Hydrangea seemannii L.Riley

This is a vigorous, potentially overbearing evergreen climbing species for milder parts of our area. Easily confused with two other evergreen species, its white flowers are at best extremely showy. It is probably the most impressive of the hardier cornidias, and one candidate for the hardiest (Samain & Martínez Salas 2015).

H. seemannii has the distinctive globular ‘drumstick’ buds of Section Cornidia, surrounded by deciduous involucral bracts. It is one of the three far-flung but closely related temperate-climate cornidias, which collectively are a sister group to the rest of the section (Granados Mendoza et al. 2021). In the wild, there can be no confusing them, with the other species growing thousands of miles away in Taiwan and the Philippines (H. integrifolia) and Chile and Argentina (the misleadingly named H. serratifolia). In the garden these three are much confused, and great care should be taken in naming anything lacking a wild provenance. Botanical descriptions in the literature are often inadequate for distinguishing them. In leaf the three are at once very similar to one another and quite variable; generalizations about length/width ratio seem unreliable in practice. None are routinely toothed, 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, although these are most strongly developed in H. serratifolia, where they sometimes form part of well defined domatia visible even on the upper surface as blisters. H. seemannii always has sterile flowers around the margin of the inflorescence with showy, well developed sepals: the most up to date study of Mexican hydrangeas (Samain, Hernández Najarro & Martínez Salas 2019) is unambiguous on this point, and large sterile-flower sepals are a feature of wild-collected herbarium specimens (pers. obs.). Sterile flowers are absent or very rare in H. serratifolia (McClintock 1957), while in H. integrifolia they tend to be smaller and more scattered through the inflorescence (Wei & Bartholomew 2001; Foster 2023) although some herbarium specimens of H. integrifolia do show a marginal ring of well developed sterile flowers (Chinese Academy of Sciences 2024; GBIF 2024). Inflorescences are normally umbel-like in H. seemannii and H. integrifolia, opening rather flat in H. seemannii, but in H. serratifolia they are much more likely to be thyrse-like, with a well defined central axis and domed or elongated outline (McClintock 1957). Along with H. integrifolia, but unlike all other cornidias, H. seemannii has truly hermaphrodite flowers (Granados Mendoza et al. 2021).

H. seemannii has clearly been known in its native range for a long time, and is sometimes grown there as an ornamental (Samain, Hernández Najarro & Martínez Salas 2019). It has been used traditionally to treat diabetes; an aqueous extract has been shown to reduce blood sugar levels in both diabetic and normal mice (Pérez-Vásquez et al. 2013). It is one of the few cornidias to have a vernacular name in Mexico; in Durango State trees covered in the white flowering climber are known as arboles vestidos (‘dressed trees’) (Samain, Hernández Najarro & Martínez Salas 2019). Herbarium specimens were collected in 1849–50, in Durango, by Berthold Seemann, botanist on the British HMS Herald expedition, who described it growing in trees ‘like ivy’ (Seemann 1857). At the time these were labelled as H. peruviana. Riley’s specific epithet, published 1924, commemorates him. Although not evaluated by the IUCN, this relatively widespread species is considered endangered by Samain, Hernández Najarro & Martínez Salas (2019), who cite declining, fragmented populations as a result of habitat destruction, with few populations in federally protected areas.

Hydrangea seemannii was apparently introduced to gardens in our area as recently as the late 1960s and became widespread much more recently. Haworth-Booth (1984) was still unaware of it being grown, and expressed surprise that North American growers had not got hold of it. Californian botanist Dennis Breedlove, an expert on the flora of Chiapas State, collected live material at least once, in Durango, on his many collecting trips which began in 1964 (Daniel & Almeda 2012). van Gelderen & van Gelderen (2004) mention already large specimens at the Strybing Arboretum, San Francisco; while the species is not currently named in the catalogue, images of a Mexican-provenance accession from 1969 attributed to Breedlove and currently labelled H. peruviana look suspiciously like H. seemannii (San Francisco Botanical Garden 2024). Two 1970s accessions still growing at Berkeley are also linked to Breedlove, one in 1974 (Breedlove 18902, Durango) and a second (without number) in 1975 (University of California Botanical Garden 2024). van Gelderen & van Gelderen (2004) and Clarke (1988) state that Cornish nurseryman and magnolia expert Neil Treseder introduced it to Europe in the 1980s. Treseder had strong links to American growers, although Clarke implies introduction directly from Mexico. This tallies with accessions of cultivated material by Sir Harold Hillier Gardens and RBG Edinburgh around this time (Royal Botanic Garden Edinburgh 2024). Roy Lancaster also brought seed from the Strybing plants to Britain in 1985 (Clarke 1988).

Grown on a wall, it forms a dense, dark green covering in sun or shade, and is perfectly capable of making both upright, shrub-like growth on top of the wall and colonizing the other side. On a mature tree it will cover the trunk and fill the canopy; it is worth noting that unlike deciduous H. petiolaris or H. hydrangeoides it is not content to remain in the inner part of the canopy – in the wild shrub-like growths may overtop the host tree (Samain, Hernández Najarro & Martínez Salas 2019). Erect, shrubby forms are also seen on forest floor boulders, the plant modifying its stems’ mechanical and structural properties to suit either liane or shrub form (Granados Mendoza et al. 2014). Such plants can be established in gardens around a rock, tree stump or some more temporary support. A luxuriant thicket with no visible support grows well and flowers modestly under closed tree canopy at Westonbirt, Gloucestershire, UK (pers. obs. 2023). Flowering is much better –sometimes spectacular – with sun and warmth, as seen on outbuildings at Crûg Farm, N Wales (Foster 2023). While it may experience –30°C in the wild, this is during the winter dry season, and damage during cold, wet periods limits its range in cultivation. It proves hardy across lowland parts of Britain, and is grown unprotected in several Belgian collections (Plantcol 2024). In North America it suits the Pacific seaboard and the Southeast.

This diploid species (2n=36 – Cerbah et al. 2001) was one of four selected as ideal candidates for distant hybridization in Hydrangea by Granados Mendoza et al. (2013), on genetic and other criteria. One well documented intersectional hybrid (with H. petiolaris) is already known, Hydrangea ‘Inovalaur’ (SEMIOLA™), q.v.. The within-section crosses H. peruviana × H. seemannii and H. seemannii × H. serratifolia have been claimed (see Hydrangea Species of Section Cornidia) but further research to verify these is needed (Samain & Martínez Salas 2015).