This has a rather narrow range, mostly in the Big Barrens region of Ky. and Tenn., plus outlying populations in Ga., Ala. and perhaps Miss. [There have been erroneous records in SERNEC (2020) from GARR and MCRE.] Typical pinnatifidum may not occur north of the Ohio Rv. (D, FNA 21). Some similar plants in Ind. and Ohio have been interpreted as forms of terebinthinaceum, perhaps introgressed with laciniatum; there are also uncertain records from Ill. and Mich. (Molano-Flores 2004; FNA 21). In Ky. pinnatifidum survives mostly along lower edges of limestone glades, along roadsides on deeper soils, and locally in old hayfields or pastures with moderate rough browsing; horses generally do not eat it (R. Seymour, pers. comm.). But farming has generally removed it. S. pinnatifidum may have originated from hybridization of lacinatum with terebinthinaceum, as reviewed by Molano-Flores (2004). Further research is needed to clarify distinction among various hybrids and introgressants involving these three species. The ranges of these species overlap in western Ky., but pinnatifidum is by far the most common one of the three withiin the Big Barrens region, and it may be the only species within central sections of this region. In addition to their deeper dissection, lower leaves of pinnatifidum are generally cuneate (versus cordate in terebinthinaceum). Apparent hybrids with typical terebinthiaceum have been collected from HART, WARR and perhaps TRIG (APSC). Possible hybrids with laciniatum have been collected from GRAY (EK), LOGA (US) and TRIG (APSC). Typical terebinthinaceum tends to flower a month or so later than laciniatum, peaking in Aug to Sep versus Jul to Aug, but there is much overlap. S. pinnatifidum may have an intermediate peak, with most colls. between 10 Aug and 3 Sep.