the plants in the genus Aralia.

Trochodendron aralioides is the only species in its genus and the only genus in the family Trochodendraceae; some group the Tetracentraceae, also with only one genus and species, with the Trochodendraceae. Some taxonomists consider the family quite primitive in origin, placing it between the magnolias and the witch hazels. The wood of wheel tree, lacking vessels, resembles that of conifers more than the wood of most broadleaf trees.

In its native habitat, wheel tree is exceedingly handsome; it sometimes starts life as an epiphyte on trunks of Japanese cedar (Cryptomeria japonica), and may remain shrubby for many years. Native to Japan, from northern Honshu southward, in the Ryukyu Islands, and into Taiwan, it is also found on Korea’s Cheju Island. It grows to sixty feet or more in height with trunks as much as sixteen feet in diameter. In cultivated gardens it tends to be much smaller, especially in cooler climates. With its small size and curious flowers best viewed from above, wheel tree is ideally suited for small urban gardens.

The exceptionally neat, spiral arrangement of the foliage catches the eye immediately. Our wheel tree at Washington Park Arboretum sits in the center of the courtyard south of the Graham Visitors Center. Especially eye-catching are the whorls of leaves holding terminal clusters of green flowers at their center.

The leaves are two to six inches long, narrowly ovate or lanceolate, leathery, and shallowly toothed at the upper end; they are dark green above and lighter below. The branchlets are brown to ash colored. The bud scales are papery.

Foliage and flowers of Trochodendron aralioides

Foliage and flowers of Trochodendron aralioides

Ten to twenty flowers are produced on each erect, raceme-like terminal inflorescence. The flowers, lacking petals and sepals, average less than three-quarters of an inch in diameter and are a vivid green. The radiating bright green stamens (often forty to seventy in number) are set around the edge of a green hemispheric disk.

In early summer, the flowers develop into a three-quarter-inch-wide fruit composed of a head of fused follicles in a fleshy brown disc. The seeds are dustlike, ellipsoid with an oily endosperm.

Wheel tree tolerates temperatures down to 14° F; in the wild it can be found in mountainous areas up to nearly 10,000 feet elevation. It requires moisture-retentive soil, and grows best with a pH in the neutral range. It prefers sun but will tolerate partial shade. Once established, it is not easily moved.