Scientific name: Trochodendron aralioides
Synonyms: Trochodendron longifolium
Common name: Wheel tree, parasol tree, birdlime tree, yamaguruma, nagaba-no-yamguruma, kun lan shu, ka-tang-lai
Family: Trochodendraceae
Native Range: Japan (south of Yamagata), Kyushu, Shikoku, Ryuku Islands, Taiwan
Height and Spread: May grow from 60-75 tall feet in the wild with trunks as large as 16 feet in diameter, typically 20-25 feet tall by 10-15 feet wide in cultivation.
Conservation status: Not Evaluated
In the Arboretum: Old grove near Woodland Garden (30-2E, 30-3E), individual trees (23-1E and 25-1E)
Accolades: Great Plants Pick, International Dendrology Society: 2009 Tree of the Year

Since 1838 when Trochodendron aralioides was first described to the west in Flora Japonica by Siebold and Zuccarini, the wheel tree has existed solely as a specimen tree for collectors. (Historical note: In 1840 the wheel tree was re-described unknowingly by another researcher and named Gymnanthus paradoxus. It took two years before another botanist realized this was the same tree.) Recently though, the wheel tree has become more available and is a common tree in public parks and gardens. T. aralioides is a handsome, evergreen tree with deep green whorls of leaves dressing its branches. The flowers of T. aralioides are quite distinctive as upright raceme-like inflorescences consisting of 10 to 20 vivid chartreuse flowers with an arrangement of many stamens (up to 70) radiating out like the spokes of a wheel. In the early summer the flowers produce a disc-like brown fruit consisting of fused follicles containing dust-like seeds. As the fruit dries and cracks open, the seeds are easily and distantly dispersed by the wind.

Botanically, Trochodendron aralioides has been of great interest to the scientific community. Currently, Trochodendron is a monotypic genus, containing just the one species: T. aralioides. Similarly, the family Trochodendraceae only contains one extant genus: Trochodendron. This, however, belies a deep and interesting past, both botanically and evolutionarily.

Early in the evolution of Angiosperms (flowering plants), the development of vessels (long open tubes) in the xylem (water and mineral conducting tissue) of plants came about. This was an important evolutionary step for flowering plants as it allowed larger quantities of water and minerals to be transported quickly into the canopy of a tree. This larger transport makes the large flush of spring growth possible for hardwood trees, not to mention the massive flower and fruit displays of some species.

It has long been known to science that Trochodendron does not contain vessels within its xylem. This originally led botanists to classify Trochodendron as a primitive evolutionary species, developing soon after the Order Magnoliidae (~80 million years ago [Mya]) and much earlier than the appearance of vessels in hardwood plants. This was before dicots and even monocots had arrived on the scene. Later phylogenic studies (studies of the evolution and diversification of species) have shown that Trochodendron is more properly placed in the early Eudicots (synonym Dicots), which is significantly later in evolutionary time and after the development of xylem vessels. These studies along with the latest scanning electron microscopy techniques (which have found clues in Trochodendron structure that vessels once did exist) have led to a theory that Trochodendron first evolved vessels, and then evolved to once again to be vessel-less. One might draw a parallel to the evolution of legs by land animals and then the evolution back to no legs by some mammals such as whales.

In addition to this odd evolutionary occurrence, it may be of interest to local gardeners to know that Trochodendron aralioides used to be indigenous to the Pacific Northwest, growing in the area of the Okanogan Highlands of eastern Washington and Idaho along the southern Canadian border and as far south as Oregon. Fossil records not only show Trochodendron leaves flowers and fruit essentially unchanged dating back 15 million years, but from the Paleocene (66-56 Mya) through to the Miocene (23-5.3 Mya) there were multiple genera present within the family Trochodendraceae (e.g. Nordenskioldia and Trochodendroxylon). Studies of these now extinct genera have confirmed that throughout the dramatic climatic changes that occurred during the last 45 million years, species within Trochodendraceae have existed in a sort of unchanging evolutionary stasis. They appear to have survived for so long by spreading to advantageous habitats, rather than evolving along with changing ecological conditions.

In the 21st century Trochodendron aralioides is native to eastern Asia, growing in mixed broadleaf and coniferous rainforests, sometimes rarely present and other times in dense stands. Its native habitat is the central forests of Japan (south of Yamagata), Shikoku, Kyushu, the Ryuku islands as well as Taiwan. Some historical reports describe the wheel tree being native to the islands and peninsula of Korea, but a recent survey has not found any indigenous populations. The genus name, Trochodendron, comes from the Greek word trochos, meaning wheel, and dendron, meaning tree. The specific epithet aralioides comes from oides, meaning similar to, and Aralia– likely in reference to the similar flowers of Aralia trees.

The wheel tree is hardy to USDA zone 6b (-5 degrees Fahrenheit), but is known to be damaged by cold, drying winter winds. Thus a protective placement within a garden is recommended. The wheel tree may be grown in the PNW in full sun as well as significant shade, but it will not tolerate drought in either location. Native to rainforests, T. aralioides would prefer some watering in the home garden during dry weather spells.
In the Arboretum, Trochodendron aralioides can be found in two areas. The first is on the slope above Arboretum Drive, just to the south of the upper Woodland Garden pond (grids 30-2E and 30-3E). Here there is a grouping of our largest wheel trees, some dating to 1941, just a few years after the creation of the Arboretum itself. Another specimen planted in 1980 is located in grid 25-1E and is just on the north side of the trail intersection above which hangs Union, the sculpture by John Grade.

Sources:

Sun, Y.M. et. al., Phylogenomic and structural analyses of 18 complete plastomes across all families of early-diverging eudicots, including an angiosperm-wide analysis of IR gene content evolution, Molecular Phylogenetics and Evolution, Dec 2015.

Bronckaers, Sven, Trochodendron aralioides, Trees and Shrubs Online, accessed 2/6/2024.

Wott, John A., Trees of the Washington Park Arboretum: The Wheel Tree, Pacific Horticulture Online, accessed 2/6/2024.

Andrews, Susyn. Tree of the Year: Trochodendron aralioides, International Dendrology Society, http://dendrology.org, 2009.

Smith, A.C., A Taxonomic Review of Trochodendron and Tetracentron, Journal of the Arnold Arboretum, Vol XXVI, No. 2, pp 123-142, April 1945.

Manchester, Steven R., et al. Newly recognized diversity in Trochodendraceae from the Eocene of western North America. International Journal of Plant Sciences 179.8 (2018): 663-676.

Manchester, Steven R. et al., Nordenskioldia and Trochodendron (Trochodendraceae) from the Miocene of Northwestern North America, Botanical Gazette, The University of Chicago, 152(3):357-368, 1991.

Li, Hong-Fang et. al., Vessel elements present in the secondary xylem of Trochodendron and Tetracentron (Trochodendraceae), Flora- Morphology, Distribution, Functional Ecology of Plants, Vol 206, Issue 6, June 2011, pp 595-600.

Hergert, H. L., and H. K. Phinney. Trochodendroxylon Beckii Gen. et Sp. Nov. from the Tertiary of Oregon. Bulletin of the Torrey Botanical Club, vol. 81, no. 2, 1954, pp. 118–22.

Trochodendron aralioides, Missouri Botanical Garden, www.missouribotanicalgarden.org