Hydrangea barbara (L.) Bernd Schulz

Probably the first climbing hydrangea grown in Western gardens, H. barbara has since become overlooked in favour of Asiatic species such as H. petiolaris, except in its native American South. It is deciduous to semi-evergreen, depending on climate, and established specimens produce abundant foliage and scented white flowers in full shade, even when other parts of the plant are in sun: this is particularly valuable when growing on a canopy tree. Abundant moisture in the soil during the spring and summer is needed for best performance.

Hydrangea barbara is a member of Section Decumaria, along with the Asiatic H. obtusifolia. A complete absence of sterile flowers (a feature shared with H. viburnoides), and the strikingly high numbers of floral parts (sepals, petals and stamens) distinguish this Section. H. obtusifolia differs in being fully evergreen; it has smaller leaves and tends to grow less vigorously.

Completely lacking sterile flowers, this plant looks less obviously hydrangea-like than most other climbing species. Full-sized inflorescences present as rather rounded masses of stamen-dominated flowers, with a sweet honey scent which carries in the air. Peak flowering is in early- to mid-summer, late May in Georgia, USA, June–July in Britain (Dirr 2021; Bean 1981). Petals fall early, but they remain free rather than fused into a cap (as in H. petiolaris and H. viburnoides for example), opening out at roughly right angles to the floral axis before being shed (Hufford 2001). The high number of floral organs (7–12 sepals; 7–12 petals – often not equal to sepal number; 20–30 stamens) is very striking. It results from having more organs per whorl rather than extra whorls, probably due to enlargement of the apical meristem. Hufford (2001) likens this to the effects of mutations in genetically well studied plants such as Arabidopsis thaliana and Tomato (Lycopersicon esculentum), which increase meristematic capital through increased cell number and/or size. Direct sunlight is not required for good flowering: a Woodvamp climbing into a canopy tree may bear flowers all the way up the tree trunk. Without sterile flowers, inflorescences do not age attractively, although close inspection is worthwhile: Bean (1981) notes how the lower part of the fruit is ‘prettily striped with numerous whitish ridges’.

Leaves are dark green and more or less glossy above, sometimes strikingly shiny as in ‘Barbara Ann’ (see below). The blade tends to lie almost flat, rather than being sharply angled at the midrib as in the narrower leaved H. viburnoides. Non-flowering extension shoots cling to the substrate by aerial roots; in subsequent years shorter rootless shoots grow out from this framework: it is these which flower, on the current year’s growth (Dirr 2021). Like most other climbing hydrangeas it can produce small leaved, non-climbing ‘juvenile’ shoots which spread quickly across the ground before colonising vertical surfaces; these terrestrial shoots never flower (Missouri Botanical Garden 2023) but provide a propagating opportunity, plus the easiest means to establish a new plant. As a wild plant it is always described as fully deciduous, but away from the American Southeast this is less clear cut. In southern England where it is often described as semi-evergreen (Edwards & Marshall 2019) some leaves may be lost quite early in autumn while others stay green far longer (pers. obs. 2023); it may be essentially evergreen in some milder Pacific Northwest gardens (Hinkley 2009). What environmental factors are at play here is unclear.

First described as Decumaria barbara (Linnaeus 1763), the specific epithet implies an origin in coastal North Africa, once known as the Barbary Coast; here Linnaeus explicitly cites speculation (badly mistaken!) by the German naturalist Johann von Schreber. 1785 is often repeated as the date of first introduction to Europe (Bean 1981; Edwards & Marshall 2019), yet Linnaeus (1763) makes clear that it was already grown. It would certainly have been encountered by colonists in the Carolinas and Georgia well before that date. It is not at all clear whether subsequent anonymous introductions have supplemented the European stock, or if more or less everything derives from the earliest plants.

In North America it is sometimes planted to cover dead or established trees, fences, house walls and outbuildings, beyond its native range. Hardiness to USDA Zone 6 is usually suggested (e.g. Missouri Botanical Garden 2023), but Dirr (2021) – a great booster of this species – claims Zone 5, citing plants surviving winter lows of around –30°C in Illinois. It is also listed in the vine collection – grown as an arbor – at the Zone 5b Maxwell Arboretum, Lincoln, NE (University of Nebraska–Lincoln 2023). Caution is required in fire-prone areas: H. barbara has been assessed more than once as highly flammable (Kays et al. 2020; North Carolina State University 2023). This is primarily a bottomland species, and available soil moisture through the growing season is needed for it to perform well.

A reputation for limited hardiness has restricted its use in Europe. Away from the mildest Atlantic coastal areas it is usually seen as a wall shrub. The changing climate makes H. barbara a good bet for wider planting on mature trees in wetter areas of large gardens and arboreta, especially as desirable American cultivars become more available.