Sp. Pl. 1: 226. 1753.
Gen. Pl. ed. 5, 106. 1754 ,.
Herbs (usually evergreen), sometimes stoloniferous; caudex or rhizome stout, often branched, scaly. Flowering stems erect or ascending, leafless or bearing 1–5 cauline leaves (H. alba, H. americana, H. bracteata, H. caroliniana, H. longiflora, H. pubescens), 3–145 cm, glabrous or stipitate-glandular, rarely viscid (H. maxima, H. micrantha, H. parishii, H. parviflora, H. pilosissima). Leaves in basal rosettes and cauline; stipules present; petiole glabrous or stipitate-glandular; blade reniform, orbiculate, ovate, cordate, oblong, or polygonal, palmately 3–9-lobed, base cuneate or cordate to truncate, ultimate margins serrate, dentate, or crenate, ciliate or glandular-ciliate, apex rounded or obtuse to acute to acuminate, apiculate, or mucronate, surfaces glabrous or long or short stipitate-glandular, rarely viscid (H. maxima, H. micrantha, H. parishii, H. parviflora, H. pilosissima); venation palmate. Inflorescences thyrses (with cymose side branches), sometimes diffuse (resembling panicles) or dense (resembling spikes), from axillary buds in rosette, 100–1000-flowered, not secund (secund in H. bracteata, H. hallii, H. parishii, H. pulchella, H. rubescens), bracteate. (Pedicels bracteolate, glabrous or long or short stipitate-glandular; bracteoles scalelike, scarious or herbaceous, margins ciliate or glandular-ciliate.) Flowers radially or bilaterally symmetric; hypanthium adnate to ovary for proximal 1/4–1/2, free from ovary 0.1–7 mm, abruptly inflated distal to adnation with ovary (H. alba, H. americana, H. caroliniana, H. chlorantha, H. longiflora, H. pubescens, H. richardsonii) or weakly inflated (H. parishii), green, white, cream, yellow, pink, purple, or red, (elongating during flowering and fruit maturation, short to long stipitate-glandular); sepals 5 (6 in H. eastwoodiae), green, white, cream, yellow, pink, purple, or red, often green or red tinged; petals (1–)5(–6 in H. eastwoodiae), sometimes minute or absent (usually absent in H. chlorantha, H. cylindrica, H. eastwoodiae), green, white, cream, pink, or purple; nectary tissue encircling base of styles at junction of ovary and free hypanthium usually white or yellow (yellow to orange in H. parvifolia), usually concealed by free hypanthium and sepals (exposed in H. parvifolia); stamens 5 (6 in H. eastwoodiae), opposite sepals; filaments terete or broader at base; (anthers orange or yellow); ovary 1/2 inferior, carpels completely connate, 1-locular; placentation parietal; styles 2; stigmas 2(–3). Capsules 2-beaked. Seeds dark brown or black, ovoid, ellipsoid, fusiform, or straight on 1 side and convex on other, spiny (smooth in H. parviflora). x = 7.
North America, Mexico.
Species ca. 37 (32 in the flora).
Heuchera, the largest herbaceous genus exclusively in North America and Mexico in the Saxifragaceae, is notoriously difficult taxonomically because of morphological intergradation among species, possibly following hybridization, and because character expression, especially in flowers and fruits, varies with the stage of development. Heuchera species usually remain distinct in nature due to geographic isolation and differences in flowering phenology, but climatic fluctuations have caused range extensions and contact among species, resulting in hybridization and persistent hybrid populations (C. O. Rosendahl et al. 1936; J. A. Calder and D. B. O. Savile 1959; S. A. Spongberg 1972; C. K. Wilkins and B. A. Bohm 1976; E. F. Wells 1979, 1984; B. G. Shipes and Wells 1996; D. E. Soltis et al. 1991). Multiple origins of autotetraploid lineages from diploid ancestors have been documented in some species, sometimes associated with increased flower size relative to diploid progenitors, causing further taxonomic difficulties (Soltis et al. 1989; K. A. Segraves and J. N. Thompson 1999). Molecular techniques [particularly rbcL, a chloroplast gene, sequence divergence and cpDNA (chloroplast DNA) restriction site analyses] have been used to help assess relationships within the genus (Soltis et al. 1991) and have suggested that Heuchera is paraphyletic. One group of species, scattered throughout sects. Rhodoheuchera Rosendahl, Butters & Lakela, Holochloa Nuttall ex Torrey & A. Gray, and Heucherella Torrey & A. Gray, appears to be sister to the remainder of the genus, including other species scattered throughout sects. Heuchera, Heucherella, and Holochloa (Rosendahl et al.), and other genera, including Elmera, Tellima, Tiarella, and Conimitella, and most species of Mitella (Soltis et al. 1991), adding confusion at the sectional and generic levels. Soltis et al. (1991) suggested that hybridization among individuals of distantly related species of Heuchera followed by subsequent backcrossing among hybrid offspring and individuals belonging to parental species might have introduced maternally inherited chloroplasts into hybrid progeny that resembled pollen (paternal) parents. The suggested capture of chloroplasts from one species by another following hybridization could explain the complex pattern of cpDNA relationships within the genus.
Some species of Heuchera are cultivated as perennial ornamentals in cooler parts of North America. The most commonly available cultivated species include H. americana, H. cylindrica, H. grossulariifolia, H. micrantha, H. richardsonii, H. rubescens, H. sanguinea (coralbells), and H. villosa. Cultivated alum-roots often exhibit white-variegated or bronze-red leaves. The sale and cultivation of these native species as landscaping ornamentals sometimes is implicated in distribution occurrences far beyond their natural ranges. Crosses between Heuchera and Tiarella, known as ×Heucherella, are also cultivated.
The flower in Heuchera has a hypanthium, or floral tube, adnate to the unilocular ovary proximally and free from the ovary distally for 0.1–7 mm. Some species have virtually no hypanthium beyond that adnate to the ovary. The hypanthium bears at its rim usually five, rarely six, sepals alternating with usually five, rarely six, petals, which are borne in the sinuses between the sepals or inserted inside the hypanthium distal to the ovary, alternating with the sepals. Stamens are attached inside the hypanthium opposite the sepals. The portion of hypanthium distal to the ovary varies from radial to bilateral in different species, and, additionally, the sepals may counter or exaggerate the effect, by being equal or unequal in length on either side of a flower. In some species, the distal portion of the hypanthium tube is radially symmetric and the flower is radial. In other species, the adaxial sepals and the adaxial side of the distal portion of the hypanthium tube are longer than the abaxial sepals and abaxial side of the hypanthium tube, and the flower is bilateral. In some species, the adaxial sepals are shorter than the abaxial, and the flower appears to be radially symmetric until examined closely.
Whereas the inferior portion of the ovary is adnate to the hypanthium, the superior portion of the ovary arises at the base of the free hypanthium tube and is not adnate to the tube. The superior portion of the ovary consists of two, rarely three, pointed hollow beaks each tapering to a solid style and terminating in a capitate stigma; the beaks of the ovary are often surrounded by yellow, rarely orange, nectariferous tissue proximally.
Floral measurements in the key are given at anthesis. Stamens and styles continue to elongate during flowering, and hypanthium, ovary, and fruit dimensions change over time. The length of the hypanthium free from the ovary is measured between the distal point of adnation to the ovary and the bases of the sinuses on the adaxial side of the flower (the longer side of the hypanthium).
In Heuchera, stamens and stigmas may be deeply included, or included to slightly exserted, or exserted 1–6 mm; the length of exsertion is measured from the distal lip of the connate portion of the hypanthium.
Virtually all hairs in Heuchera end in multicellular glands. Surfaces are described as short, medium, or long stipitate-glandular.
|1||Styles to 1.5 mm; hypanthia usually short stipitate-glandular, sometimes sparsely or densely, medium or long stipitate-glandular; stamens included (rarely incurved); w North America||> 2|
|1||Styles 0.2-7 mm; hypanthia short to long stipitate-glandular, rarely glabrous; stamens exserted or barely included (when hypanthium 5+ mm); e, w North America||> 11|
|2||Hypanthia flat, saucer-shaped; sepals reflexed||> 3|
|2||Hypanthia campanulate, broadly campanulate, or broadly turbinate; sepals erect or incurved||> 4|
|3||Petioles long stipitate-glandular; hypanthia green; New Mexico.||Heuchera wootonii|
|3||Petioles glabrate or short stipitate-glandular; hypanthia greenish or cream to yellow; w North America.||Heuchera parvifolia|
|4||Sepals and stamens 6; petals usually absent or (1-)6 and much reduced.||Heuchera eastwoodiae|
|4||Sepals and stamens 5; petals rarely absent or (1-)5, ± equaling or longer than sepals||> 5|
|5||Hypanthia 6-11 mm; petals absent or (1-)5||> 6|
|5||Hypanthia to 6 mm; petals usually 5||> 8|
|6||Hypanthia green; filaments ca. 3 times length of anthers.||Heuchera chlorantha|
|6||Hypanthia cream or yellow, often tinged with red or green; filaments shorter than to 2 times length of anthers||> 7|
|7||Filaments to 2 times length of anthers.||Heuchera cylindrica|
|7||Filaments shorter than anthers.||Heuchera grossulariifolia|
|8||Filaments equaling or longer than anthers, erect or incurved distally (not concealed by anthers); styles 0.5-1.5 mm||> 9|
|8||Filaments shorter than anthers, strongly incurved (almost concealed by anthers); styles to 0.5 mm||> 10|
|9||Filaments longer than anthers; hypanthia 4-5.5 mm; petioles glabrous or short stipitate-glandular; Colorado.||Heuchera hallii|
|9||Filaments equaling anthers; hypanthia 3-5 mm; petioles glabrous or long stipitate-glandular; Arizona, New Mexico.||Heuchera novamexicana|
|10||Hypanthia 3.5-5 mm, yellowish green, densely long stipitate-glandular mixed with short stipitate-glandular; Arizona, New Mexico.||Heuchera glomerulata|
|10||Hypanthia 4-7 mm, cream, short stipitate-glandular; Idaho, Montana, Nevada, Oregon, Washington.||Heuchera grossulariifolia|
|11||Hypanthia pink to rose, purple, or red (petals white, cream, or pink), 3-8 mm, short to long stipitate-glandular; sepals often green- or dark red-tipped||> 12|
|11||Hypanthia green, greenish white, greenish yellow, pink, or white (petals green, greenish white, white, pink, or purple), 1-14 mm, short to long stipitate-glandular, rarely glabrous; sepals green- or red-tipped||> 20|
|12||Hypanthia dark pink to red; stamens included 1.5-3 mm.||Heuchera sanguinea|
|12||Hypanthia pink or rose to red or purple; stamens to 2 mm included to 3 mm exserted||> 13|
|13||Petals relatively narrow||> 14|
|13||Petals relatively wide||> 15|
|14||Hypanthia slightly bilaterally symmetric; petals ± equal; styles to 0.1 mm diam.; w North America.||Heuchera rubescens|
|14||Hypanthia strongly bilaterally symmetric; petals unequal; styles 0.1+ mm diam.; San Bernardino Mountains, California.||Heuchera parishii|
|15||Hypanthia campanulate; New Mexico.||Heuchera pulchella|
|15||Hypanthia cylindric to narrowly campanulate or urceolate; California||> 16|
|16||Stamens barely included to 1.5 mm exserted; leaf blades ovate, deeply lobed; petioles glabrous or short to medium stipitate-glandular.||Heuchera abramsii|
|16||Stamens included or to 0.5 mm exserted; leaf blades orbiculate or reniform, shallowly lobed; petioles short to long stipitate-glandular||> 17|
|17||Stamens included 1 mm; hypanthia free from ovary 1.5-2.2 mm on adaxial side; Laguna Mountains, San Diego County, California.||Heuchera brevistaminea|
|17||Stamens included 0.5 mm to 0.5 mm exserted; hypanthia free from ovary 1.8-3.5 mm on adaxial side; Mount San Jacinto, San Gabriel Range, w Transverse Ranges of Kern, Los Angeles, Riverside, San Bernardino, Tulare, and Ventura counties and vicinity||> 18|
|18||Sepals to 0.5 mm on adaxial side of hypanthium.||Heuchera hirsutissima|
|18||Sepals all well developed, 1-2 mm on adaxial side of hypanthium||> 19|
|19||Hypanthia cylindric or cylindric-urceolate; sepals unequal.||Heuchera elegans|
|19||Hypanthia narrowly campanulate, widening at mouth; sepals equal.||Heuchera caespitosa|
|20||Hypanthia usually long stipitate-glandular, rarely glabrous or short stipitate-glandular, white, greenish white, or pink (petals white or pink), free to 1.6 mm||> 21|
|20||Hypanthia short stipitate-glandular, green or greenish yellow (petals green, greenish white, white, pink, or purple), free 0.5-7 mm||> 27|
|21||Hypanthia hemispheric, densely long stipitate-glandular; styles slightly exserted; w North America||> 22|
|21||Hypanthia obconic, broadly turbinate, campanulate, subglobose, or hemispheric, usually moderately long stipitate-glandular, rarely glabrous; styles much exserted; e, w North America||> 24|
|22||Leaf bases shallowly cordate to truncate; petioles 2.4-5 cm, moderately long stipitate-glandular; Siskiyou Mountains of Oregon and California, Trinity Mountains of California||Heuchera merriamii|
|22||Leaf bases deeply cordate; petioles 7-22 cm, densely long stipitate-glandular; Coast Ranges of California and Oregon, n Channel Islands of California||> 23|
|23||Leaf surfaces moderately to densely long stipitate-glandular adaxially; Coast Ranges of California and Oregon.||Heuchera pilosissima|
|23||Leaf surfaces glabrous adaxially except long stipitate-glandular near margins; n Channel Islands, Santa Barbara County, California.||Heuchera maxima|
|24||Leaf blades rounded-cordate, orbiculate, or oblong, often ± polygonal, lobes usually triangular; seeds spiny||> 25|
|24||Leaf blades reniform to orbiculate or polygonal, lobes rounded; seeds smooth or spiny||> 26|
|25||Petioles and hypanthia glabrous or short to moderately long stipitate- glandular; Pacific Northwest, Alaska.||Heuchera glabra|
|25||Petioles and hypanthia usually long stipitate-glandular, rarely glabrous; Arkansas, e of Mississippi River.||Heuchera villosa|
|26||Leaf blades reniform to orbiculate; seeds smooth; Arkansas, e of Mississippi River.||Heuchera parviflora|
|26||Leaf blades orbiculate to polygonal; seeds spiny; British Columbia, California, Idaho, Oregon, Washington.||Heuchera micrantha|
|27||Inflorescences dense||> 28|
|27||Inflorescences diffuse||> 29|
|28||Hypanthia strongly bilaterally symmetric, 5-14 mm, free 2-7 mm, abruptly inflated distal to adnation to ovary; prairies and dry woods, British Columbia to Ontario to Illinois, Indiana, and Oklahoma.||Heuchera richardsonii|
|28||Hypanthia weakly bilaterally symmetric, 3-5 mm, free 0.5-1.5 mm, not inflated distal to adnation to ovary; rocky ledges and outcrops in Rocky Mountains and foothills, Colorado and Wyoming.||Heuchera bracteata|
|29||Hypanthia free from ovary to 2 mm, weakly bilaterally symmetric||> 30|
|29||Hypanthia free from ovary 2-7 mm, weakly to strongly bilaterally symmetric||> 31|
|30||Stamens exserted 3-5 mm; styles exserted 2.6-6.4 mm; hypanthia urceolate or campanulate; e North America.||Heuchera americana|
|30||Stamens exserted 0.2-1.5 mm; styles included or exserted to 1.1 mm; hypanthia subglobose; North Carolina, South Carolina, Virginia.||Heuchera caroliniana|
|31||Styles included 1.3-5.3 mm; hypanthia gibbous-tubular; sepals and petals inflexed (closing mouth of flower)||Heuchera longiflora|
|31||Styles from 0.5 mm included to moderately exserted; hypanthia subglobose or campanulate; sepals and petals erect, spreading, or inflexed||> 32|
|32||Hypanthia 2.8-4.5 mm, subglobose.||Heuchera caroliniana|
|32||Hypanthia 5-14 mm, campanulate||> 33|
|33||Petioles densely or sparsely stipitate-glandular; hypanthia strongly bilaterally symmetric, sinuses between sepals wider than petals; petals usually green or greenish white, rarely pink.||Heuchera richardsonii|
|33||Petioles glabrous or short stipitate-glandular; hypanthia moderately bilaterally symmetric, sinuses between sepals narrower than petals; petals white, pink, or purple||> 34|
|34||Petals pink or purple; c Appalachian Mountains of Maryland, North Carolina, Pennsylvania, South Carolina, Virginia, West Virginia.||Heuchera pubescens|
|34||Petals white; sandstone ridges, Ridge and Valley Province of Virginia and West Virginia.||Heuchera alba|