This narrow-leaved species is widespread across North America, but relatively infrequent in the Ohio and central Mississippi Valleys. It is easily confused with foliosus, but fruits are distinctive and hybrids are unknown; 2n = 26 in pusillus versus 28 in foliosus (FNA 22). There has also been confusion with closely related plants known as berchtoldii; see notes under that name. Further study is needed.