A rarely visited corner at the Arnold Arboretum is nestled beneath the tall stone wall that separates the hickory collection from traffic on Centre Street. In late summer, the area feels otherworldly. The heavy overstory filters the light and cools the air; the humidity seems to increase; and densely planted shrubs block out the surrounding views and noises. The corner is dominated by a planting of seemingly colossal hybrid wingnuts (Pterocarya × rehderiana), with their drooping Spanish moss-like fruits and twisted forms. Standing next to their large multistemmed trunks can make you feel miniature.

Wingnuts are closely related to hickories (Carya) and walnuts (Juglans). There are six species of Pterocarya, with native ranges clustered in China, Japan, Southeast Asia, and the Caucuses. In addition to cultivating representatives of five of the six species, the Arnold Arboretum has eight specimens of this unusual hybrid, all of which grow in this out-of-the-way corner.

The oldest of the eight originated at the Arboretum from seed sent, in 1879, by Pierre Alphonse Lavallée of the Arboretum de Segrez, outside of Paris. At the time, the Arboretum de Segrez was one of the largest in the world (and a noteworthy landscape where Marcel Proust once suffered an asthma attack but still managed to write a poem about its beauty). Lavallée collected the seeds from a Chinese wingnut (P. stenoptera) in his arboretum, and, once they germinated in Boston, the seedlings were planted along Centre Street.

Two decades later, Alfred Rehder, an Arnold Arboretum taxonomist, noticed that the trees didn’t look quite like the Chinese wingnut. “The trees in the Arnold, known as Pterocarya stenoptera … I can no longer consider, after much study, as the real species of that name,” Rehder wrote to the German Dendrological Society in 1903, “but now consider [them] a cross between this and P. fraxinifolia [the Caucasian wingnut], which in its characteristics almost exactly stops between the two species.”

Rehder hypothesized that pollen from a Caucasian wingnut growing at the Arboretum de Segrez must have landed on the flowers of a Chinese wingnut growing nearby. We don’t know who collected and brought the Chinese and Caucasian wingnuts to Paris, but it may well have been the first time that the two species, normally separated by the thousands of miles between the Caucasus Mountains and eastern China, were growing in the same place.

Rehder conferred with Camillo Schneider, a taxonomist working at the Vienna Natural History Museum, who agreed with Rehder’s assessment. Based on their correspondence, Schneider published the first botanical description of the new hybrid in 1906. Writing in German in the Illustriertes Handbuch der Laubholzkunde, he identified the unique characteristics of the buds and rachises of the “Bastardes” growing at the Arnold Arboretum and officially named the hybrid for his friend, choosing the Latin name Pterocarya × rehderiana.

Four trees (accession 1191) from Lavallée’s 1879 shipment still grow along the Centre Street wall, hidden behind the hickory collection. In addition, four neighboring trees (23119) were accessioned as seedlings from the original trees. When the wingnuts fruit in midsummer, they offer a dazzling display of long, pendulous clusters of winged nutlets (hence the common name) that dangle from what seems like every branch. One particularly large specimen, accession 1191*E, has an incredible form, with leaders that shoot up more than 125 feet and droop over the Works Progress Administration-constructed bus shelter on Centre Street.

As with many hybrids, Pterocarya × rehderiana seems to display hybrid vigor and, according to Rehder, are “much hardier and more satisfactory than their supposed parents.” A windstorm in October 2020 took out one of the leaders from accession 1191*E, but overall, the hybrids don’t seem terribly affected by the cold New England winter, even after more than 140 years growing at the Arboretum. While the hybrids are a product of a chance cross that would likely have never been possible in the wild, the trees have more than claimed their uncanny home.

Jared Rubinstein is an associate project manager at the Arnold Arboretum. For more on the taxonomic history of the Rehder wingnut, see his 2020 article with Michael Dosmann in Novon, issue 28(4).

Citation: Rubinstein, J. 2021. Otherworldly Wingnuts: Pterocarya × rehderiana. Arnoldia, 78(5-6): 68–69.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

It takes resources to gather and nurture these new voices, and we depend on the support of our member-subscribers to make it possible. But membership means more: by becoming a member of the Arnold Arboretum, you help to keep our collection vibrant and our research and educational mission active. Through the pages of Arnoldia, you can take part in the life of this free-to-all landscape whether you live next door or an ocean away.

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