Unraveling the Mystery of Quercus ×introgressa P.M. Thomson

Originally published in Oak News & Notes, Vol. 19, No. 1

This unusual hybrid was described by Paul Thomson in 1977 from a location in Lafayette County, Missouri. Thomson discovered a hybrid swarm of 17 oaks; three hybrids of Q. muehlenbergii Willd. × prinoides Engelm., one Q. bicolor Willd., and 13 putative hybrids of this unique mix. Q. ×introgressa was distinguished on the site by long peduncles bearing small, rounded fruit. Its leaves were found to be fairly narrow and long. Individuals ranged from 39-65 ft tall, and were between 18-33 in in diameter.

Thomson described the complicated matter of the similarities between Q. muehlenbergii and Q. prinoides. Q. muehlenbergii was once considered a varietal form of Q. prinoides (Q. prinoides var. acuminata (Michx.)

Leaves an acorns on my tree of Q. ×introgressa

Gleason). This was thoroughly examined in the study and the morphological differences well explained. The research leaves little doubt as to the certainty of the three-way hybrid.

However, since this research was carried out, this group of trees was cut down (sometime during the 1990s) for reasons unknown to me. I have searched for a few years to find anyone who may have grafts of one of these original trees. Many in the area have grown seedlings that supposedly originated from this grove (the Missouri Department of Conservation has even offered seedlings for sale), but these are now F3 or F4 trees, allowing for ample opportunity for outcrossing with other species. I found one local nurseryman who had a grafted plant and he was gracious enough to share cuttings with me. However, as I found out later, this tree was a seedling found at the site after the original trees had been removed, so it is not totally clear if this remaining tree is a fair representation of this hybrid. My speculation is this plant was from one of the seedlings found across the fence, out of reach of cows and pigs that grazed this property. Thomson describes this situation in his research. My tree bears characteristics morphologically in line with Q. bicolor. Not surprising as Q. bicolor did represent 50% of the genetic makeup in the original hybrids.

Lanny Rawdon, the nursery man who shared the scion with me, next his graft of possibly the last surviving seedling from the original site. There is a very strong bicolor influence in this plant.

It is only about 3 ft tall but bore 15 acorns this past fall. Perhaps this is because I grafted mature wood, but not knowing how old the remaining tree is, it is possible that this is a trait inherited from Q. prinoides, a species well known to produce flowers and seed in three growing seasons. Hopefully my search will yet yield a grafted plant of one of the original trees from Thomson’s study, and I continue to track down possible leads. This hybrid may exist elsewhere, and it likely does, but given the similarities in the parent group, especially between Q. muehlenbergii and Q. prinoides, it would take a very discerning eye to spot it. My goal is to find a remnant plant and make sure its genetics are preserved.

Further reading:
Paul Thomson's original description can be read here.
An interesting discussion on a forum, including photographs of a specimen, can be seen here.
The entry for Q. ×introgressa on oaknames.org, including Jan de Langhe's high resolution scan of leaves from a plant at the Sir Harold Hillier Gardens, UK: see here.

Green and mature Q. ×introgressaacorns on my tree. (All photos © Ryan Russell)