Cirsium quercetorum occurs in the north and south Coast ranges of California from Mendocino to San Luis Obispo counties. It overlaps in range and habitat with several other thistle species and has been reported to hybridize with C. andrewsii, C. douglasii, C. occidentale, C. remotifolium var. odontolepis, and C. fontinale var. fontinale (F. Petrak 1917; J. T. Howell 1960b). Considerable variation occurs within the range of C. quercetorum, and two of the variants have been given taxonomic recognition as vars. walkerianum and xerolepis. Additional study over the range of the species is needed to determine whether these or other variants should be recognized formally.