narrowleaf mountainmint

Pycnanthemum tenuifolium Schrad. 

Alternate Common Name: slender mountain mint

Scientific Synonyms: Koellia flexuosa auct. non (Walter) MacMill., Pycnanthemum flexuosum auct. non (Walter) Britton, Sterns & Poggenb., Pycnanthemum linifolium

Family: ‌mint family (Lamiaceae)

Functional Group: forbs (wildflowers)

Description 

Life cycle and growth form

Rhizomatous perennial that forms vegetative colonies. 

Height: 1-3 ft  

‌

Leaves and stem

Very narrow (1/8 in or less), opposite leaves with a strong mint/citrus odor when crushed; stems are slender, 4-sided, branched above, and entirely hairless, helping to distinguish this species from our other mountain mints.

Flower, fruit and seedhead

Flower: Small (1/4 in long) two-lipped, white flowers with scattered purple dots, in clusters at stem tips. 

Fruit/seed head: Seedheads are light brown when mature in contrast to the grey color of P. virginianum and P. pilosum seed heads. 

Pollination: Insects such as bees, butterflies, wasps, and beetles

Seed

 

Appendix D

‌Seed characteristics 

Seeds per ounce: 378,000 (IA NRCS)

1000 seed weight: 0.08 g (Seed Information Database)

Description: Seed units are tiny nutlets (nearly 1 mm long), developing within each tube-like calyx of the inflorescence.

Typical seed test 

PLS: 90% (n = 13)

Purity: 94% (n = 13)

Germination: 36% (n = 9)

Dormant: 45% (n = 10)

(averages obtained from n tests of purchased seed lots)

Habitat and range

‌Habitat: Mesic to dry-mesic soils in upland prairies; full sun; prairies, moist meadows, limestone glades, thickets, woodland openings, abandoned fields; Wetland Indicator Status is Facultative (FAC) for the Midwest.

Conservation status: Global- G5, secure; Nebraska- S1, critically imperiled (NatureServe) 

General Comments

Narrowleaf mountainmint, like other members of the genus Pycnanthemum, is visited by numerous and diverse pollinators including native bees, wasps, and flies. This species can be distinguished from the other members of the genus in Iowa by its hairless stems, narrower leaves (not exceeding 1/4 in), brown seed heads (not gray), and somewhat later flowering time. Plants are relatively easy to grow, yield heavily for a few years, and produce seed that stores well under refrigeration.

Recommendations for Seed Production 

Establishment for seed production

‌Direct seeding

Not recommended for this species.

 

‌‌Greenhouse

Seed pre-treatment: Dry/cold stratify 12 weeks at 40° F.

Sowing: Surface sow seed in greenhouse two months before last frost free date.  Water carefully (fine mist) to prevent seed from splattering out of containers.

Transplanting: Transplant into a weed barrier at 8-12 in intervals.  Plants spread clonally, so the weed barrier can be removed by the third season, but seed production typically declines by the fourth season.

‌Appendix A

Stand management

Weeds: Plastic mulch or weed barrier suppresses many weeds during the first year or two. Hand rogue weeds, being careful not to uproot seedlings or disturb roots and rhizomes of the mountainmint. An anecdotal report from a commercial native seed grower suggests that cultivation within mountainmint rows weakens plants and can cause loss of the crop.

Pests: None noted.

Diseases: None noted.

Seed production

First harvest: Some flowering and seed production first year from greenhouse grown transplants.

Yield: 10-80 bulk pounds/acre

Stand life: Peak harvests second-third year. Stand persists but seed production may decline significantly fourth year and after.

Flowering date: Flowering occurs mid-July into August.

Seed maturity/Harvest date: Mid September to early October.

Seed retention: Holds seed well, shattering occurs mid to late October.

Harvest date range at TPC (2004-2007): Sept 21 - Oct 18

Recommended harvest method: Combine, no air. 

Appendix B

Seed cleaning and storage

Cleaning process: Pre-clean by scalping thru 1/2 in and 1/4 in mesh to remove large particles and make flowable, then air-screen. (No awns or appendages to remove).

Seed storage: cool/dry (33-50° F, 30-50% RH)

Appendix C

Released Germplasm 

Source Identified material: Natural Selections/Iowa Ecotype Zone 3 (southern Iowa)

References

Gleason, H. A., & Cronquist, A. (1991). Lamiaceae. In Manual of Vascular Plants of Northeastern United States and Adjacent Canada (2nd ed., p. 443). The New York Botanical Garden. 

Hilty, J. (2019). Slender mountain mint - Pycnanthemum tenuifolium. Illinois Wildflowers. https://www.illinoiswildflowers.info/prairie/plantx/slm_mintx.htm 

Houseal, G. A. (2007). Forbs Wildflowers. In G. A. Houseal (Eds.), Tallgrass Prairie Center’s native seed production manual (pp. 44–45). Tallgrass Prairie Center - University of Northern Iowa.

Kartesz, J.T., The Biota of North America Program (BONAP). 2023. North American Plant Atlas. (http://bonap.net/napa). Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2023. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)]

NatureServe. 2024. NatureServe Network Biodiversity Location Data accessed through NatureServe Explorer [web application]. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/. (Accessed: February 29, 2024).

USDA NRCS National Plant Data Team. (n.d.). Pycnanthemum tenuifolium Schrad. USDA plants database. https://plants.usda.gov/home/plantProfile?symbol=PYTE

Species Guide Updated 12/5/2025