(horse mint, wild bergamot) Plants annual or more commonly perennial herbs, with slender taproots or rhizomes. Stems erect or ascending, bluntly to sharply 4-angled, unbranched or branched, sparsely to densely hairy or rarely glabrous. Leaves sessile or petiolate, the petiole unwinged or winged, usually with a strong fragrance when crushed. Leaf blades variously linear to ovate or ovate-triangular, the margins sharply toothed, the surfaces glabrous or hairy, also with conspicuous, sessile glands. Inflorescences terminal (some of the lower clusters sometimes associated with relatively large bracteal leaves and thus appearing axillary), consisting of 1 dense flower cluster or 2–7 clusters that are noticeably separate along the axis forming a spike, each cluster subtended by a conspicuous involucre of numerous bracts (the outer bracts more or less leaflike, but often whitened or strongly pinkish-, reddish- or purplish-tinged), the inner bracts shorter, narrower, and more scalelike or hairlike), each with numerous flowers, these sessile or nearly so, sometimes with 1 slender bractlet smilar to the innermost bracts at the base. Calyces actinomorphic or nearly so, lacking a lateral projection, more or less symmetric at the base, more or less cylindric, the tube strongly 13–15-nerved (-ribbed), glabrous or hairy in the mouth, hairy externally, not 2-lipped, the lobes all similar, much shorter than the tube (longer elsewhere), triangular to narrowly triangular-tapered, not spinescent, not becoming enlarged at fruiting, but usually becoming papery. Corollas 5–50 mm long, strongly zygomorphic, white to cream-colored, pale yellow, pink, lavender, purple, or red, the lower lip usually with darker spots or blotches, the outer surface glabrous or more commonly moderately to densely pubescent with fine, spreading, glandular and/or nonglandular hairs, in some species also with sessile glands, the tube narrowly funnelform with a tubular lower portion and a usually elongate, expanded throat, 2-lipped, the upper lip entire or shallowly notched at the tip, relatively elongate and slender, straight or arched downward, often somewhat hooded at the tip, the lower lip spreading or strongly arched downward, 3-lobed or appearing more or less entire, sometimes with short, terminal, toothlike extension(s). Stamens 2, ascending under the upper lip, not or only slightly exserted, the anthers small, the connective very short and inconspicuous, the pollen sacs 2, spreading, white, pink, purple, or yellow to yellowish brown. Ovary deeply lobed, the style appearing basal or nearly so from a deep apical notch. Style exserted, unequally 2-branched at the tip. Fruits dry schizocarps, separating into usually 4 nutlets, these 1.2–2.0 mm long, oblongobovoid (rounded at the tip, angled or tapered at the base), the surface yellowish brown to brown or black, glabrous, smooth or finely pebbled. Fifteen to 20 species, U.S., Canada, Mexico.

The genus Monarda has been monographed taxonomically twice (McClintock and Epling, 1942; Scora, 1967), yet some workers have suggested that more research needs to be done on the group (Gill, 1977; B. L. Turner, 1994). Hypotheses of species limits and relationships based on morphological features and crossability should be tested using molecular markers. A number of species are cultivated as ornamentals for their showy flowers. Native Americans used most of the species ceremonially and in a number of medicinal treatments, including as a cold and sore throat remedy, for sores, burns, and other skin problems, and for aches and fevers. A few of the species, notably M. didyma, are used in herbal teas. A few of the species are among several members of the Lamiaceae that have been investigated as sources of natural rubbers (Buchanan et al., 1978). Dried material of some species once had limited use as a fragrant stuffing for pillows, and more recently has been used in sachets, potpourri, perfumes, and aromatherapy. The genus also has been used as a spice and flavorant. However, the bergamot oil used to flavor Earl Grey tea is extracted from a variant of a widely cultivated tree native to southeastern Asia, the sour orange, Citrus ×aurantium L. ssp. bergamia (Risso & Poit.) Wight & Arn. ex Engl. (Rutaceae).

An aberrant specimen was collected by Jean-Baptiste Duerinck in St. Louis, possibly in the late 1830s (see the chapter on botanical history in the introductory section of Volume 1 of the present work [Yatskievych, 1999]). As discussed in detail by Dorr (1986), one duplicate of this collection in the National Botanical Garden of Belgium Herbarium was the basis for the description of M. villosa M. Martens, which today is considered a synonym of M. bradburiana. However, during his studies of the genus Monarda, Carl Epling annotated a duplicate accessioned in the Missouri Botanical Garden Herbarium as not corresponding to M. bradburiana, but instead perhaps representing M. media Willd. Monarda media was treated as a species by McClintock and Epling (1942), but Scora (1967) and Gill (1977) provided evidence that it represents a variable series of hybrids between M. didyma and either M. clinopodia or M. fistulosa. Such hybrids, if they exist in Missouri, almost certainly represent cultivated material rather than wild plants, given the rarity of M. clinopodia and M. didyma in the state. Plants attributed to this name otherwise are encountered sporadically in the northeastern United States, no closer to Missouri than northern Illinois and central Kentucky. This hybrid is thus excluded from the Missouri flora for the present.