Erysimum, commonly known as wallflowers, is a large genus of around 150–200 recognised species (with approximately 410 total taxa across all ranks) in the mustard family Brassicaceae. Plants range from short-lived annuals to herbaceous perennials and sub-shrubs, usually growing 25–53 cm tall. The genus is immediately distinguished by its characteristic malpighiaceous (two-rayed or star-shaped) trichomes covering the stems and leaves, and by its elongate siliques (seed pods) containing many seeds. Petals are typically obovate to spatulate, narrowing abruptly into a long claw, and are most commonly yellow or orange, though cultivated selections span white through red and russet.
The genus name derives from the Ancient Greek erysimon, which may trace to eryo ("to drag") or rhyomai ("to ward off" or "to heal"), reflecting early medicinal associations. The familiar common name "wallflower" alludes to the plant's natural ability to colonise loose mortar in old walls and rocky crevices.
Erysimum was formally published by Linnaeus in Species Plantarum (1753), building on Tournefort's earlier circumscription — hence the authority Tourn. ex L. The cultivated wallflower long circulated under the name Cheiranthus cheiri, but that genus is now treated as a synonym and the plant is accepted as Erysimum cheiri.
The genus is distributed across the temperate Northern Hemisphere — primarily Eurasia, North Africa, Macaronesia, and North America south to Costa Rica — with many species confined to narrow endemic ranges such as the slopes of Mount Etna, the California coast, the Canary Islands, and Cyprus. In Europe, diverse species grow from the Mediterranean to alpine habitats; Switzerland alone hosts at least 14 species and subspecies.
Ecologically, Erysimum species are visited by a broad suite of pollinators including bees, bee flies, hoverflies, butterflies, beetles, and ants, though some species show near-specialist pollination relationships (E. scoparium on Tenerife is pollinated almost exclusively by the bee Anthophora alluadii). The genus produces glucosinolates and, in at least 48 species, cardiac glycosides (cardenolides) that deter generalist herbivores. The evolutionary radiation of Erysimum from its Brassicaceae relatives is dated to the Pliocene (approximately 2.33–5.2 million years ago).
Etymology
The genus name Erysimum comes from the Ancient Greek word erysimon, which may derive either from eryo, meaning "to drag," or from rhyomai, meaning "to ward off" or "to heal," reflecting the plant's early use in traditional medicine. The familiar English common name "wallflower" refers directly to the plant's habit of growing in loose mortar and rocky wall crevices, where it can take root and flower in apparently inhospitable conditions.
Distribution
Erysimum has a broad temperate distribution centred on Eurasia but extending to North Africa, Macaronesia (Canary Islands and Madeira), and North America south to Costa Rica. The genus reaches greatest species richness in central and eastern Eurasia and around the Mediterranean basin. Numerous species are narrow endemics: notable examples include populations restricted to the slopes of Mount Etna (Sicily), the California coastline, the Canary Islands (e.g. E. scoparium on Tenerife), and Cyprus (E. kykkoticum). North American diversity includes species such as E. asperum, E. capitatum, and E. inconspicuum, among more than 50 species recorded from the continent. In Switzerland, at least 14 species and subspecies are documented, including E. cheiranthoides, E. cheiri, E. crepidifolium, E. repandum, E. rhaeticum, E. sylvestre, and E. virgatum.
Taxonomy
Erysimum was formally established by Linnaeus in Species Plantarum (1753), based on the earlier generic concept of Tournefort, giving the authority Tourn. ex L. The genus belongs to family Brassicaceae, order Brassicales, class Magnoliopsida. GBIF records approximately 410 total descendant taxa; accepted species counts cited in literature range from around 150 (Wikipedia) to 200 (Flora of North America / SEINet). The chromosome base number is variable: x = (6) 7, 8 (9–17).
The genus Cheiranthus, long used for the cultivated wallflower, is now treated as a synonym of Erysimum; accordingly, the garden wallflower is accepted as Erysimum cheiri. Molecular dating places the diversification of Erysimum from other Brassicaceae relatives in the Pliocene, approximately 2.33–5.2 million years ago.
Ecology
Erysimum species occupy a wide range of habitats including dry rocky slopes, walls, coastal cliffs, alpine meadows, and disturbed ground. The flowers attract a diverse pollinator assemblage — bees, bee flies (Bombyliidae), hoverflies (Syrphidae), butterflies, beetles, and ants — though some species have evolved near-specialist pollinator relationships. On Tenerife, E. scoparium is pollinated almost exclusively by the solitary bee Anthophora alluadii.
The genus employs multiple chemical defences. At least 48 species produce cardiac glycosides (cardenolides) that make them toxic or unpalatable to generalist vertebrate herbivores; E. crepidifolium is notably toxic to deer and ibex. Glucosinolates are broadly present across the genus. Despite these defences, Erysimum is grazed by various insect herbivores including Lepidoptera larvae, weevils, and beetles; E. cheiranthoides resists oviposition by the cabbage white butterfly (Pieris).
Cultivation
Erysimum species and hybrids are popular ornamental garden plants valued for their fragrant, warm-toned flowers in late winter through summer. Most cultivated selections derive from or are closely related to E. cheiri (southern European origin). They thrive in well-drained soils in full sun and tolerate poor, alkaline, or rocky substrates; they dislike acid conditions. On sunny walls they may persist for several years beyond the typical biennial cycle, though they are susceptible to fungal and bacterial diseases as well as clubroot (Plasmodiophora brassicae). USDA hardiness zones 5–9; hardy to UK hardiness zone 6. The cultivar 'Bowles's Mauve' is widely grown and holds the RHS Award of Garden Merit.
Propagation
Seed is sown in spring either directly outdoors or in cold frames; germination typically occurs within approximately three weeks. Biennials intended for spring flowering are sown in early summer and transplanted to their flowering position in autumn. Some perennial cultivars and selections (such as 'Bowles's Mauve') are more reliably propagated from softwood cuttings, as they may not come true from seed.
Conservation
Several Erysimum species face serious conservation threats due to their extremely restricted ranges. E. kykkoticum, a Cyprus endemic, is considered nearly extinct. E. teretifolium, confined to Santa Cruz County, California, is federally listed as endangered. The narrow endemism characterising many Erysimum species — often tied to a single mountain, island, or coastline — makes them especially vulnerable to habitat loss, invasive species pressure, and climate-driven range shifts.
Cultural uses
Ethnobotanical use of Erysimum spans at least two millennia. Pliny the Elder documented applications in his Naturalis Historia (c. 77 CE) and Dioscorides in De Materia Medica (c. 70 CE). Medieval herbals continued to describe therapeutic uses of wallflowers for various ailments. In traditional Chinese medicine, E. cheiranthoides (wormseed wallflower) was used to treat heart disease, likely owing to its cardenolide content. Ukrainian ethnobotany employed E. diffusum medicinally. Documented folk applications across cultures include poultices for wounds and rheumatic joints, topical washes for muscle pain, inhaled crushed leaves for headaches, warmed root poultices for toothache, crushed seed infusions for digestive cramps, and seed paste applied to relieve sunburn.
