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Montana Field Guides

Sweetflag - Acorus americanus
Other Names:  Acorus calamus [misapplied name]

Species of Concern
Native Species

Global Rank: G5
State Rank: S1S2
(see State Rank Reason below)

Agency Status
MNPS Threat Rank:
C-value: 9

External Links

State Rank Reason (see State Rank above)
This species occurs at the edge of its range in Montana, where it has been collected from two localities in the vicinity of Flathead Lake. Current status of these populations is largely unknown. The species has likely been negatively impacted by hydrologic alterations and devolopment in the area.
  • Details on Status Ranking and Review
    Sweetflag (Acorus americanus) Conservation Status Review
    Review Date = 02/11/2013
    View State Conservation Rank Criteria
    Population Size

    Score2-3 - Very Small to Small: Population size is imprecisely known but is believed to be <10,000 individuals.

    CommentPopulation size is undocumented but is likely to be very small.

    Range Extent

    Score3 - Local Endemic or Very Small Montana Range: Generally restricted to an area <10,000 sq. miles (equivalent to the combined area of Phillips and Valley Counties) or <6 Sub-basins (4th code watersheds) Range-wide OR limited to one Sub-basin in Montana

    Area of Occupancy

    Score3 - Very Low: Generally occurring in 3 or fewer Subwatersheds (6th Code HUC’s).

    Environmental Specificity

    Score1-2 - Moderate to High.


    Score1-3 - Declining: Species is likely declining though the magnitude of declines is uncertain. Declines may be based upon range extent and/or occupied area in the recent past (approximately 30 years).

    CommentTrend data are lacking. However, it would appear likely that declines in the species' distribution and abundance have occurred.


    Score2-3 - High to Very High.

    CommentHabitat loss from development, hydrologic alterations and invasive species.

    Intrinsic Vulnerability

    Score1 - Moderate Vulnerability: Specific biological attributes, unusual life history characteristics or limited reproductive potential makes the species susceptible to extirpation from stochastic events or other adverse impacts to its habitat and slow to recover.

    Raw Conservation Status Score

    Score 15 to 18 total points scored out of a possible 19.

General Description
Scapose rhizomatous perennial with a pleasant odor. Leaves basal, folded, flattened, linear, erect, sheathing below, 50–100 cm long. Inflorescence 20–60 cm; flowers densely clustered on a narrowly cylindrical spike (spadix), 4–7 cm long, subtented by an erect, linear, leaf-like spathe, 15–70 cm long, that appears to be a continuation of the stem. Flowers perfect, tepals 6, separate, brown, ca. 1 mm long; stamens 6; ovary superior, 1- to 3-celled. Fruit a leathery, oblong, glabrous, several-seeded berry 3–5 mm long embedded in the spadix (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Flowering occurs in late July and early August. Montana collections were made in early and mid-August.

Diagnostic Characteristics
Sweetflag bears some resemblance to cattail (Typha spp.), but cattail lacks a spathe that extends beyond the inflorescence, and its inflorescence is usually longer and divided into male and female sections. Hardstem bulrush (Scirpus acutus) may appear vegetatively similar, but its inflorescence is not a finger-like spadix. Acorus calamus, introduced from Europe, is very similar, but the spadix tends to be longer than 7 cm.

Species Range
Montana Range


Range Comments
AK to NL south to WA, MT, CO, IA and NY. Known from Flathead and Lake counties (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Observations in Montana Natural Heritage Program Database
Number of Observations: 9

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Sweetflag occurs in the shallow water of ponds and marshes at lower elevations. Historical collections of this species in Montana are from marshes along lakes and streams, with cattails (Typha sp.) and Scirpus acutus. Rangewide, it occurs in shallow waters and wetlands, including ponds, marshes, swamps, and quiet riverbanks or floodplains.

Sweetflag occurs in the shallow water of ponds and marshes at lower elevations.

Sweetflag grows in relatively stable wet or shallow-water habitats, and would be vulnerable to hydrologic changes associated with impoundments or drainage. Its response to nutrient pollution is not known.

  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • Packer, J.G. and G.S. Ringius. 1984. The distribution and status of Acorus (Araceae) in Canada. Canadian Journal of Botany 62:2248-2252.
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Citation for data on this website:
Sweetflag — Acorus americanus.  Montana Field Guide.  .  Retrieved on , from