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Thinsepal monkeyflower - Mimulus hymenophyllus

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Species of Concern
Native Species

Global Rank: G2
State Rank: S1S2
(see State Rank Reason below)
State Threat Score: No Known Threats

Agency Status


External Links

State Rank Reason (see State Rank above)
See rank details. Surveys of the previous collection sites are needed to document the species' status. Without additional data, a rank of "SH" will be applicable.
  • Details on Status Ranking and Review
    Thinsepal monkeyflower (Mimulus hymenophyllus) Conservation Status Review
    Review Date = 11/07/2012
    View State Conservation Rank Criteria
    Population Size

    Score3 - Vey Small: Generally <2,000 individuals.

    Range Extent

    Score2-3 - Local or Regional Endemic: Species has a small to very small range, though its distribution is either imprecisely documented or information to precisely categorize it was lacking at the time its status was reviewed.

    Area of Occupancy

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

    Environmental Specificity

    Score1-2 - Moderate to High.


    Score0-3 - Population trends are unknown.


    Score0-3 - Threats levels are Unknown.

    Intrinsic Vulnerability

    Score1-2 - Moderate to High Vulnerability.

General Description
PLANTS: An annual with ascending stems that grow from 2 to 10 cm tall and a slender taproot. Plants have sparsely glandular-pubescent hairs. Source: Lesica et al. 2012.

LEAVES: Leaves are simple and arranged opposite on the stem. The leaf petiole is long and blades are ovate with dentate margins and 4 to 12 mm long. Source: Lesica et al. 2012.

INFLORESCENCE: Yellow flowers grow on long stems (pedicels) from the upper leaf axils. Source: Lesica et al. 2012.

The specific epithet hymenophyllus is a combination of the Greek words hymen meaning “membrane” and phyllus meaning “leaf” (Merriam-Webster 2019). Mimulus is derived from the Latin word mime, meaning “actor” or “mimic,” and the male diminutive -ulus (Merriam-Webster 2019). This is most likely referring to the mask-like appearance of the flowers.

Thinsepal Monkeyflower flowers from April through August (Nesom et al. 2012).

Diagnostic Characteristics
Montana has 14 native Mimulus species (Lesica et al. 2012). The following species share characteristics of being short annuals, often less than 15 cm tall, with yellow flowers.

Thinsepal Monkeyflower-Mimulus hymenophyllus
*Stems are generally more prostrate and at the basal nodes are sharply bent.
*Hairs: Stems and calyx with sparsely glandular-pubescent. Hairs are single-celled (not septate).
*Leaves: Long-petiolate. Petiole is mostly longer than the blade. Blades are ovate, dentate, and 4-12 mm long.
*Flower Stem: Pedicel of flower is 3-4 times longer than the calyx. In fruit the pedicel bends to form about a 90-degree angle with stem and is generally pressed against the substrate.
*Corolla: 7-20 mm long, yellow, and nearly regular.
*Calyx: 3-5 mmm long. Teeth about equal with rounded to ovate tips. About 1 mm long.

Stalk-leaved Monkeyflower-Mimulus ampliatus
*Hairs: Stems and calyx glabrate to glandular pubescent. Hairs are single-celled (not septate).
*Leaves: Petiolate. Blades are ovate, dentate, 2-10 mm long.
*Flower Stem: Pedicel is 2-3 times as long as the calyx.
*Corolla: 10-15 mm long, yellow, flares open, strongly bilabiate, and lower lip is longest.
*Calyx: Purplish, 4-8 mm long. Teeth (or lobes) equal, about 0.5 mm long.

Short-flowered Monkeyflower-Mimulus breviflorus
*Hairs: Stems and calyx with glandular-puberulent. Hairs are single-celled (not septate).
*Leaves: Petiolate, usually longer than the calyx. Blades are narrowly elliptic, entire, and 5-10 mm long.
*Flower Stem: Pedicel is 1-3 times as long as the calyx.
*Corolla: 5-8 mm long, yellow, and slightly bilabiate with subequal lobes.
*Calyx: 3-5 mm long. Teeth about equal, 1 mm or less long.

Floriferous Monkeyflower-Mimulus floribundus
*Hairs: Stems and calyx with glandular-villous, usually multi-cellular (septate), but occasionally unicellular hairs.
*Leaves: Petiolate, mostly shorter than the leaf blades. Blades are ovate, not noticeably dentate, and 4-12 mm long.
*Flower Stem: In fruit the pedicel ascends or is curved, and is not pressed against the substrate.
*Corolla: 7-11 mm long, yellow, bilabiate with a larger lower lobe and a red-spotted palate.
*Calyx: 4-7 mm long. Teeth equal, 1 mm or less long.

Short-flowered Monkeyflower-Mimulus suksdorfii
*Hairs: Stems and calyx glabrate to glandular-puberulent. Hairs are single-celled (not septate).
*Leaves: Sessile. Blades are narrowly elliptic, entire, and 5-10 mm long.
*Flower Stems: Calyx about as long as the pedicel.
*Corolla: 5-8 mm long, yellow, and slightly bilabiate with subequal lobes.
*Calyx: Purplish, 3-5 mm long. Teeth about equal, 0.5 mm or less long.
*Habitat: In drier habitats than most Monkeyflowers.

Common Large Monkeyflower-Mimulus guttatus
*Plants found in temporarily moist areas, may grow as short annuals while those in permanently moist areas tend to be taller perennials; Sometimes plants become stoloniferous (Lesica et al. 2012).
*Hairs: Stems and calyx glabrate to glandular-puberulent. Hairs are single-celled (not septate).
*Leaves: Petiole is short. Blades are ovate, serrate, and 0.5-9 cm long.
*Corolla: 15-40 mm long, yellow with red-spots, and strongly bilabiate with spreading lips.
*Calyx: 6-14 mm long. Teeth (or lobes) acute, 0.5-3 mm long, and unequal; the upper calyx lobe is largest.

Species Range
Montana Range


Range Comments
Thinsepal Monkeyflower can be found in northeast Oregon, central Idaho, and western Montana (Lesica et al. 2012).

Observations in Montana Natural Heritage Program Database
Number of Observations: 5

(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)

Thin-sepal Monkeyflower prefers to inhabit cool, moist cliffs in montane zones of Montana (Lesica et al. 2012). Elsewhere in their range, plants grow on cliffs in steep, basalt terrain above riparian habitats (Meinke 1983).
Predicted Suitable Habitat Model

This species has a Predicted Suitable Habitat Model available.

To learn how these Models were created see

In eastern Oregon, Thinsepal Monkeyflower can often be found with other plants that are common on basalt-cliffs (Meainke 1992). This may include Borsch’s Stonecrop (Sedum leibergii), Wilcox’s Beardtongue (Penstemon wilcoxi), Spiny Greasebush (Glossopetalon nevadense), and Brittle Bladder Fern (Cystopteris fragilis) (Meinke 1992). The hills around the cliffs are usually dominated by native bunchgrasses (Agropyron spicatum and Festuca idahoensis) or mixed conifer forests with trees and shrubs including Ponderosa Pine (Pinus ponderosa), Douglas Fir (Pseudotsuga menziesii), Grand Fir (Abies grandis), and Oceanspray (Holodiscus discolor) (Meinke 1992).

Few plants successfully colonize cliff faces. Thin-sepal Monkeyflower appears to maintain or possibly create its specialized micro-habitat (Meinke 1995). When individuals die, their plant material adds organic nutrients and helps build soil in what is otherwise an exposed habitat. Dense layers of Thin-sepal Monkeyflower create cover and raise humidity leaves under their canopy, which also creates suitable conditions for bryophyte communities to flourish.

The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus bifarius, Bombus centralis, Bombus flavifrons, and Bombus pensylvanicus (Thorp et al. 1983, Colla and Dumesh 2010).

Reproductive Characteristics
Mature flowers of Thin-sepal Monkeyflower are nearly regular with a yellow corolla of 7 to 20 mm long (Lesica et al. 2012). The calyx is 3 to 5 mm long with equal teeth that are about 1 mm long (Lesica et al. 2012). The pedicels are 3 to 4 times as long as the calyx (Lesica et al. 2012).

Mature fruit of Thinsepal Monkeyflower are ellipsoidal capsules, 3-6 mm long, and contain many seeds (Lesica et al. 2012).

Thin-sepal Monkeyflower responds to light in ways that change over the course of their life (Meinke 1992). In the beginning of their life as they are developing, plants are heliotrophic meaning they grow towards the sun. After pollination, their light-sensitive pedicels are adapted to grow away from light presumably to help capsules disperse their seeds towards the cliff faces. This adaptation is crucial for survival in their vertical habitat. Some seeds may be lost to gravity before plants re-orient themselves, but many manage to establish themselves in rock crevices and germinate the following growing season (Meinke 1995).

Seed collection and storage may be considered as a precautionary conservation technique due to the very narrow distribution of Thinsepal Monkeyflower (Meinke 1995).

Threats or Limiting Factors
Thin-sepal Monkeyflower's extremely narrow distribution and it vertical and isolated habitat suggest that it is not particularly vulnerable to disturbance (Meinke 1995).

Threat impact not assigned because threats in Montana are not known.

  • Literature Cited AboveLegend:   View Online Publication
    • 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.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • 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.
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Citation for data on this website:
Thinsepal monkeyflower — Mimulus hymenophyllus.  Montana Field Guide.  .  Retrieved on , from