3. Spiraea tomentosa Linnaeus, Sp. Pl. 1: 489. 1753.
Hardhack, steeplebush, spirée tomenteuse{Attr:E F W}
Shrubs, 3–15 dm. Stems erect to ascending, rarely branched. Leaves: petiole 2–3 mm; blade ˂bicolorous˃, ovate-lanceolate, 3–6 × 1–2.5 cm, coriaceous, base cuneate, margins coarsely serrate to crenate on distal 3/4, (secondary teeth on large and long shoot leaves), venation pinnate craspedodromous, secondary veins prominent, apex acute or rounded, abaxial surface densely ˂white to gray, tan, or rusty˃, tomentose, adaxial glabrous or puberulent. Inflorescences predominantly terminal, narrow, conic panicles, ˂150–15,000+-flowered˃, 5–30 × 3–20 cm, ˂branching varying from compact to wide and spreading˃; branches tomentose. Pedicels 0.1–1.5 mm, tomentose. Flowers 4–6 mm diam.; hypanthia hemispheric, 0.5–1 mm, abaxial surface tomentose, adaxial glabrous; sepals triangular, 0.7–1.1 mm; petals usually light to dark pink, sometimes white or purple, ovate to orbiculate, 1–1.5 mm; staminodes 0; stamens 15–20, 1 times petal length. Follicles cymbiform, 2–2.5 mm, tomentose to arachnoid. 2n = 24, 36.
Varieties 2 (2 in the flora): e North America; introduced in Europe.
P. J. Salamun (1951) conducted a comprehensive study of variation of the inflorescence across much of the range of Spiraea tomentosa and concluded that the two varieties could be segregated based upon the number of flowers or fruits per centimeter (density) of a lateral branch of the inflorescence. Intermediate populations have been identified, particularly in western New York and Pennsylvania. A. Gille (1949) discussed the overall distribution of S. tomentosa without evaluating varieties, and conducted a detailed study of its ecology in Quebec. The occurrences of this species in Oregon are as a weed in cranberry bogs; it has the potential to spread to natural wetlands. The species is reported as invasive in Belgium and Poland.
