Tiarella, commonly known as foamflowers, is a genus of perennial herbaceous flowering plants in the family Saxifragaceae, order Saxifragales. The genus comprises seven species distributed across Asia and North America: one species (T. polyphylla) occurs in eastern Asia, one (T. trifoliata) in western North America, and five in eastern North America.
Plants grow from short, slender rhizomes and produce basal leaves that are often lobed or compound. Three morphological features are used to distinguish species: the presence or absence of stolons, the size and shape of basal leaves, and the presence or absence of stem (cauline) leaves. Two species — T. austrina and T. stolonifera — bear stolons; plants from the southern Blue Ridge Mountains southward tend to have larger basal leaves with an extended terminal lobe.
The genus was established by Carl Linnaeus in 1753 with two founding species, T. cordifolia and T. trifoliata. A third, T. polyphylla, was added by David Don in 1825. Several species and numerous garden hybrids are widely cultivated as shade-tolerant ornamentals; T. cordifolia, T. wherryi, and several cultivars hold the Royal Horticultural Society's Award of Garden Merit. The taxonomy of eastern North American species remains an area of active research, with treatments recognizing between two and six species.
Etymology
The generic name Tiarella is a Latin diminutive of tiara, meaning "little turban" — a reference to the shape of the seed capsules, which resemble the tiered headgear of the same name.
Distribution
Tiarella is native to Asia and North America. One species, T. polyphylla, occurs in eastern Asia; one species, T. trifoliata, is found in western North America; and five species are distributed across eastern North America, with the greatest diversity in the Appalachian region. The genus has also been introduced into Norway.
Taxonomy Notes
The genus was founded by Carl Linnaeus in 1753 with two species, T. cordifolia and T. trifoliata. In 1840, Torrey and Gray proposed two sections — Anthonema (western, leafy-stemmed, paniculate) and Eutiarella (eastern, naked-stemmed, racemose) — but these sections have largely fallen out of use because T. polyphylla does not fit the dichotomy cleanly. Accepted taxonomy now draws on three sources: the Flora of China treatment for T. polyphylla, the Flora of North America treatment for T. trifoliata and related taxa, and a 2021 revision by Guy Nesom for eastern North American taxa. As of 2022, the eastern North American taxonomy remains in flux, with different authorities recognizing between two and six species.
Cultivation
Numerous hybrids are in cultivation, particularly as shade-tolerant groundcovers in temperate gardens. The following taxa have received the Royal Horticultural Society's Award of Garden Merit: Tiarella Angel Wings (cv. 'Gowing'), T. 'Spring Symphony', T. cordifolia, and T. wherryi.
Conservation
Tiarella trifoliata is globally secure (NatureServe rank G5) in western North America, with each recognized variety also ranked G5. Tiarella cordifolia sensu lato is likewise globally secure (G5) across most of its eastern North American range but becomes rare at the periphery of its distribution, including in Wisconsin, the western Upper Peninsula of Michigan, Nova Scotia, New Jersey, and Mississippi.
