10. Spiraea douglasii Hooker, Fl. Bor.-Amer. 1: 172. 1832.
Douglas’ or rose spiraea, western hardhack
Shrubs, 10–30 dm. Stems erect to ascending, branched. Leaves: petiole 3–5 mm; blade ˂bicolorous˃, elliptic to obovate, 3–10 × 0.5–4 cm, chartaceous, base cuneate, margins serrate distally from middle, venation pinnate craspedodromous, secondary veins prominent, apex acute to obtuse, abaxial surface lanate to tomentose, glabrous, or pubescent, adaxial glabrous. Inflorescences mostly terminal, conic panicles, [5–]8–25 × 3–6[–10] cm ˂height 2–4 times diam.); branches glabrous or glabrescent to tomentose. Pedicels 1–3 mm, glabrous or tomentose. Flowers 2–4 mm diam.; hypanthia hemispheric, 1 mm, abaxial surface tomentose, adaxial glabrous; sepals ovate, 1 mm; petals light to dark pink, ovate to obovate, 1–2 mm; staminodes 0; stamens 25–35, 2–3 times petal length. Follicles oblanceoloid, 2.5–3 mm, ˂shiny˃, glabrous, ˂adaxial suture sometimes ciliate˃.
Varieties 3 (3 in the flora): w North America; introduced in Europe.
Spiraea douglasii has been popular in the garden trade and was used to develop a number of hybrids. In North America, the hybrid most likely to be encountered is S. ×billardii Hortus ex K. Koch, because the parental species are both native, although S. salicifolia was widely imported from Europe and thus S. ×pseudosalicifolia Silverside would also likely be encountered.