Hybrids of Thuja occidentalis and T. plicata (Cupressaceae) – Is this a fact or misunderstanding?

Thuja occidentalis and T. plicata belong to alien trees most frequently cultivated in Europe. Morphologically they differ significantly from each other and their natural ranges are geographically isolated, hence the taxonomic status of both taxa has never been questioned. In cultivation, however, these species are often very difficult to distinguish so speculations appeared that they can hybridize with each other. Reports about their hybrids growing in the Kórnik Arboretum, Poland, have been delivered several times; for the first time in 1933, then in 1957 and in 1981. In the latter publication, based on those hybrids, a new nothospecies Thuja ×plicatoides was described. That name, however, was not validly published, because its Latin diagnosis was limited only to a few words and no mention was made of how the hybrid differed from its parental species. The analysis of numerous arborvitae specimens growing in the Kórnik Arboretum, and in many other places in Poland, has shown that the earlier reports about the occurrence of Thuja hybrids in cultivation were fully justified. The results of this study were documented by numerous photographs showing the main diagnostic features of T. occidentalis, T. plicata and their hybrids. Special attention was paid to the characteristics of cones, which are rarely used in the Thuja taxonomy. The shape of cone scales, the length of the scale mucro and the structure of small 'lips' on the scale apices turned out to be significant cone features; the latter was used for the first time when distinguishing between both Thuja species. The cone and leaf features made it possible to recognize not only 'pure' species, but also their hybrids. The name Thuja ×plicatoides Seneta, for formal reasons has been rejected, so for the hybrids a new name Thuja ×senetiana has been proposed, which commemorates Włodzimierz Seneta, an outstanding Polish dendrologist.