Hydrangea integrifolia Hayata

This vigorous evergreen climber is one of the three hardiest cornidias, though still only suitable for milder parts of our area. Easily confused with two other species, its glossy dark green leaves, bronzed when young, together with white flowers in early summer, make it at its best a desirable plant. Rather neglected for many years, it is now finding favour in the gardens of America’s Pacific Northwest.

H. integrifolia has the fat, globular buds surrounded by deciduous involucral bracts which characterize Section Cornidia. It is the only Asiatic cornidia, and one of the trio of closely related temperate-climate species (with H. seemannii and H. serratifolia), which together are the sister group to the rest of the section (Granados Mendoza et al. 2021). As wild plants the three are widely separated, making identification easy, but in gardens they are sometimes confused, and great care should be taken naming anything without a trustworthy wild provenance. Botanical descriptions in the 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, 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. Floral characters prove more helpful in identification. Inflorescence form is normally umbel-like in H. integrifolia and H. seemannii, typically roughly domed or rounded in H. integrifolia, opening rather flat in H. seemannii, but in H. serratifolia it is much more likely to be thyrse-like, with a well defined central axis and domed to elongated outline (McClintock 1957). Sterile flowers are developed to different degrees in the three. H. seemannii always has sterile flowers around the margin of the inflorescence, with showy, well developed sepals (Samain, Hernández Najarro & Martínez Salas 2019). 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). Like H. seemannii, H. integrifolia is unusual among cornidias in having truly hermaphrodite flowers (Granados Mendoza et al. 2021).

As a wild plant H. integrifolia grows in the high mountains of Central Taiwan and in the Philippines. It climbs high into canopy trees, sometimes intimately mixed with H. anomala, as well as on cliffs (Crûg Farm Plants 2024; Hinkley 2003). It became known to science early in the 20th century, when several new species, all now considered synonymous, were described. Bunzō Hayata described first H. integrifolia (Matsumura & Hayata 1906) then H. integra with narrower leaf bases (Hayata 1908), both from Central Taiwan. Probably in ignorance of these, Adolph Elmer went on to describe H. glandulosa and its ‘very close relative’ H. cuneatifolia from two islands in the Philippines (Elmer 1908, 1915). Taxonomic lumper Elizabeth McClintock swept the lot into synonymy (McClintock 1957) and her view has generally been accepted.

The species has probably been in Western gardens only since the mid-20th century. In North America Nevling (1964) implied that he knew it first hand as a little-tried plant. In Europe a stock at Sir Harold Hillier Gardens, UK (illustrated by van Gelderen & van Gelderen 2004) was received by Hilliers from the USDA Plant Introduction Station at Glenn Dale, MD, in 1970 (B. Clarke pers. comm. 2024), potentially the origin of all older plants in Europe. More recent collections include Dan Hinkley’s DJHT 99055 (Taiwan, 1999 – Hinkley 2023). In the 1990s Crûg Farm made several collections in the high mountains of Central Taiwan (BSWJ 22, 68 and 6967) as well as of a plant found growing terrestrially in northern Luzon, Philippines, collected under BSWJ 4031 as H. glandulosa (Crûg Farm Plants 2024). It has always been a Cinderella species in Europe, grown far less than H. seemannii. However, in North America long established plants at University of British Columbia, Vancouver, and at the University of Washington’s Washington Park and Center for Urban Horticulture, Seattle, have proven its worth as a ‘gorgeous gem’ and ‘one of very few evergreen vines for gardeners in the Pacific Northwest’ (Reyes 2014), where it is now quite widely offered by nurseries. Several West Coast growers confidently assert hardiness to USDA zone 7.

In gardens it can be grown into mature trees as at Washington Park, Seattle (Dirr 2021) and White House Farm, Kent, UK; like other cornidias it is most unlikely to cross into the crowns of adjoining trees but may form thickets at ground level which then colonize neighbours. It suits walls, flowering even in full shade in Seattle (Reyes 2014), but at Crûg Farm in higher latitude North Wales where it rampages across farm buildings, it flowers best with more sun (B. Wynn-Jones pers. comm. 2023). It can also be effective as a free-standing bush, or clipped into a tall hedge (Foster 2023).

This species was one of four selected as ideal candidates for distant hybridization in Hydrangea by Granados Mendoza et al. (2013), on genetic and other criteria.