Mallow family (Malvaceae)
Description: This herbaceous perennial wildflower is 3-7' tall and largely unbranched, except for the short flowering stalks along the upper one-fourth of its length. The central stem is terete, light green, and either glabrous or canescent. The alternate leaves are 3-8" long and and 2-4" across; they are ovate to broadly ovate and crenate-dentate along their margins. Some leaves may have a pair of lobes that are shallow and broad. The upper leaf surface is yellowish green to medium green and glabrous (or nearly so), while the lower leaf surface is pale green and canescent. The slender petioles are 2-5" long and either light green or reddish green. At the base of each petiole, there is a pair of small linear stipules. The upper flowering stalks are 1½–3" long and canescent; each stalk has a single leaf and a single flower.
Individual flowers are 4-6" across and a similar length, consisting of 5 white or pink petals, 5 light or yellowish green sepals, a reproductive columnar structure with numerous stamens along its length and 5 styles with knobby stigmata at its apex. The persistent sepals are ovate and canescent (or tomentose); they are united at the base. Each flower usually has a reddish purple throat at the base of its petals, although sometimes this is absent. Directly underneath the sepals, there are about 12 linear bracts that curve upward; they are 1" in length. The blooming period occurs from mid-summer to early fall and lasts about 1 month. Individual flowers are short-lived; usually only 0-2 flowers are in bloom at the same time on a single plant. The flowers are replaced by seed capsules that are about 1–1¼" long and a little less across; they are globoid-ovoid, short-beaked, and glabrous. Each seed capsule contains a ring of seeds; individual seeds are reniform, flattened, and hairless. This wildflower spreads by reseeding itself.
Cultivation: The preference is full to partial sun, wet to consistently moist conditions, and soil containing loam, silt, or some sand with organic material.
Range & Habitat: The native Swamp Rose Mallow is found in widely scattered localities along the eastern half of Illinois, where it is uncommon. Illinois lies near the NW range-limit for this species in North America. Swamp Rose Mallow is also found in east Asia, where some additional color forms exist. Habitats include marshes, open swamps, and low soggy areas along rivers and ponds. Along the Atlantic coast of the U.S., this wildflower is also found in brackish marshes. Because of its attractive flowers, Swamp Rose Mallow is cultivated occasionally in gardens; some cultivars display evidence of hybridization with other Hibiscus spp. (Rose Mallows).
Faunal Associations: The flowers are cross-pollinated by bumblebees and other long-tongued bees, including Melitoma taurea and Ptilothrix bombiformis (Rose Mallow bee). The Rose Mallow bee is a specialist pollinator (oligolege) of native Hibiscus spp. (Rose Mallows); it sucks nectar and collects pollen from the flowers, while other long-tongued bees suck nectar primarily. The Ruby-Throated Hummingbird is another nectar-seeking visitor of the flowers. Some insects feed on various parts of Rose Mallows. The introduced pest, Popillia japonica (Japanese Beetle), gnaws on the flowers and young leaves, while the aphids Aphis gossypii and Macchiatiella rhamni suck plant juices. The caterpillars of Pyrgus communis (Common Checkered Skipper) and the butterfly Vanessa cardui (Painted Lady) sometimes feed on the leaves, while the caterpillars of the butterfly Strymon melinus (Gray Hairstreak) feed on the developing seeds. The caterpillars of several moths also feed on Rose Mallows, including Acontia delecta (Delightful Bird-Dropping Moth), Anomis erosa (Yellow Scallop Moth), Automeris io (Io Moth), and Eudryas unio (Pearly Wood Nymph).
Photographic Location: A prairie swale at Meadowbrook Park in Urbana, Illinois.
Comments: Swamp Rose Mallow has exotic oversized flowers that are very striking, particularly when they occur in a colony of plants. Other native Hibiscus spp. (Rose Mallows) in Illinois are similar to Swamp Rose Mallow in appearance and they are found in similar habitats. These species include Hibiscus lasiocarpos (Hairy Rose Mallow) and Hibiscus laevis (Halberd-Leaved Rose Mallow). Hairy Rose Mallow has felty leaves that are conspicuously hairy on both their upper and lower surfaces; Swamp Rose Mallow has leaves that are hairless (or nearly so) on their upper surfaces. Hairy Rose Mallow also has hairy ovaries/capsules, while the ovaries/capsules of Swamp Rose Mallow are glabrous. Sometimes Hairy Rose Mallow is regarded as a subspecies of Swamp Rose Mallow, or Hibiscus moscheutos lasiocarpos. The remaining similar species, Halberd-Leaved Rose Mallow, has leaves with divergent basal lobes (shaped like a halberd) or palmate lobes (maple-leaf shaped), leaf surfaces that are hairless on both the lower and upper sides, and seeds that are quite hairy. In contrast, Swamp Rose Mallow lacks such strongly lobed leaves, the lower surfaces of its leaves are canescent (or tomentose), and its seeds are hairless. In the past, the pink-flowered form of Swamp Rose Mallow that lacks a reddish purple throat was classified as a distinct species, Hibiscus palustris. However, it is now considered one of several color forms of Swamp Rose Mallow.