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A genus of three species of small, summer-flowering, deciduous, suckering shrubs, spreading underground by means of runners or stolons and forming colonies. Leaves simple, opposite, serrate, 5–18 cm long, leaf scars of 3 bundle scars. Buds with loosely imbricate scales. Inflorescences 3- or 5-flowered, forming axillary or terminal panicles. Flowers small, 1.1-2.1 cm long, yellow, two lipped. Corolla funnel-shaped, 5-lobed. Stamens 5, filaments hairy, style and stamens protruding from corolla. Fruit slender woody capsules, calyx persisting on the fruit, the whole fruit persisting for up to two years before dehiscing, with many small glossy seeds. Sometimes confused with Weigela, many species of which used to be placed here, but differing from Weigela in their smaller, two-lipped flowers, borne on the current season's growth (Bean 1981a; Lance 2004; Cullen et al. (eds) 2011).
All three Diervilla species are native to eastern North America and are typically found as suckering shrubs growing in rocky habitats or open woodland where they are pollinated by insects such as bees and hawkmoths (Lance 2004). Although Diervilla have traditionally been significantly less popular in cultivation than the closely related and more showy Weigela, which is native to Asia, they are popular in North America and increasingly so in Europe as a group of tough, reliable, small shrubs. They will grow in almost any soil and are noted for their drought tolerance, leading to their increased use in municipal planting schemes. They grow well with little maintenance, will flower all through summer, and propagation is made easy by their suckering habit: suckers can be removed and re-planted or potted in the spring, or soft-wood cuttings can be taken in summer. They are generally trouble free, but can suffer from leaf spot and powdery mildew. Besides their use in municipal situations they are occasionally seen in gardens in the shrub border, as a small hedge, or in the woodland garden where they naturalise well (Hillier & Coombes 2002; Brickell 2003). A National Plant Collection of Diervilla is held at Sheffield Botanical Gardens, South Yorkshire.
Recent years have seen much work to improve the range of Diervilla available to the horticulturist and many new selections, such as the D. rivularis KODIAK® Series cultivars, are being trialled and registered in the United States with new names appearing in the catalogues each year. As Larry Hatch (2017) points out they have many virtues – several new cultivars exhibit interesting foliage characteristics, and many have outstanding autumn colour. He also enthuses over a new intergeneric cross between D. sessifolia 'Butterfly' and Weigela florida, which will doubtless yield many extraordinary plants in the coming years: 'These taken together with the best variegated, bronze-tipped, purple-leaved and compact clones of pure Diervilla give this genus a truly vivid future' (Hatch 2017).
Diervilla is named in honour of the French surgeon Diereville (or Dierville) who observed the genus during his visit to Canada in 1699–1700. On his return to France he introduced D. lonicera to European cultivation and the genus was later named after him. By 1739 it was being cultivated in England by Miller. D. rivularis wouldn't be introduced to Britain until 1902 when material arrived at Kew. The introduction of D. sessilifolia seems to have passed without obvious record, thought it must have arrived in Europe by the mid-19th century, for it was in European cultivation that the hybrid D. × splendens arose at about this time. Bean described D. lonicera as 'the least ornamental of Diervillas' and praised D. sessilifolia for being 'much superior' on account of the larger and more showy inflorescences. He advocated that 'It should be pruned back in spring before growth commences, when it will send up a dense mass of shoots that will blossom during the summer' (Bean 1981a).
Shoots desnsely hairy, leaves hairy beneath
Shoots glabrous or hairy only at nodes, leaves glabrous beneath
Petiole >5 mm, shoots angular or nearly square in cross-section
Petiole <5 mm, shoots rounded, margins ciliate