Roadsides, fields, clearings, disturbed sites; dry open forests, meadows, banks; a less common immigrant than H. aurantiacum and H. piloselloides. First collected in Michigan in 1936 (Cheboygan Co.), and spreading most rapidly since the 1960’s.

The ± glaucous nature of leaves of H. piloselloides is often not evident on dried specimens, making separation from H. caespitosum more difficult. The conspicuous long, often stiff hairs on leaves and stem, often larger leaves, and darker inflorescence of larger heads give H. caespitosum a reasonably distinct aspect hard to express in a key and open to exception on any one character. The leaves are often sparsely stellate-pubescent beneath (in addition to long hairs), but are more acute at the apex than the ± obtuse to rounded ones of H. flagellare.

 A few collections from Allegan, Berrien, Calhoun, Kalamazoo, Lenawee, Luce, Newaygo, and Oceana Cos. appear intermediate between this species and H. flagellare; one from Iosco Co. appears intermediate with H. aurantiacum.