E-Flora BC: Electronic Atlas of the Flora of British Columbia

Acorus americanus (Raf.) Raf.
American sweet-flag (sweetflag)
Acoraceae (Calamus family)

Introduction to Vascular Plants

© Daniel Stewart  Email the photographer   (Photo ID #71231)

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Distribution of Acorus americanus
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American sweet flag is a rhizomatous, emergent wetland plant that occurs across Canada and the northern United States. It is found in open wet stillwater areas, such as marshes, quiet edges of lakes, ponds, and swales. It flowers in late spring to mid-summer. It is a fertile species (diploid) that produces mature fruits, unlike the introduced, sterile Acorus calamus (found in Washington, Oregon and California, and eastern North America). The two species are readily separated by variation in the prominence of the veins on the leaves--major veins in this species are equally raised above leaf surface (Thompson 2010). The earliest UBC Herbarium record for this species in BC is a 1945 collection from Shuswap Lake by G. A. Muirhead.

In his rare plants of the Fraser Valley article, Lomer (2011) says: "Rare and widespread in the BC interior, very rare in the Fraser Valley. Sweetflag has been collected from two wetland sites in Pitt Meadows, but had not been reported since 1973 (UBC: V.C. Brink s.n., T. Barnard 107). Recent search efforts in the Pitt River area over the past fifteen years [were unsuccessful] until 2009 when a dense stand was rediscovered at the site where Vernon Brink collected it near South Gilley Slough, Pitt Meadows (UBC: Lomer 7083). The introduced Acorus calamus from Europe may also occur in the Fraser Valley. It can be told from the native species by its sterility; it cannot produce ripe fruit, whereas the native can." Extracted from Botanical Electronic News #432, January 2011, with permission.

Flora North America provides a detailed description of this species.

Species Information

Click on the image below to view an expanded illustration for this species.

Perennial aquatic herb from a thick rhizome; stems 20-60 cm tall.
All basal, linear, erect, 40-80 cm long, 0.8-2 cm wide, the midveins offset.
Inflorescence a dense, fleshy, linear, 5- to 10-cm long spike subtended by a leaflike bract, the bract as long as or longer than the flower stem; flowers perfect; perianth of 6, scale-like, yellow or brown sepals.
Broadly pyramidal, hard, dry, gelatinous within.
Packer and Ringius (1984) have shown that our fertile diploid species is different than the sterile triploid Eurasian species (A. calamus L.) which is introduced in E North America.

Source: The Illustrated Flora of British Columbia

USDA Species Characteristics

Flower Colour:
Blooming Period:
Late Spring
Fruit/Seed characteristics:
Colour: Green
Present from Summer to Fall
Source:  The USDA

Habitat / Range

Shallow water in the montane zone; rare in S BC east of the Coast-Cascade Mountains; N to AK and NT, E to PQ and NB and S to ME, PA, VA, IL, IA, NE, MT, ID and WA.

Source: The Illustrated Flora of British Columbia


Ecological Framework for Acorus americanus

The table below shows the species-specific information calculated from
original data (BEC database) provided by the BC Ministry of Forests and Range.
(Updated August, 2013)

Site Information
Value / Class




Elevation (metres) 1170 1525 1880
Slope Gradient (%) 0 32 65
Aspect (degrees)
[0 - N; 90 - E; 180 - S; 270 - W]
270 270 270
Soil Moisture Regime (SMR)
[0 - very xeric; 4 - mesic;
8 - hydric]
3 3 4
Modal Nutrient Regime
Number of field plots
 species was recorded in:
Modal BEC Zone Class
All BEC Zones (# of stations/zone) species was recorded in:


The climate type for this species, as reported in the: "British Columbia plant species codes and selected attributes. Version 6 Database" (Meidinger et al. 2008), is not evaluated, unknown or variable.

Synonyms and Alternate Names

Acorus calamus L.
Acorus calamus var. americanus (Raf.) H.D. Wulff.

Taxonomic and Nomenclatural Links

Additional Photo Sources

Species References

MacKinnon, Andy, Linda Kershaw, John Thor Arnason, Patrick Owen, Amanda Karst, Fiona Hamersley-Chambers. 2009. Edible & Medicinal Plants of Canada. Lone Pine Publishing, Edmonton.

General References