© Jörg Ochsmann 2001 

During recent studies on spotted knapweed (Centaurea stoebe L.) in Europe it became quite obvious that fertile hybrids with the closely related diffuse knapweed (Centaurea diffusa LAM.) are far more frequent than expected. Although both parents are noxious weeds in North America only assumptions of the occurrence of the hybrid are made, no reports of this hybrid occur in literature. Studies of American literature on invasive knapweed species provided evidence that this hybrid occurs in North America: In two cases figures assigned to diffuse knapweed clearly showed characters of the hybrid. Studies in several herbaria (including MO and MICH) revealed a number of specimens of C. ´ psammogena from seven different states of the USA.
Two facts may be of importance for biological control of spotted knapweed, diffuse knapweed and their hybrid: 1) Introgression of diffuse knapweed into spotted knapweed was proofed by molecular data. This may result in a change of ecological characters and special adaptations. 2) While all American plants of spotted knapweed are reported to be tetraploid (2n = 36), only diploid plants (2n = 18) are known of C. ×psammogena so far. This might indicate an introduction of the hybrids from Europe, as triploid hybrids are extremely rare and sterile.