Tiarella

Plants of genus Tiarella are perennial, herbaceous plants with short, slender rhizomes.^[4] Three morphological features are used to distinguish Tiarella species: 1) presence or absence of stolons; 2) size and shape of basal leaves; and 3) presence or absence of stem leaves (also called cauline leaves). Two species of Tiarella have stolons (T. austrina, T. stolonifera) while two other species have stem leaves (T. nautila, T. austrina). Plants from the southern Blue Ridge Mountains and southward have relatively large basal leaves with an extended terminal lobe (T. austrina, T. nautila, T. wherryi).^[6]

The following identification key was published by Guy Nesom in 2021:^[6]

In 1753, Swedish botanist Carl Linnaeus established genus Tiarella by recognizing two species, Tiarella cordifolia and Tiarella trifoliata.^[7][8] A third species, Tiarella polyphylla, was described by David Don in 1825.^[9] Together these three species form the taxonomic backbone of the genus. In 1840, in the first critical treatment of Tiarella since Linnaeus, John Torrey and Asa Gray described two new sections:^[10]

• Tiarella sect. Anthonema Nutt. apud Torr. et Gray: flowering stem leafy with alternate leaves; flowers paniculate; petals filiform or subulate; western North America
• Tiarella sect. Eutiarella Torr. et Gray: flowering stem naked; flowers racemose; petals oblong with a small claw or stalk; eastern North America.

Olga Lakela highlighted the section names in 1937,^[11] but they have since fallen out of favor with botanists, mainly because Tiarella polyphylla is inconsistent with the dichotomy,^[12] but perhaps also because there are taxa with leafy flowering stems in both western and eastern North America.

In Asia, the genus is represented by one species (Tiarella polyphylla).^[4] In North America, there have been numerous major treatments of genus Tiarella, with taxonomies recognizing from two to six species, some including infraspecific taxa.

Currently accepted taxonomies are based on three sources:^[16][17][15]

• Tiarella polyphylla in Flora of China
• Tiarella trifoliata and related taxa in Flora of North America
• Tiarella cordifolia and related taxa in a paper published by Guy Nesom in 2021

The treatment in the first source is near-universally accepted,^{[18][19][20][21][22]} the second is widely recognized,^{[17][23][24][25][26][27][28]} while the third is new and growing in acceptance.^{[29][30][31][32]} A few authorities (with global scope) accept all three.^[2][33]

Tiarella is native to Asia and North America. It has been introduced into Norway.^[2]

In western North America, Tiarella trifoliata is globally secure (G5).^[25] Each variety is globally secure as well.

In eastern North America, Tiarella cordifolia sensu lato is globally secure (G5).^[41] It is frequent to common throughout most of its wide distribution but becomes rare at the edges of its range, in Wisconsin and the western Upper Peninsula of Michigan, Nova Scotia, New Jersey, and Mississippi.^[42]

Many hybrids are known and cultivated. The following have been given the Royal Horticultural Society's Award of Garden Merit:

• Tiarella Angel Wings = 'Gowing'^[43]
• Tiarella 'Spring Symphony'^[44]
• Tiarella cordifolia^[45]
• Tiarella wherryi^[46]

• 
  Tiarella 'Cygnet' flowers
• 
  Tiarella 'Cygnet' leaves