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Carlos collecting Quercus ×alentejana (Q. faginea × Q. pyrenaica) in northeastern Portugal for his PhD thesis © Carlos Vila-Viçosa
An interview with Portuguese oak conservationist Dr. Carlos...
Amy Byrne | Apr 19, 2024
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Roderick Cameron | Apr 13, 2024
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It was a great pleasure for me to be able to write about my...
Gert Fortgens | Feb 15, 2024

Plant Focus

Quercus crassipes acorns with inrolled cupule margin
One of the more well-known Mexican oaks in cultivation.

Hybrid Highlight: Quercus ×jackiana C.K. Schneid.

Quercus ×jackiana, or Jack’s oak, is a naturally occurring North American hybrid. Its parent species, Q. alba and Q. bicolor, share overlapping ranges from Missouri north to Wisconsin and east into New England. Although individual hybrids are not exceedingly common, Jack’s oak is fairly well known among foresters, botanists, and oak enthusiasts.

Leaves and acorn
Late summer foliage with nearly mature acorn © Ryan Russell

Leaves of Q. ×jackiana are medium green above with whitish tomentum below as in Q. bicolor, generally with 7–9 rounded lobes with shallow sinuses as in Q. alba. Bases are rounded to cuneate with a yellow petiole and midrib. Flaky, peeling bark can be found on young stems and branches, as in Q. bicolor. Cupules are thick and warty in appearance like Q. alba, with a longer (but stouter) peduncle like Q. bicolor. Acorns are light brown, glabrous, 3 cm long by 2 cm wide. (This description is based on a mature, wild-source individual of Q. ×jackiana. Of course, variations on this description should be anticipated, as with all hybrids.)

Acorns
Mature acorns and caps (note the elongated but stout peduncle) © Ryan Russell

This hybrid can grow to massive proportions. The veteran tree in the photo below is 29 m tal and 30 m wide with a trunk circumference of 4.5 m, and is likely ~200 years old.

A mature Quercus xjackiana
A mature Quercus ×jackiana, located in Boone County Missouri © Ryan Russell

Selections of Jack's oak have been made as well. The cultivar Q. ×jackiana ‘Guy’ was selected by Dirk Benoit from a tree growing at Starhill Forest Arboretum (Petersburg, Illinois, U.S.) and named in honor of its founder and fellow IOS member Guy Sternberg. Another cultivar, Q. ×jackiana ‘Jefmir’ (Admiration®) was selected from a mature street tree found in Saskatoon, Saskatchewan, Canada.

Leaves in fall
Fall color on Q.×jackiana (note rounded lobes with shallow sinuses) © Ryan Russell

The hybrid's name was originally published as Q. jackiana (sans hybrid notation) in Illustriertes Handbuch der Laubholzkunde by German botanist Camillo Karl Schneider in 1904, as the hybrid Q. platanoides (a synonym of Q. bicolor) × alba. The epithet honors Canadian-born American dendrologist John George Jack, who reportedly first found the hybrid in 1894. Schneider wrote that, according to Alfred Rehder, Jack discovered it in his home town of Châteauguay, Quebec, then a farming community and now a suburb of Montreal on the south shore of the St. Lawrence River; according to Palmer (1948), what he considered to be the type was collected by Jack in Boston's Franklin Park in 1895, and a herbarium specimen, verified by Schneider, is at the Herbarium of the Arnold Arboretum.

 

Trunk
Trunk of a mature Quercus ×jackiana © Ryan Russell 
Twig
Flaky bark on smaller branches is indicative of Quercus bicolor parentage. © Ryan Russell
Possible type of Q. xjackiana
Type (?) specimen of Quercus ×jackiana, collected by J.G. Jack in Franklin Park, Boston, 1895

Further reading

Jablonski, E., and R. Russell. New and Recently Published Cultivars. International Oaks 29: 113–124. (Quercus ×jackiana ‘Guy’ described pp. 117–118)

Palmer, E.J. 1948. Hybrid Oaks of North America. Journal of the Arnold Arboretum 29(1): 1–48. (Quercus ×jackiana described p. 31)

Pearson, L. 2014. John George Jack: Dendrologist, Educator, Plant Explorer. Arnoldia, 71(4): 2–11