Species Spotlight: Quercus hypoleucoides A. Camus

For this Species Spotlight we train our follow spot on an oak that is quite a star of the quercine scene: Quercus hypoleucoides (stage name: silverleaf oak). It has received several rave reviews: “very rare and remarkable” (The Hillier Manual of Trees and Shrubs), “this high-altitude oak [is] a beautiful ornamental tree” (Shaun Haddock), “one of the most beautiful oaks we have had the opportunity to observe” (le Hardÿ de Beaulieu and Lamant), “our favorite oak” (Cistus Nursery). And like many a star of stage and screen, the secret of its success lies in a hidden talent: the silver underside of its leaves.

Silverleaf oak is a small evergreen tree, usually 6 to 10 m tall, though specimens up to 26 m have been recorded. It can be a shrub in very dry, exposed places, forming extensive thickets. It tolerates drought and is particularly resistant to low winter temperatures for a broad-leaved evergreen. Its leaves are thick and leathery, narrowly oblong to lanceolate, with entire, revolute margins or with an occasional tooth. Their upper surface is dark green and shiny, while beneath they are densely and permanently white and tomentose (for the technically minded: covered with fasciculate sessile trichomes; in the non-technical terms favored by Sean Hogan: “bright as a freeway reflector”). The new growth is pink above and yellowish below, both surfaces covered in indumentum. The species belongs to the Lobatae (Red Oaks) section, and acorns (ellipsoidal to oblong, sometimes striped, and chestnut brown when mature) can ripen in the first or second year. Quercus hypoleucoides grows principally in northern Mexico (Chihuahua, Coahuila, and Durango) but also in southwestern United States (Arizona, New Mexico, and Texas). It is found in pine-oak forests, often in association with Pinus cembroides, at elevations between 1,100 and 2,700 m.

he name Quercus hypoleuca (from Greek hypo “beneath” and leuca “white”) was used by Engelmann in 1875 to describe the species, which earlier had been confused with Q. confertifolia Bonpl. by Torrey (1859). It is clear that Torrey did not intend to coin a new name, he only thought the specimens he listed belonged to Q. confertifolia Bonpl. (now considered to be Q. crassipes). In any case, Q. hypoleuca was an illegitimate name because it had been used by Miquel in 1855 for what was later to become Lithocarpus korthalsii. Aimée Camus set the record straight in 1932 in the Bulletin of the National Natural History Museum of Paris by introducing the name Q. hypoleucoides to replace previous illegitimate names (the ending ‑oides means “resembling”).

Writing in 2006, le Hardÿ de Beaulieu and Lamant noted that in spite of its attractions, it was almost impossible to find this oak in cultivation, a lack they ascribed to the inaccessibility of most populations and to reduced acorn crops in recent years due to drought. Though still rare a decade later, specimens of silverleaf oak can be found in several collections around the world. A tree originally thought to be Q. hypoleucoides growing in Hillier Gardens since 1968 is now considered to be a hybrid with Q. emoryi. It was registered as a cultivar by Eike Jablonski in 2012, under the name Quercus ‘Piers Trehane’, in honor of the British horticulturist and plantsman, longtime IOS member, ICRA registrar for oaks, and creator of the www.oaknames.org website. The cultivar is more vigorous than its parents, and though it has a similar white tomentose underside to its leaves, the gray hairs rub off easily on Q. ‘Piers Trehane’, while on the leaves of Q. hypoleucoides they are firmly attached. Several specimens of the cultivar are growing in collections in the United Kingdom.

Silverleaf oak was one of the many attractions in the recent IOS Tours to New Mexico (2016 and 2017), and it appears to be gaining in popularity. According to Sean Hogan it has been successfully planted at low elevations in the U.S. Southwest and along the West Coast, and it also thrives in Denver Botanical Gardens, suggesting it may be one of the few broadleaved evergreens successful below zone 6.

his spectacular oak has evidently achieved star status among the cognoscenti. Time will tell whether it will break into the mainstream.

With thanks to Allen Coombes for assistance with matters nomenclatural.

BIBLIOGRAPHY                    

Amory, Michael Heathcoat. The Oaks of Chevithorne Barton. London: Adelphi Publishers, 2009.

Beaulieu, Antoine le Hardÿ de, and Thierry Lamant. Guide Illustré Des Chênes. Paris: Éditions du 8ème, 2006.

Camus, A. Les Chênes. Monographie des Genres Quercus (Texte: Tomes I, II, III; Atlas: Tomes I, II, III) et Lithocarpus (Texte: Tome III; Atlas: Tome III). Paris: Paul Lechevalier Editeur. 1936-1954.

Coombes, Allen J., and Zsolt Debreczy. The Book of Leaves: a Leaf-by-Leaf Guide to Six Hundred of the World's Great Trees. Chicago: The University of Chicago Press, 2011.

Haddock, S. The Fifty Shades of Gray Garden: Part Two. Hortus 117 (Spring 2016): 65-75.

Hillier, John, et al. The Hillier Manual of Trees & Shrubs. London: The Royal Horticultural Society, 2014.

Hogan, Sean. Trees for All Seasons: Broadleaved Evergreens for Temperate Climates. Portland: Timber Press, 2008.

Jablonski, E. Quercus ‘Piers Trehane’ - A New Oak Cultivar. International Oaks No.23 (2012): 109-111.

Romero Rangel, S., E.C. Rojas Zenteno, and L.E. Rubio Licona. Encinos de México. Mexico City: Universidad Nacional Autónoma de México, 2015.