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Epilobium densiflorum

(Lindley) P. Hoch & Raven

Dense-flower Spike-primrose

G5Secure Found in 12 roadless areas NatureServe Explorer →
G5SecureGlobal Rank
UnknownThreat Impact
Identity
Unique IDELEMENT_GLOBAL.2.153786
Element CodePDONA01020
Record TypeSPECIES
ClassificationSpecies
Classification StatusStandard
Name CategoryVascular Plant
Endemicoccurs (regularly, as a native taxon) in multiple nations
KingdomPlantae
PhylumAnthophyta
ClassDicotyledoneae
OrderMyrtales
FamilyOnagraceae
GenusEpilobium
Synonyms
Boisduvalia densiflora(Lindl.) S. Wats.
Other Common Names
Denseflower Willowherb (EN) denseflower willowherb (EN) Épilobe densiflore (FR)
Concept Reference
Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Conservation Status
Rank Method Rank calculation - Biotics v2
Review Date2025-03-27
Change Date1988-08-01
Edition Date2025-03-27
Edition AuthorsJohnson, J. (2025)
Threat ImpactUnknown
Range Extent200,000-2,500,000 square km (about 80,000-1,000,000 square miles)
Number of Occurrences> 300
Rank Reasons
Epilobium densiflorum is an annual herb found in seasonally wet habitats in western North America. With a large range extent, more than 600 occurrences, abundant habitat, broad habitat preferences, and no obvious intrinsic vulnerabilities, this species is considered secure.
Range Extent Comments
Epilobium densiflorum is native to western North America from southern British Columbia to northern Baja California. Most occurrences are found in California and Oregon with scattered observations in Nevada, Arizona, Montana, and Idaho. Range extent was estimated using herbarium specimens, photo-based observations, and NatureServe Network occurrence data collected between 1994 and 2025 (RARECAT 2024, GBIF 2025, iNaturalist 2025, NatureServe 2025, SEINet 2025).
Occurrences Comments
By applying a 1 km separation distance to herbarium specimens, photo-based observations, and NatureServe Network occurrence data documented between 1994 and 2025, it is estimated that there are more than 600 occurrences rangewide (RARECAT 2024, GBIF 2025, iNaturalist 2025, NatureServe 2025, SEINet 2025).
Threat Impact Comments
Although threats are not widely documented, Epilobium densiflorum is potentially threatened by development, road maintenance, invasive species, and other threats in some places. However, their overall impact is likely negligible given the species' broad range, large number of occurrences, tolerance of light disturbance, and affinity for typically abundant habitats, as well as no obvious intrinsic vulnerabilities.
Ecology & Habitat

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.

Diagnostic Characteristics

B. GLABELLA, the other member of this genus in our area, has petals 1.5-4 mm long and capsules that are slightly curved with 24-56 seeds. Species of GAYOPHYTUM have 2-celled ovaries, EPILOBIUM have seeds with tufts of hair at the tips, and CLARKIA have stalked flowers.

Habitat

Epilobium densiflorum grows in vernally wet places, moist pastures, woodlands, meadows, along streams and ditches, alluvial valleys, often on low ground in volcanic or sandy soils (FNA 2021).

Reproduction

Unlike many Epilobium species, Epilobium densiflorum does not have a tuft of hairs on the seed to facilitate wind dispersal (FNA 2021).
Terrestrial Habitats
Forest/WoodlandGrassland/herbaceousOld field
Palustrine Habitats
HERBACEOUS WETLANDSCRUB-SHRUB WETLANDRiparian
Other Nations (2)
CanadaN2
ProvinceRankNative
British ColumbiaS2Yes
United StatesN5
ProvinceRankNative
MontanaSHYes
IdahoSNRYes
WashingtonSNRYes
CaliforniaSNRYes
NevadaS3Yes
ArizonaSNRYes
OregonSNRYes
UtahS1Yes
Threat Assessments
ThreatScopeSeverityTiming
Unknown/undeterminedUnknownUnknownUnknown

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Plant Characteristics
DurationANNUAL
Economic Value (Genus)No
Roadless Areas (12)
California (9)
AreaForestAcres
Barker ValleyCleveland National Forest11,940
Bear MountainLos Padres National Forest913
CajonSan Bernardino National Forest7,548
Granite ChiefTahoe National Forest6,546
Horse Creek RidgeSan Bernardino National Forest8,969
Mill PeakSan Bernardino National Forest7,884
Sill HillCleveland National Forest5,294
TrabucoCleveland National Forest23,341
TuleAngeles National Forest9,861
Nevada (3)
AreaForestAcres
ButtermilkHumboldt-Toiyabe National Forest19,373
Mahogany RidgeHumboldt-Toiyabe National Forest16,765
Santa RosaHumboldt-Toiyabe National Forest54,555
References (8)
  1. Flora of North America Editorial Committee (FNA). 2021. Flora of North America north of Mexico. Vol. 10. Magnoliophyta: Proteaceae to Elaeagnaceae. Oxford Univ. Press, New York. xxiv + 456 pp.
  2. Global Biodiversity Information Facility (GBIF). 2025. Global Biodiversity Information Facility (GBIF) data portal. Online. Available: https://www.gbif.org/ (accessed 2025).
  3. Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
  4. Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.
  5. NatureServe. 2025. NatureServe Network Biodiversity Location Data. NatureServe, Arlington, Virginia.
  6. NatureServe's Rapid Analysis of Rarity and Endangerment Conservation Assessment Tool (RARECAT). 2024. Version: 1.1.1 (released Oct 01, 2024).
  7. Raven, P. H. and D. M. Moore. 1965. A revision of BOISDUVALIA (ONAGRACEAE). Brittonia 17:238-254.
  8. Southwest Environmental Information Network (SEINet). 2025. Collections Databases. Online. Available: https://swbiodiversity.org/seinet/collections/index.php (accessed 2025).