Diplacus is a genus of flowering plants in the lopseed family, Phrymaceae, traditionally placed in Scrophulariaceae before molecular work moved the broader monkeyflower complex out of the figworts. The genus was first described by the English-born American botanist Thomas Nuttall in 1838 and went on to have a famously turbulent taxonomic history: by 1905 most of its members had been sunk into the catch-all genus Mimulus, where they remained for more than a century until DNA evidence and a 2012 revision by Barker and colleagues restored Diplacus as a distinct lineage with roughly 46 to 49 accepted species.
Members of the genus are nearly all native to the western United States and northwestern Mexico, with California serving as the center of diversity. They are characteristically plants of dry, sunny, rocky places — chaparral slopes, granite outcrops, desert mountains, and serpentine soils — and most are perennial subshrubs or annual herbs with showy, two-lipped tubular flowers. Within the family, Diplacus is one of the two largest genera, sharing the dominant monkeyflower role with Erythranthe (about 111 species), while the narrowly redefined Mimulus now contains only a handful of taxa. The defining technical features that hold Diplacus together are parietal placentation in the ovary and sessile (stalkless) flowers, both inherited from the older Mimulus sensu lato circumscription.
The genus is best known horticulturally through Diplacus aurantiacus, the sticky monkey-flower, an orange-flowered subshrub of the California coast and Sierra foothills that holds the Royal Horticultural Society's Award of Garden Merit and supports hummingbirds, native bees, and the caterpillars of common buckeye and variable checkerspot butterflies. Other species, such as the diminutive alpine Diplacus mephiticus and the Channel Islands endemic Diplacus parviflorus, illustrate the genus's range from high-elevation specialists to maritime annuals. As of the most recent treatments, Diplacus is not tracked as an invasive group anywhere in the world; the conservation pressure runs the other way, with several narrow California endemics on regional rare-plant lists.
Taxonomy
Diplacus was established by Thomas Nuttall in 1838 (Ann. Nat. Hist. Ser. I. 1: 137) and is currently accepted by GBIF and POWO-aligned checklists as a genus in Phrymaceae (order Lamiales), with about 85 descendant taxa recorded by GBIF. The genus had been merged into Mimulus by 1905 and stayed there until a 2012 molecular revision by Barker and colleagues split Mimulus sensu lato into several segregate genera; Diplacus was resurrected as one of the two largest of these (~46 species), distinguished within the family by parietal placentation and sessile flowers. The closely related Erythranthe absorbed the bulk of the remaining American monkeyflowers, while Mimulus in its modern, narrow sense contains only a small handful of species. Many older herbarium records and field guides still use the Mimulus combinations (e.g. Mimulus parryi for what is now Diplacus parryi), which is reflected in regional databases such as SEINet.
History
The genus has had an unusually visible taxonomic seesaw. Nuttall described Diplacus in 1838 from western American material, but the genus was sunk into Mimulus by 1905 and remained there throughout most of the 20th century, so generations of botanists and gardeners knew the plants only as "shrubby Mimulus". Between 2002 and 2012 a series of molecular studies, culminating in the Barker et al. 2012 treatment, demonstrated that Mimulus sensu lato was polyphyletic. The resulting split moved most North American shrubby and sessile-flowered species back into Diplacus, while the herbaceous, pedicellate species shifted into Erythranthe and a much narrower Mimulus was retained for the type species and its closest relatives.
Distribution
Diplacus is essentially a western North American genus. Wikipedia describes the ~49 accepted species as native to the western United States and northwestern Mexico, and SEINet's specimen-backed records concentrate the genus across California, Oregon, and the southwestern states, with California holding the lion's share of the diversity. Several species reach into Baja California and the Channel Islands, while a few high-elevation taxa (e.g. Diplacus mephiticus) extend up the Sierra Nevada and into adjacent ranges. The genus is not naturally present outside this region.
Ecology
Diplacus species are characteristic plants of dry, open, rocky habitats — chaparral, coastal scrub, granite outcrops, serpentine slopes, and high-desert mountains. The well-studied Diplacus aurantiacus tolerates a wide range of substrates from sandy and rocky soils to serpentine, and its tubular flowers are visited by hummingbirds and native bees; despite a phenolic leaf resin that deters most herbivores, it serves as a larval host for the common buckeye and variable checkerspot butterflies. These ecological traits — drought tolerance, pollination by hummingbirds and bees, and reliance on rocky or disturbed ground — recur across many members of the genus.
Cultivation
At genus level, Diplacus is cultivated almost exclusively as "shrubby Mimulus" or "bush monkey-flower", and references to "Mimulus" in older horticultural literature typically refer to species now placed in Diplacus or Erythranthe. The Phrymaceae treatment notes that about 16 species across the family are still in cultivation, with a handful of particular importance to gardeners. The flagship horticultural species is Diplacus aurantiacus, an evergreen subshrub used in water-conserving and California-native gardens that holds the Royal Horticultural Society's Award of Garden Merit; its hybrids and color forms (yellow, orange, red, cream) are widely sold. The genus as a whole prefers full sun, sharp drainage, and lean rocky soils — conditions that mirror its native habitats.
Conservation
Diplacus is not listed in the IUCN Global Invasive Species Database, which explicitly returns no record for the genus, so the group poses no recognized invasion risk outside its native range. Conservation concern within Diplacus tends to run in the opposite direction: several California species have very narrow ranges (Channel Islands and serpentine endemics, for example) and appear on state and regional rare-plant inventories, though formal status varies species by species rather than at genus level.
Cultural uses
Cultural use within the genus is best documented for Diplacus aurantiacus: Indigenous peoples of California, including the Miwok and Pomo, used the plant medicinally for minor ailments such as sores and burns, and incorporated the bright tubular flowers into decorative arrangements.
