Spiraea

Shrubs, 1–40 dm; usually rhizomatous. Stems 5–20+, usually erect to ascending or arching, sometimes spreading, prostrate, or decumbent; bark reddish to dark brown, gray, or gray-black, exfoliating or not; long and short shoots present; young stems tan or reddish brown to brown, glabrous or villous. Leaves deciduous (tardily so in S. thunbergii, partly persistent in S. cantoniensis), cauline, alternate, dimorphic with shoot type, simple; petiole present; blade obovate to oblanceolate, rhombic, elliptic, or linear to lanceolate, ovate, or suborbiculate, 1–10 cm, membranous, chartaceous, or coriaceous, margins flat, serrate to serrulate, dentate, or entire, venation pinnate, surfaces glabrous or hairy. Inflorescences mostly terminal or mostly axillary, (2–)3–1000+-flowered, panicles or corymbiform or racemiform, hairy or glabrous; bracts present or absent; bracteoles present or absent. Pedicels present. Flowers opening before or after full foliation, 2–15 mm diam.; epicalyx bractlets 1 or 2; hypanthium usually hemispheric or campanulate, sometimes turbinate or crateriform, 0.5–2(–5) mm, ± densely hairy or glabrous; sepals 5, usually erect, spreading, or reflexed, sometimes ascending, deltate, triangular, ovate-triangular, or ovate; petals 5, greenish, yellowish, chalky, or translucent white, white, pink [rose], or purple, ovate to obovate, suborbiculate, or orbiculate, sometimes elliptic; staminodes present or reduced to annulus, nectariferous; stamens 10–50 in 2–4 series, shorter or longer than petals; torus absent or reduced; carpels [3 or]4 or 5[–8], free, glabrate to tomentose, styles terminal or subterminal, stigmas capitate or discoid; ovules 2[–4]. Fruits aggregated follicles, 4 or 5, cymbiform, ellipsoid, falcate, fusiform, or oblanceoloid, (0.5–)1.5–4 mm, coriaceous, glabrous or tomentose; hypanthium persistent; sepals persistent or deciduous, spreading, ascending, erect, or reflexed; styles persistent or deciduous. Seeds 2–4 per follicle, fusiform to oblong, 2–4 mm. x = 9.

Distribution

North America, Europe, Asia, introduced widely, especially in the northern hemisphere, where often cultivated.

Discussion

Species 100–120 (17, including 3 hybrids, in the flora).

Spiraea has been a popular decorative plant in North America, Europe, and Asia. Horticultural hybrids have been named. Hybridization also occurs in natural settings, complicating the identification of the species and varieties. For specimens that are suspected of being naturalized or of hybrid origin, additional references for identification include horticultural works such as L. H. Bailey et al. (1949), W. J. Bean (1970–1988, vol. 4), A. J. Rehder (1927), H. S. Maxwell and S. G. Knees (1989), and A. Huxley et al. (1992, vol. 4), or the major floristic treatments by A. I. Pojarkova (1971) and Lu L. T. and C. Alexander (2003), and references therein.

The position of Spiraea within the Rosaceae has been the subject of considerable speculation; treatments by D. Potter et al. (2002, 2007b) are particularly useful. The floral ontogeny and morphology of S. alba and S. trilobata Linnaeus were examined and compared to other members of the Amygdaloideae by R. C. Evans and T. A. Dickinson (1999b).

Spiraea hypericifolia Linnaeus has been found naturalized in eastern Texas (J. N. Mink et al. 2011) and has been reported in Mississippi (plants.usda.gov). The species is native to central and southwestern Asia and southeastern Europe; it has been widely cultivated in Europe and North America, but there are few reports of its escape and naturalization. The invasive potential of S. hypericifolia is uncertain and will likely vary with local site conditions. In its native habitat, it can form thickets on dry slopes and in riparian zones and wet meadows.

A description of Spiraea hypericifolia follows: Shrubs 5–15 dm, usually rhizomatous. Stems erect to ascending. Leaves: petiole 1–4 mm; blade oblong-obovate or obovate lanceolate, 1.5–2 × 0.3–0.8 cm, length 2–4 times width, base acute, margins entire or with a few reduced teeth at apex, apex acute to obtuse, surfaces usually glabrous. Inflorescences axillary, 4–12-flowered, 0.75–2.5 × 1–3 cm. Flowers 5–8 mm diam.; hypanthia campanulate, 2–4 mm; sepals triangular, 1.5–2 mm; petals white, obovate to suborbiculate, 3–4 mm; staminodes 10–12; stamens 16–22, 0.8 times petal length. Follicles oblanceoloid, 1.4–2 mm, glabrous.

In North America, Spiraea hypericifolia would most resemble S. prunifolia or S. thunbergii and can be distinguished from these two species in that they have a leaf blade margin that is sharply serrulate, whereas in S. hypericifolia the margin is entire or with only a few teeth at the apex. Other characters that are helpful are leaf shape; see the descriptions of 12. S. prunifolia and 13. S. thunbergii for comparative details.

Selected References

None.

Lower Taxa

Spiraea alba, Spiraea cantoniensis, Spiraea chamaedryfolia, Spiraea corymbosa, Spiraea douglasii, Spiraea japonica, Spiraea lucida, Spiraea prunifolia, Spiraea salicifolia, Spiraea splendens, Spiraea stevenii, Spiraea thunbergii, Spiraea tomentosa, Spiraea virginiana, Spiraea ×hitchcockii, Spiraea ×pyramidata, Spiraea ×vanhouttei

Key

1	Inflorescences racemiform, corymbiform, or panicles, sometimes reduced or simple fascicles (S. prunifolia), usually mostly axillary	> 2
1	Inflorescences corymbiform or conic, cylindric, hemispheric obconic, pyramidal, or pyriform panicles, mostly terminal	> 6
2	Leaves: venation pinnate simple craspedodromous.	Spiraea chamaedryfolia
2	Leaves: venation pinnate cladodromous, eucamptodromous, simple craspedodromous, or suprabasal actinodromous	> 3
3	Leaves: venation suprabasal actinodromous.	Spiraea ×vanhouttei
3	Leaves: venation pinnate cladodromous, pinnate eucamptodromous, or pinnate simple craspedodromous	> 4
4	Leaf blades linear to lanceolate, venation pinnate eucamptodromous.	Spiraea thunbergii
4	Leaf blades ovate, elliptic, oblanceolate, or rhombic-lanceolate, venation pinnate cladodromous or pinnate simple craspedodromous	> 5
5	Leaf blade margins serrulate, usually from near base to apex, sometimes only distally from middle, rarely nearly entire with relatively few teeth apically, venation pinnate cladodromous.	Spiraea prunifolia
5	Leaf blade margins coarsely serrate or irregularly 3-fid to slightly lobed distally, venation pinnate simple craspedodromous.	Spiraea cantoniensis
6	Petals usually pink, sometimes white or purple; leaves: secondary veins prominent	> 7
6	Petals usually greenish, yellowish, or translucent white, white, or chalky white, sometimes pink-tinged to pink; leaves: secondary veins not prominent	> 11
7	Stamens 15–20, 1 times petal length; follicles tomentose to arachnoid.	Spiraea tomentosa
7	Stamens (10–)20–40, (1–)2–3 times petal length; follicles glabrous or adaxial sutures sometimes sparsely ciliate	> 8
8	Inflorescences corymbiform or elongate-conic to elongate-pyriform or hemispheric panicles, height 0.5–1(–2) times diam.; stamens 10–40, 1–2 times petal length; leaf abaxial surfaces glabrous or puberulent	> 9
8	Inflorescences conic or hemispheric to obconic panicles, height 1–4 times diam.; stamens 20–35, 2–3 times petal length; leaf abaxial surfaces lanate to tomentose or glabrous or puberulent to pubescent	> 10
9	Stamens 35–40; leaf abaxial surfaces glabrous or puberulent.	Spiraea splendens
9	Stamens 10–20; leaf abaxial surfaces puberulent.	Spiraea ×hitchcockii
10	Panicles conic, height 2–4 times diam.; stamens 25–35; staminodes 0.	Spiraea douglasii
10	Panicles hemispheric to obconic, height 1–1.5 times diam.; stamens 20–30; staminodes 10–15.	Spiraea ×pyramidata
11	Leaves: number of primary and secondary serrations 3–5 times number of secondary veins (excluding inter-secondary veins).	Spiraea japonica
11	Leaves: primary and secondary serrations 0–1 times number of secondary veins (excluding inter-secondary veins)	> 12
12	Shrubs 1–10(–15) dm; inflorescences corymbiform, height usually 0.4–1.1 times diam.; inflorescence branches rarely in axils of leaves	> 13
12	Shrubs 10–40 dm; inflorescences usually conic, cylindric, obconic, or pyramidal panicles, height 1.4–5 times diam., sometimes corymbiform, then ± hemispheric, height 0.5–1 times diam.; inflorescence branches usually in axils of leaves	> 15
13	Leaves coriaceous, margins irregularly, coarsely and sharply doubly serrate, teeth acute and mucronate.	Spiraea corymbosa
13	Leaves membranous, margins usually regularly to irregularly serrate, serrulate, or crenate, rarely entire, teeth rounded	> 14
14	Stems erect to ascending, often dying to ground annually; leaf bases acute.	Spiraea lucida
14	Stems ascending to prostrate, rarely dying to ground, rarely branched; leaf bases obtuse.	Spiraea stevenii
15	Leaves: blade length 3 times width, margins entire or dentate, (teeth 1–4 in distal 1/4 of blade).	Spiraea virginiana
15	Leaves: blade length 3–5 times width, margins serrulate to serrate	> 16
16	Leaf blades narrowly lanceolate to oblanceolate or broadly oblanceolate to obovate, secondary veins irregularly terminating in primary teeth, inter-secondary veins usually 8–12+ per leaf; inflorescence branches puberulent to pubescent.	Spiraea alba
16	Leaf blades narrowly rhombic to rhombic or lanceolate to narrowly elliptic, secondary veins regularly terminating in primary teeth, inter-secondary veins usually 1–4 per leaf; inflorescence branches puberulent or glabrous.	Spiraea salicifolia