Resolution of Sweet Flag Species Identity Provides Basis for Managing Contrasting Impacts of Native and Introduced Sweet Flag in North American Ecosystems

The long-standing practice of some botanists, taxonomists, and natural areas managers to refer to the 2 North American species of sweet flag (Acorus calamus and Acorus americanus) collectively as A. calamus has created much confusion. A current reference text, The Manual of Vascular Plants of Northeastern United States and Adjacent Canada (Gleason and Cronquist 1991), describes only A. calamus and includes the other species name, A. americanus, in parentheses, without comment, treating the 2 names as synonymous. In addition, Les and Mehrhoff (1999) noted the long-standing confusion in a review of some of the earliest floras of the U.S. Josselyn (1672) described sweet flag growing in southeastern Maine during 1638 to 1663 as one ‘of such plants as are proper to the country,’ noting that these American plants had the appearance of the sterile European hybrid but were ‘not barren’, a comment that revealed the potential for significant taxonomic confusion. Les and Mehrhoff (1999) also observed that, in spite of this early historic reference, some botanists routinely applied the name A. calamus to populations of sweet flag while others recognized 2 species, a native sweet flag and a sterile, introduced hybrid.

Thompson’s (2002) publication provides an authoritative description of the significant differences between the species. The clear distinction is based on morphology, essential oil biochemistry, cytology, and the structural difference between enzymes that have the same functional activity (Packer and Ringius 1984, Thompson 1995). The native species is fertile, with 2 sets of chromosomes and is clearly distinguished by seed production and unique leaf vein morphology (Thompson 2000). The midvein plus between 1 and 5 other veins are more or less equally raised to form a smooth leaf surface (Figure 1). The nonnative A. calamus is a sterile hybrid with 3 sets of chromosomes. Leaves have a midvein that alone is prominently raised above the leaf surface. A. calamus typically has significantly longer and wider leaves and a longer non-fertile club-like flower spike than A. americanus (Figure 1). Although A. calamus is sterile, it is readily propagated from rhizomes.

A. americanus is a northern species, occurring in 11 Provinces of Canada and 26 States in the U.S. Populations occur as scattered clumps in wetland communities (Figure 2). In Ohio, the species has a coefficient of conservatism of 6 on a scale of 1 to 10 (Andreas et al. 2004), indicating that there is a rather high probability that this species can persist in quality remnant wet meadows. A population of A. americanus in a wetland in Holmes County, OH is growing among giant bur-reed (Sparganium eurycarpum) as part of a botanically-rich sedge meadow community. Other high quality species present include upright sedge (Carex stricta), bluejoint reedgrass (Calamagrostis canadensis), swamp lousewort (Pedicularis lanceolata), white turtle-head (Chelone glabra), closed bottle gentian (Gentiana andrewsii), and eastern prairie fringed orchid (Platanthera leucophaea) (Don Beam, personal communication).

A. calamus occurs in northeastern Canada and into the southern reaches of the U.S., from Minnesota south to Texas. A disjunct set of western populations is also found in California, Oregon, and Washington (Thompson 2000). Populations are found growing in open wet areas, marshes, swales, and along the edges of quiet water. Under favorable conditions plants spread by branching rhizomes and form dense monotypic mats. Populations generally are found throughout Ohio in farm fields and near human habitation but have invaded some natural areas (Rick Gardner, personal communication). Several populations of A. calamus have been identified within disturbed areas in natural wetlands of Greene and Montgomery Counties (Ohio). At the periphery of the massive rhizome mats, numerous slender rhizomes tipped with young shoots advance into the surrounding native vegetation and continue to displace native species. Alerted by the clear species recognition, natural area managers are beginning to control these invasive populations.

Controlling aggressive growth of extensive rhizomatous networks of A. calamus calls for effective management strategies. We investigated both chemical and mechanical methods for controlling dense populations covering 1000 m² in a...