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Montana Field Guides

Dense Spike-primrose - Epilobium densiflorum
Other Names:  Boisduvalia densiflora

Potential Species of Concern
Native Species

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

Agency Status


External Links

State Rank Reason (see State Rank above)
Known from one historical collection in Sanders County from 1938.
General Description
Dense Spike-primrose is an annual with simple or branched stems that are 5-30 cm high. The sessile lower leaves are narrowly lance-shaped and 15-30 mm long with entire or sparsely-toothed margins, while the upper leaves are shorter and broader. Foliage is covered with soft, white, short or long hairs and may also be glandular. Flowers are borne in the axils of the upper leaves in dense spike-like inflorescences at the branch tips. Each flower has 4 pink to purple, 2-lobed petals, 2-8 mm long, and a 4-lobed calyx borne on top of the seed-bearing ovary. The 4-celled ovaries develop into straight, club-shaped capsules, 6-10 mm long, with 12-24 naked seeds.

Flowering in July or August.

Diagnostic Characteristics
Epilobium pygmaeum has petals 1.5-4 mm long and capsules that are slightly curved with 24-56 seeds. Species of Gayophytum have 2-celled ovaries, other species of Epilobium have seeds with tufts of hair at the tips, and Clarkia have stalked flowers.

Species Range

Range Comments
BC, MT, south to CA, UT, Mexico; one collection from Sanders County (Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Observations in Montana Natural Heritage Program Database
Number of Observations: 4

(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)

Vernally wet soil, often around ponds in the valleys.

The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus bifarius, Bombus centralis, Bombus fervidus, Bombus flavifrons, Bombus frigidus, Bombus melanopygus, Bombus mixtus, Bombus rufocinctus, Bombus sylvicola, Bombus ternarius, Bombus sitkensis, Bombus occidentalis, Bombus griseocollis, Bombus impatiens, Bombus insularis, Bombus suckleyi, and Bombus kirbiellus (Thorp et al. 1983, Mayer et al. 2000, Colla and Dumesh 2010, Wilson et al. 2010, Colla et al. 2011, Koch and Strange 2012, Koch et al. 2012, Pyke et al. 2012, Williams et al. 2014).

  • Literature Cited AboveLegend:   View Online Publication
    • Colla, S., L. Richardson, and P. Williams. 2011. Bumble bees of the eastern United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 103 p.
    • Colla, S.R. and S. Dumesh. 2010. The bumble bees of southern Ontario: notes on natural history and distribution. Journal of the Entomological Society of Ontario 141:39-68.
    • Koch, J., J. Strange, and P. Williams. 2012. Bumble bees of the western United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 143 p.
    • Koch, J.B. and J.P. Strange. 2012. The status of Bombus occidentalis and B. moderatus in Alaska with special focus on Nosema bombi incidence. Northwest Science 86:212-220.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • Mayer, D.F., E.R. Miliczky, B.F. Finnigan, and C.A. Johnson. 2000. The bee fauna (Hymenoptera: Apoidea) of southeastern Washington. Journal of the Entomological Society of British Columbia 97: 25-31.
    • Pyke, G.H., D.W. Inouye, and J.D. Thomson. 2012. Local geographic distributions of bumble bees near Crested Butte, Colorado: competition and community structure revisited. Environmental Entomology 41(6): 1332-1349.
    • Thorp, R.W., D.S. Horning, and L.L. Dunning. 1983. Bumble bees and cuckoo bumble bees of California (Hymenoptera: Apidae). Bulletin of the California Insect Survey 23:1-79.
    • Williams, P., R. Thorp, L. Richardson, and S. Colla. 2014. Bumble Bees of North America. Princeton, NJ: Princeton University Press. 208 p.
    • Wilson, J.S., L.E. Wilson, L.D. Loftis, and T. Griswold. 2010. The montane bee fauna of north central Washington, USA, with floral associations. Western North American Naturalist 70(2): 198-207.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Raven, P.H. and D.M. Moore. 1965. A revision of Boisduvalia (Onagraceae). Brittonia 17:238-254.
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Citation for data on this website:
Dense Spike-primrose — Epilobium densiflorum.  Montana Field Guide.  .  Retrieved on , from