Hybridization in Quercus (as Seen through the Eyes of an Enthusiast)

Ever since the genus Quercus grabbed my attention, I have been amazed by the ability of oaks to hybridize easily (sometimes too easily), even when the species involved don't share habitat in the wild. As we know, oaks occur almost exclusively in the Northern Hemisphere, with only a few species occurring south of the Equator. In many cases different species of oak share the same habitat and hybridize frequently, which can make finding a plant that is “true” to type difficult. Especially in mixed forests where different species of the same section occur, many hybrids are known, for example, Q. ×bebbiana (Q. alba × macrocarpa) among White Oaks and Q. ×heterophylla (Q. phellos × rubra) among Red Oaks.

What makes this fascinating is that when oaks are planted together in a city park or, even better, an arboretum, very interesting things can happen! Even when two oaks don't share habitat in the wild, they are able to hybridize. A well-known example is Q. Pondaim Group, a group of hybrids resulting from the cross between the East Asian Q. dentata and the Caucasian Q. pontica. Another example, which as far as I know is the only known case of intersectional hybridization, is Q. ×kewensis, where two oaks of different sections seem to have found each other: Q. cerris (Cerris Section) and Q. wislizeni (Lobatae or Red Oak Section). A third example, well known in the tree trade, is Q. ×bimundorum (Q. robur × Q.alba), which has proven hybrids can be valuable as well, because some problems can be solved. In this case, the hybrid is resistant to mildew, a notorious problem on Q. robur and other species related to it; also, the autumn color has been improved, as Q. robur usually does not have good autumn color.

As I said before, the ability of oaks to hybridize easily can also be annoying, because when you plant acorns from an arboretum it is very difficult to raise good offspring, and identifying oaks raised from arboretum seed can be very difficult, or even impossible. For example, Q. rysophylla, or loquat-leaved oak, is known to hybridize easily with many deciduous Red Oak species. One of its hybrids has been named Q. 'Chocha', grown from acorns of a specimen of Q. rysophylla in Arboretum de Chocha in France. It has slightly leathery leaves that are tardily deciduous and turn orange in autumn, but their shape was inherited from its putative pollen parent, the deciduous Q. rubra. Knowing the identity of at least the seed bearing parent is crucial if you like to experiment with open-pollinated seed, like me!

Last example: during the Post-Conference Tour of the 2012 IOS Conference, I collected some acorns of a Q. phillyreoides in Shaun Haddock’s Arboretum de la Bergerette. I expected to get normal Q. phillyreoides seedlings, as it was in a slightly isolated part of the garden. One seedling, however, was already different when it sprouted its very first leaves, which made me curious enough to give it its own pot and watch it for a while. Recently Shaun told me that in that earlier that year, in March 2012, his Q. franchetii, which was planted very close to the Q. phillyreoides, was frozen to the ground. I then realized my seedling is probably a hybrid of Q. phillyreoides and Q. franchetii, and the latter may have donated its pollen a year earlier (as both oaks mature their seed in two years, the pollination would have taken place in spring 2011).

Planting hybrid acorns can be very interesting, but be careful: if you don’t keep data of the seeds you plant, your hybrids will be worthless!

All photos © Jeoren Braakman

This article has been corrected. In the original version, the caption in the photograph of Quercus 'Chocha' indicated it was at Arboretum de Chocha. It was grown from seed collected in Arboretum de Chocha, but the tree in the photo is in Jardín Botánico de Iturraran.