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Species Spotlight: Quercus rugosa Née

Cupped leaves of netleaf oak (Quercus rugosa

According to the Guide illustré des chênes, Quercus rugosa is found in 27 of the 32 Mexican states, making it the most widely distributed of the Mexican oaks. Not content with dominating Mexico, it spills over national boundaries, southwards into the highlands of Guatemala and northwards into the Southwestern United States, where it is found in Arizona, New Mexico, and trans-Pecos Texas. It occurs in upland areas next to deserts or in moist high-altitude forests. Sources differ as to the altitude range, placing it between 1,700-2,000 m and 2,900-3,500 m above sea level.

It is as wide-ranging in form as in distribution: evergreen or semievergreen, a tall, suckering shrub or medium tree up to 20 m tall (depending on available moisture), with a broadly conical to columnar or spreading habit. Its leaves, one of several attractive features, are especially variable: in dry conditions they are barely 4 cm long, but can be as long 23 cm in moister climates. Their salient feature is the lustrous, dark green upper surface, coriaceous and sometimes bearing sparse pubescence.  In shape they are mostly obovate or pandurate (fiddle-shaped) but can also be circular or elliptical, rarely strictly obovate. They are rigid in texture and usually cupped, and this convexity coyly conceals their principal ornamental asset: a wooly indumentum on the underside of the leaf that ranges from nearly white to cream to golden. The leaf margins are thick, almost like cartilage, and slightly revolute, with mucronate teeth near the apex, which is broad and rounded. On emerging, they are covered with dense tomentum, dark red above with veins contrasting in yellow, and pale cream below.

New growth covered with dark red indumentum (click on images to enlarge)

Venation is conspicuously impressed and particularly prominent on the underside of the leaves. Above, the secondary veins contribute to the epithetical rugosity, creating a leathery, almost puckered effect, while below the reticulate pattern of veins is responsible for one of its synonyms (Q. reticulata) and its common name in English: netleaf oak. Acorns are borne in groups of up to three on a long peduncle to 10 cm or more, and are between half to three-quarters enclosed in the cupule, which is cup- or saucer-shaped. The mature acorn turns brown or reddish brown and the flesh is a purplish pink, a distinctive characteristic. The acorn cup is covered in oval, loosely-appressed, often pubescent scales, which makes the surface look warty. The flaky bark of netleaf oak is another attractive feature.

Underside of a Quercus rugosa leaf, showing pale yellow indumentum and reticulate pattern of prominent secondary veins

Quercus rugosa was first described by Luis Née, the French-born Spanish botanist, who collected it in 1791 in Mexico while on the famed five-year Malaspina expedition. He published a description of the species in 1801, in the Anales de Ciencias Naturales, and it is one of 12 currently accepted oak names described by Née in that publication. According to the Guide illustré des chênes, the epithet rugosa, (meaning wrinkled, from ruga, Latin for wrinkle, no etymological connection to the English word “rugged”), refers to the dense indumentum on the underside of the leaf, but this appears to incorrect, as Née’s description clearly refers to the coriaceous upper surface of the leaves when using the Spanish word “rugosas”. Humboldt and Bonpland published their description of Q. reticulata in 1809, and it was under this name that it was first introduced to Europe 30 years later. Despite its early introduction, it remains rare in cultivation, but it can be found in specialist collections around the world.

A Q. rugosa acorn split open to reveal the purple flesh of the cotyledons, Centro de Educación Ambiental Ecoguardas, Mexico City, Mexico 
© Charles Snyers

Sean Hogan describes netleaf oak as a “tough plant,” hardy to -18° C, and colder in the case of plants sourced from high altitude. The Hillier Manual of Trees & Shrubs says it is “hardy given reasonable shelter,” and indeed Michael Heathcoat Amory reported “it is one of the white oaks at Chevithorne Barton we have had difficulty getting established,” while at Starhill Forest in Illinois, USA it is cut to the ground each winter and survives only through annual shoots from the base. However, it has thrived in arboreta in the milder parts of the United Kingdom and France. Despite being described as slow-growing by Hillier, it has put on impressive growth in some collections: at Hackfalls Arboretum in New Zealand, a specimen planted in 1985 had reached 16 m in 2004 and an estimated 25 m by 2016; in Grigadale Arboretum in Argentina, a tree planted in 1998 has reached close to 14 m, in the last few years growing about a meter per year on average.

Quercus rugosa in Hackfalls Arboretum, New Zealand, grown from seed collected near Arroyo Zarco, Mexico State, Mexico © Bob Berry

In Mexico this oak is prized for its medicinal properties: the bark is used to treat dysentery, toothache, and hemorrhages, and to strength the teeth. Tea brewed from its leaves alleviates muscle pain and coughing. The acorns are toasted for human consumption, or fed raw to cattle, pigs, and goats, or used to make a coffee-like drink. The wood is used to manufacture tools, as pulp for paper, and as firewood and charcoal.

Suspected Quercus rugosa hybrid, photographed during the 2017 IOS New Mexico Tour, Mogollon Mountains, near Glenwood 
© Rebecca Dellinger-Johnston

Quercus rugosa is known to hybridize in the wild with several white oaks: Q. ×basaseachisensis, originally described as a species by C.H. Muller, is believed to be the cross of Q. rugosa and Q. depressipes; other hybrids occur with Q. laeta, Q. arizonica, and Q. obtusata (an oak in the Chevitorne Barton collection is thought to be Q. obtusata × Q. rugosa, with leaves that are less convex and have on occasion produced very good autumn color). Quercus obtusata is in fact a very similar species, but it can be distinguished from Q. rugosa by comparing the mucronate teeth on the leaf margins, which are shorter in Q. obtusata than in netleaf oak, where they can reach 3 mm. On the trip to New Mexico led by Michael Meléndrez in 2017, several oaks were observed that appeared to be hybrids of netleaf oak, possibly with Q. toumeyi and with Q. grisea. Another suspected hybrid, though of cultivated origin, is Q. ×warburgii, a putative cross with Q. robur. Most of the known specimens are thought to originate from trees distributed by a nurseryman named Smith of Worcester, UK in the 1870s. He received seed from the Genoa Botanic Garden in 1869 under the name Q. rugosa, and propagated the issue as grafted plants under the name Q. rugosa genuensis. Subsequently it was referred to as Q. obtusata, then in 1933 E.F. Warburg proposed Q. genuensis, and six years later Aimée Camus described it as Q. warburgii. While both Warburg and Camus suggested it may be a hybrid, neither proposed Q. robur as the other parent. Hillier describes its leaves as being “remarkably like those of Q. robur,” and it is now listed as being Q. robur × rugosa on the IOS’ Oak Names Checklist and in the Guide illustrée des chênes.

Leaves and acorn of Quercus ×warburgii, Grigadale Arboretum, Argentina

An oak often associated with Q. rugosa, though more likely to be Q. greggii, is known unofficially as “La Siberica” or “La Siberia” form. It was collected by Sean Hogan of Cistus Nursery (Portland, Oregon, USA) in the mountains of southern Nuevo León, Mexico, close to the border with Tamaulipas, and originally thought to be a form of Q. rugosa, with fewer marginal teeth and a paler indumentum on the undersurface of the leaves. In his book Trees for All Seasons, Hogan describes the leaves as “particularly attractive, deep green and fluted, with a very warm, creamy golden indumentum on the underside.” Trees from acorns of this form have been grown successfully as street trees in Portland, Oregon where they have reached about 8 m with a rounded crown. Though Cistus Nursery still advertises it on its website as “Quercus aff. rugosa”, where they have reached about 8 m with a rounded crown. Though Cistus Nursery still advertises it on its website as “Quercus aff. rugosa”, Allen Coombes believes it to be Q. greggii and Sean Hogan described it as such in a 2015 newsletter of the Pacific Horticulture Society. There seems to be confusion regarding the name of the form: according to Hogan, it was named after the small mountain village close to where it was collected: La Siberia (so called, one assumes, in allusion to its frigid winters). The plant often appears as “La Siberica”, but the confusion may have arisen in association with the botanical epithet meaning “of Siberia”: sibirica. In any case, the village in Nuevo León, situated at 2,617 m above sea level and boasting a population of 216, is definitely called Siberia.

La Siberia form of Quercus greggii, originally thought to be Q. rugosa © Heritage Seedlings

Given its rugged beauty and hardiness, Quercus rugosa is recommended for gardens, parks, and arboreta in most regions. As it grows naturally in such an extended area and across a variety of habitats and altitudes, there is also scope for further experimentation with seed sourced from different provenances.


Photos © Roderick Cameron unless specified

BIBLIOGRAPHY

Beaulieu, A. le Hardÿ de, and T. Lamant. Guide illustré des chênes. Geer, Belgique: Edilens, 2010.
Berry, R. Hackfalls Arboretum, Catalogue of Plant Collection. Privately published, 2016.
Coombes, A. J. The Book of Leaves. London: Ivy Press, 2017.
Grimshaw, J., and R. Bayton. New Trees: Recent Introductions to Cultivation. Kew: Kew Publications, 2009.
Heathcoat Amory, M. The Oaks of Chevithorne Barton. London: Adelphi, 2009.
Hogan, S. “Oaks are the Answer!” Pacific Horticulture. Fall 2015.
Hogan, S. Trees for All Seasons: Broadleaved Evergreens for Temperate Climates. Portland, Or.: Timber Press, 2008.
Hillier, J. The Hillier Manual of Trees & Shrubs. London: Royal Horticultural Society, 2014.
Née, L.,"Descripción de varias especies nuevas de encina", Anales de Ciencias Naturales, nº 3, (1801): 260-278.
Romero Rangel, S., E.C. Rojas Zenteno, and L.E. Rubio Licona. Encinos de México. Mexico City: Universidad Nacional Autónoma de México, 2015.