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Montana Field Guide

Montana Field Guides

Toothcup - Rotala ramosior

Species of Concern
Native Species

Global Rank: G5
State Rank: S1S2
(see State Rank Reason below)
State Threat Score: No Known Threats
CCVI: Highly Vulnerable
C-value: 6

Agency Status


External Links

State Rank Reason (see State Rank above)
Rare in Montana, where it is known from approximately a half-dozen wetland sites in the valley bottoms in the western portion of the state. Potential threats and impacts to the known occurrences, as well as population trends, need to be evaluated.
General Description
Toothcup is a small, glabrous annual with simple or branched, erect to prostrate stems that are up to 10 cm high. The opposite, narrowly lance-shaped leaves have short petioles and are 15-30 mm long. The tiny, solitary flowers are sessile in the axils of upper leaves. The cup-shaped, shallowly 4-lobed calyx is 1-3 mm long in flower but up to 4 mm long in fruit. The 4 stamens are shorter than the 1 mm long, white petals. The fruit is a globose capsule that is ca. 3 mm across.

Flowering in late July-August.

Diagnostic Characteristics
Ammannia robusta has sessile, clasping leaf bases. Members of the genus Gratiola (Plantaginaceae) have short-stalked flowers and separate sepals. A hand lens will be necessary to discern the flower characters.

Species Range
Montana Range Range Descriptions


Range Comments
Throughout U.S. south to S. America (Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Observations in Montana Natural Heritage Program Database
Number of Observations: 41

(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)

Open, wet, gravelly soil around ponds and sloughs in the valley zone.
Predicted Suitable Habitat Model

This species has a Predicted Suitable Habitat Model available.

To learn how these Models were created see

Ecological Systems Associated with this Species

Threats or Limiting Factors
Threat impact not assigned because threats are not known (MTNHP Threat Assessment 2021).

  • Literature Cited AboveLegend:   View Online Publication
    • Hitchcock, C. L., A. Cronquist, M. Ownbey, and J. W. Thompson. 1961. Vascular Plants of the Pacific Northwest, Part 3. Saxifragaceae to Ericaceae. Seattle, WA and London, England: University of Washington. 614 pp.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Cook, C.D.K. 1979. A revision of the genus Rotala (Lythraceae). Boissiera 29:1-?.
    • Fassett, N. C. 1985. (3rd ed.) A Manual of Aquatic Plants, with Revision Appendix by E. C. Ogden. University of Wisconsin Press, Madison. 405 pp.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2022. Manual of Montana Vascular Plants, Second Edition. Fort Worth, TX: BRIT Press. viii + 779 p.
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Citation for data on this website:
Toothcup — Rotala ramosior.  Montana Field Guide.  .  Retrieved on , from