Sp. Pl. 1: 446. 1753.
Herbs, annual or perennial,or subshrubs, terrestrial or amphibious, 0.5–15(–30) dm, green, gray-green, or gray-white glaucous, usually glabrous, sometimes puberulent. Stems erect, weakly erect, or procumbent, usually branched, youngest growth narrowly 4-ridged or winged, submerged stems sometimes thickened by spongy tissue. Leaves opposite, subopposite, alternate, subalternate, or whorled; sessile or subsessile; blade ovate, obovate, oblong, oblong-lanceolate, lanceolate, oblanceolate, linear, linear-oblong, linear-lanceolate, orbiculate, or suborbiculate, base rounded, cordate, or attenuate. Inflorescences terminal and spikelike or racemes. Flowers sessile, subsessile, or short-pedicellate, actinomorphic, mono-, di-, or tristylous; floral tube perigynous, usually cylindrical or obconic (campanulate in L. portula), 8–12-ribbed; epicalyx segments shorter to or up to 2 times longer than sepals; sepals 6, narrowly deltate to subulate, obtuse and thickened in L. tribracteatum; petals caducous, (0 or)6, purple, lavender, rose, rose purple, pink, or white, sometimes with purple or red midvein; nectary encircling base of ovary or absent; stamens (2–)4–6(–12), usually 6 or 12, in 1 or 2 whorls, complementing style lengths; ovary 2-locular; placenta elongate; style slender, included or well exserted; stigma capitate. Fruits capsules, walls thin and dry, usually dehiscent, dehiscence septicidal or septifragal (indehiscent and splitting irregularly in L. portula, placenta and seeds remaining within capsules). Seeds 10–90(+), obovoid to fusiform or subglobose, to 1 mm; cotyledons ± complanate.
North America, Mexico, South America, Europe, Asia, Africa, Australia, introduced in West Indies.
Species ca. 35 (12 in the flora).
Lythrum is represented in North America by equal numbers of native and introduced species. Lythrum thymifolia Linnaeus, native to the Mediterranean, was reported from Mobile, Alabama, on ballast in 1893 (C. T. Mohr 1901). It is similar to L. hyssopifolia but consistently has stamens reduced to two or three, leaves usually less than 2 mm wide, and is monostylous.
Lythrum is one of six genera in Lythraceae with a heterostylous breeding system and one of three in the family with tristylous species (also Decodon and the incipiently tristylous tropical genus Adenaria Kunth). Lythrum includes mono-, di-, and tristylous species. All native North American Lythrum species are distylous. Among the introduced species, L. junceum, L. salicaria, and L. virgatum are tristylous and L. hyssopifolia, L. portula, and L. tribracteatum are monostylous. The native North American species represent a single lineage corresponding taxonomically to subsect. Pythagorea Koehne. Some species of the subsection are taxonomically difficult and have shown little molecular divergence (J. A. Morris 2007). Hybridization and introgression are suspected where two native species co-occur, and also possibly between non-native species or native with non-native (B. Ertter and D. Gowen 2019).
|1||Floral tubes broadly campanulate, 1 × 1.5 mm, widths greater than lengths; capsules indehiscent, splitting irregularly; stems mostly decumbent to creeping.||Lythrum portula|
|1||Floral tubes cylindrical or obconic, 3–7 × (0.4–)1–3 mm, lengths distinctly greater than widths; capsules septicidal or septifragal; stems erect, decumbent, or prostrate.||> 2|
|2||Inflorescences spikelike, terminal (in L. virgatum racemose proximally, spikelike distally); flowers tristylous; stamens 12.||> 3|
|3||Leaf blades lanceolate, bases cordate or rounded; plants usually gray-puberulent, sometimes glabrate.||Lythrum salicaria|
|3||Leaf blades lanceolate to narrowly linear, bases attenuate; plants glabrous.||Lythrum virgatum|
|2||Inflorescences racemose, diffuse, leafy; flowers mono-, di-, or tristylous; stamens (2–)4–12.||> 4|
|4||Herbs or subshrubs 0.5–6 dm; stems decumbent or procumbent to erect, unbranched, branched from near base, or sparsely branched (much-branched distally in L. ovalifolium), sometimes with short accessory branches distally.||> 5|
|5||Floral tubes obconic, with red spots on proximal half; epicalyx segments about equal to and more prominent than sepals; flowers tristylous; stamens 12.||Lythrum junceum|
|5||Floral tubes obconic or cylindric, without red spots; epicalyx segments shorter than, equal to, or longer than sepals; flowers mono- or distylous; stamens (2–)4–6(–12).||> 6|
|6||Floral tubes obcconic, lengths 8–10 times width at tube base; epicalyx segments as long as or longer than sepals; flowers monostylous.||Lythrum tribracteatum|
|6||Floral tubes cylindric or slightly obconic, lengths 5 times or less widths; epicalyx segments about 2 times longer than sepals; flowers mono- or distylous.||> 7|
|7||Leaves opposite throughout, equal to or shorter than internodes; flowers distylous.||Lythrum flagellare|
|7||Leaves mostly alternate, sometimes opposite proximally, mostly longer than internodes and closely overlapping distally; flowers mono- or distylous.||> 8|
|8||Plants gray-green glaucous; leaf blade bases rounded; petals pink or rose, 1/2 length of floral tube; flowers mono- stylous.||Lythrum hyssopifolia|
|8||Plants green to slightly gray glaucous; leaf blade bases attenuate; petals pale purple to purple, sometimes with red midvein, about as long as floral tube; flowers distylous.||Lythrum ovalifolium|
|4||Herbs or subshrubs (3–)5–15 dm; stems erect, much-branched distally; flowers distylous.||> 9|
|9||Leaves usually opposite or subopposite throughout, rarely alternate; floral nectaries absent.||Lythrum lineare|
|9||Leaves usually opposite to subopposite proximally, alternate distally or sometimes throughout; floral nectaries encircling bases of ovaries.||> 10|
|10||Branch leaves abruptly and conspicuously smaller than those on main stems; floral tube obconic; pedicels slender; epicalyx segments and sepals about equal length.||Lythrum curtissii|
|10||Branch leaves gradually smaller than those on main stems; floral tube cylindrical; pedicels stout; epicalyx segments equal to or to 2 times length of sepals.||> 11|
|11||Herbs or subshrubs green or slightly gray glaucous; leaf blades ovate to oblong and bases subcordate to rounded, or lanceolate to linear- lanceolate and bases attenuate.||Lythrum alatum|
|11||Herbs whitish gray glaucous; leaf blades oblong-lanceolate proximally, mostly linear or linear-oblong distally, bases rounded.||Lythrum californicum|