Hydrangea serratifolia (Hook. & Arn.) F.Phil.

Hydrangea serratifolia is one of the three hardiest cornidias, a very vigorous – potentially house-swallowing – evergreen vine. It is grown for its dark green, glossy leaves, sometimes purple-tinged when young, and elongated inflorescences of creamy white, all fertile flowers in mid to late summer. It was the first hardy cornidia grown in our area, but has been overtaken in popularity by H. seemannii and H. integrifolia.

Like all members of Section Cornidia, H. serratifolia has fat, round buds surrounded by deciduous involucral bracts, on non-rooting side branches from the main climbing stems. One of the three similar and closely related temperate-climate species (with H. integrifolia and H. seemannii), it grows far to the south of any other wild hydrangea. Without wild provenance, the three are easily (and often) confused in gardens. Great care should be taken in identification; descriptions in the botanical and especially the garden literature are often inadequate for distinguishing them. In leaf the three are at the same time very similar to one another and quite variable; generalizations about length/width ratio seem unreliable in practice. None are routinely toothed, even H. serratifolia despite its name, although leaves with some teeth towards the apex do sometimes appear in any of them (pers. obs.). All may have tufts of hair in the vein axils beneath, but these are most strongly developed in H. serratifolia, where they sometimes form part of well defined domatia visible as hairy pits on the lower surface, and even on the upper surface as blisters. Floral characters are more useful. Unlike the other two, H. serratifolia is functionally dioecious (Granados Mendoza et al. 2021); functionally male plants have a reduced gynoecium, females reduced stamens. The H. serratifolia inflorescence is typically thyrse-like, with a well defined central axis – fluffy balls bunched together on a stick. H. integrifolia and H. seemannii have umbel-like inflorescences, domed or flat, although these are sometimes seen in H. serratifolia. Sterile flowers are absent or very rare in H. serratifolia (McClintock 1957; Gardner, Hechenleitner & Hepp 2015), while H. seemannii has showy sterile flowers around the margin of the inflorescence (Samain, Hernández Najarro & Martínez Salas 2019) and H. integrifolia tends to have sterile flowers with smaller sepals scattered through the inflorescence (Wei & Bartholomew 2001; Foster 2023).

H. serratifolia grows in mountain forests from the Valparaiso Region of Chile south to northern Patagonia, both in the Andes and the Chilean Coastal Ranges (Gardner, Hechenleitner & Hepp 2015). It is a conspicuous component of temperate rainforests, reaching 35 m in canopy trees. In one study area 54% of trees large and small hosted this species (Jiménez-Castillo & Lusk 2009). Young plants on the forest floor colonize boulders (Gardner, Hechenleitner & Hepp 2015); negatively phototropic shoots grow into the darkness at a tree’s base, from where they can climb into the canopy (Rodriguez-Quintero et al. 2022).

Multiple scientific descriptions in the 19th century, coupled with new combinations in Hydrangea of names first published in Cornidia (see synonyms above), led to a confusing name trail. The upshot is that Hydrangea serratifolia is the unfortunate but correct name, despite the protestations of garden writers (“an enormous plant covers the north wall, a chimney stack and part of the roof of my house, and not one of its leaves is serrated” – Haworth-Booth 1984).

H. serratifolia was introduced to cultivation through seed collected in 1926 by Harold Comber, a young man with plantsmanly horticulture in his blood, on an expedition sponsored by the great and the good of British gardening (Comber 564, Llolli, Araucanía Region, Chile – Gardner, Hechenleitner & Hepp 2015). This, coupled with the easy vegetative propagation of cornidias, ensured an early, wide distribution at least in Britain. By the early 1950s large plants were flowering as far apart as Benenden, Kent, and Crarae, Argyll & Bute (Bean 1981); one certainly from Comber 564 still grows at Nymans, West Sussex (Hsu 2011). Interestingly, a cultivated stock long grown at RBG Edinburgh (19390436, including a publicly accessible plant on the north walls of Inverleith House) and presumed to be from Comber’s collection, includes both female and male plants (Royal Botanic Garden Edinburgh 2024; Gardner, Hechenleitner & Hepp 2015). Later collections – all from Chile – made by botanists and botanical gardens from the 1970s to 90s include R. Schmid 146, Los Rios Region (University of California Botanical Garden 2024), R. Nicholson 1-F-1, Los Lagos Region (Arnold Arboretum 2024), and an un-numbered seed collection from Los Lagos Region by Frieda Billiet, grown at Meise Botanical Garden, Belgium (Botanical Collections 2024). Dan Hinkley collected the species in southern Chile, 1998, including HCM 98056 and 98166 available commercially on both sides of the Atlantic (Hinkley 2024; Crûg Farm Plants 2024). Despite readily available propagating stocks this species is today offered by nurseries far less than H. seemannii in Europe or H. integrifolia in North America.

H. serratifolia proves hardy across much of the British Isles, France and Belgium; in North America it is well suited to the Pacific Northwest and is just occasionally grown on the East Coast (Arnold Arboretum 2024). Like other cornidias it grows successfully into mature trees, and is well suited to walls where (skyscrapers aside) only wall height limits its ambition. Very dry sites are not ideal, while shelter from cold, drying winds is an advantage (Bean 1981). It seems to grow equally well in sun or shade, but flowers more freely with more light.

Where only a female plant is grown, any seed set should be suspected of hybridization, although the possibility of some residual male fertility is real. The hybrid H. seemannii × H. serratifolia has been claimed (see Hydrangea Species of Section Cornidia) but further research to verify it is needed (Samain & Martínez Salas 2015).