Hydrangea strigosa Rehder

This is a member of the Hydrangea aspera complex (see H. aspera for general discussion). H. strigosa varies a great deal in characteristics which interest the gardener; indeed, molecular evidence hints at it not being a single biological species at all. What defines the concept of H. strigosa is the strigose indumentum – short, stiff, appressed, white hairs – which covers stems, peduncles, pedicels and the undersides of leaf blades, quite different from the softer, spreading hairs of H. villosa. One other common feature which matters in the garden is its late flowering, in some forms too late for areas with a short growing season.

In describing H. strigosa for the first time, Alfred Rehder (in Sargent 1913) made clear both the central importance of strigose indumentum, and the variability in other features such as leaf shape (several explicitly intergrading varieties described by Rehder are rarely if ever accepted today). Rehder’s concept was based on specimens from Hubei and Sichuan, mostly Ernest Wilson’s, but later botanists have identified vast numbers of specimens from right across southern and central China as H. strigosa (Chinese Academy of Sciences 2024). Even arch-lumper Elizabeth McClintock (1957) accepted it as a subspecies of H. aspera. Leaf and stem indumentum is a character clearly visible in almost every dried specimen, so it is understandable that herbarium botanists would cling to it in a sea of variation. However, a DNA-based study of the H. aspera complex casts doubt on H. strigosa being a natural, monophyletic grouping. Specimens from Sichuan clustered with H. aspera s.s. and H. robusta, while those from Hubei formed a sister group to H. kawakamii (De Smet et al. 2017). Perhaps H. strigosa is best thought of as purely a morphospecies.

H. strigosa does not reach such high altitudes as others in the complex, and it quickly gained a reputation for limited hardiness and late flowering in gardens (Bean 1981). Ernest Wilson made several collections; as usual with Wilson it is difficult to be certain how many included seed as well as herbarium specimens, but an October-flowering plant from W 757 (W Hupei, 1907 – Sargent 1913) was growing at Glasnevin National Botanic Garden, Dublin, in the 1930s (Bean 1981). Another was raised by H.J. Elwes (Colesbourne, UK) from a stray in a packet of seeds of climbing H. obtusifolia collected by Wilson during his 1910–11 collecting trip (Bean 1981). This fairly widespread plant may have been gathered much more often by the early 20th century collectors, but the only other collection we can trace is Kingdon Ward 20940 (Bean 1981). More recently seed sent from Shanghai Botanical Garden has reached Western botanic gardens, including Meise, Belgium, and the Arnold Arboretum, USA (Botanical Collections 2024; Arnold Arboretum 2024). Crûg Farm and collaborators have made several commercial collections in N Vietnam which they assign – sometimes tentatively – to H. strigosa. These include HWJ 653, BSWJ 8201 (sometimes named ‘Sapa’, see Hydrangea aspera Species Complex Cultivars), WWJ 11915, KWJ 12151 and BWJ 15152 (Crûg Farm Plants 2024). HWJK 2043 (Nepal, narrowly elliptic leaves) was made so far from the known range of the species that further examination seems warranted.

Forms in gardens range disconcertingly from relatively small, narrow leaved shrubs to large bushes with broad ovate leaves on upright stems, with pale or darker bark, peeling or not. Maurice Foster (Kent, UK) was tempted to prune a 4.5 m white-barked example as a small tree (Foster 2023). While some forms are exceptionally late flowering, others are not: Bean (1981) noted that the Kew herbarium contains garden specimens collected in flower from June to October. As with other asperas light shade and year-round moisture availability are ideal. This remains an uncommon species in gardens, with hardiness probably restricting its use in continental Europe and much of North America, where it grows well in the Pacific Northwest and the Southeast (Dirr 2021).

Mophead forms are known from Emei Shan, Sichuan. Wilson for Veitch 4902 of 1904 (Sargent 1913) did not become established in gardens, but Dan Hinkley and Ozzie Johnson’s ‘Elegant Sound Pavilion’ collected almost 100 years later is doing the rounds of collectors, if not impressing everyone (described under Hydrangea aspera Species Complex Cultivars).