Forsythia saxatilis (Nakai) Nakai

One of four Korean endemic forsythias with tiny, relic populations, F. saxatilis has a particularly confused nomenclatural history. It was first described, by Takenoshin Nakai in 1919, as a variety of the then newly-discovered Japanese F. japonica, and was treated as synonymous with F. japonica in the Provisional Checklist of Korean Flora (Chang, Kim & Chang 2014). Plants of the World Online, meanwhile, places it in synonymy with another Korean species, F. ovata (Plants of the World Online 2022). Visually, it is perhaps most similar to a third Korean Forsythia, F. koreana, and a presumed wild hybrid between these taxa has been reported (Lee, Kim & Hong 1982). Two recent phylogenetic analyses (Ha et al. 2018; Choi & Kim 2020) have both suggested that the species’ closest ally is the green-twigged Chinese F. viridissima and that it is only distantly related to F. japonica or to F. ovata, underlining the limits inherent in trying to organise such a uniform genus by the use of visual clues alone.

Although it has not yet been assessed by the IUCN, Forsythia saxatilis seems likely to be extremely rare in the wild. It was originally described from a single population on limestone rocks near Seoul; a second population, in the Bukhansan National Park in the same part of Korea, was identified in 2020 by Sang Chui Choi and Sangtae Kim (Choi & Kim 2020).

Although the confusion surrounding this plant makes it particularly hard to trace examples in cultivation, it seems likely to be the forsythia that was introduced, via Tokyo, to the Arnold Arboretum in 1924 (Arnold Arboretum 2018), and which was used to breed F. ‘Arnold Dwarf’. The same stock was introduced from the Arnold Arboretum to Kew (Bean 1981), but if it survives here at all it seems likely to have been lost within the records for F. ovata.