Trichostema dichotomum
Bastard pennyroyal
Image of Trichostema dichotomum

Photo by Ryan P. O'Connor 

Key Characteristics

Annual forb of dry barrens and prairies; stem branched and finely glandular-hairy with opposite elliptic leaves; tubular bluish-purple flowers have open throats and long, curving stamen filaments.

Status and Rank

  • State Status: T
  • State Rank: S2
  • Global Rank: G5


County NameNumber of OccurrencesYear Last Observed
Van Buren82008
Distribution map for Trichostema dichotomum

Updated 5/15/2018. Information is summarized from MNFI's database of rare species and community occurrences. Data may not reflect true distribution since much of the state has not been thoroughly surveyed.


Bastard pennyroyal occurs in oak savanna areas in southern Lower Michigan.

Natural Community Types


For each species, lists of natural communities were derived from review of the nearly 6,500 element occurrences in the MNFI database, in addition to herbarium label data for some taxa. In most cases, at least one specimen record exists for each listed natural community. For certain taxa, especially poorly collected or extirpated species of prairie and savanna habitats, natural community lists were derived from inferences from collection sites and habitat preferences in immediately adjacent states (particularly Indiana and Illinois). Natural communities are not listed for those species documented only from altered or ruderal habitats in Michigan, especially for taxa that occur in a variety of habitats outside of the state.

Natural communities are not listed in order of frequency of occurrence, but are rather derived from the full set of natural communities, organized by Ecological Group. In many cases, the general habitat descriptions should provide greater clarity and direction to the surveyor. In future versions of the Rare Species Explorer, we hope to incorporate natural community fidelity ranks for each taxon.

Associated Plants

Tumble grass, little bluestem, big bluestem, fall witch-grass, ground juniper, prairie dock, sheep sorrel, bitter milkwort, dwarf-dandelion, poverty grass, nut-grass, shining sumac, and sassafras.


This species requires open habitat. Prescribed fire or selective harvesting to maintain openings will likely be beneficial.

General Survey Guidelines

Random meander search covers areas that appear likely to have rare taxa, based on habitat and the judgment of the investigator.

Survey Methods

Page Citation

Michigan Natural Features Inventory. 2007. Rare Species Explorer (Web Application). Available online at [Accessed Aug 16, 2018]

More Information

See MNFI Species Abstract


Survey References

Technical References

Facebook link