Plants of catchweed bedstraw from different Norwegian locations and from three other countries were compared with respect to morphological factors, herbicide sensitivity, and genetic variation. For the morphological comparison of cotyledons, whorls, leaves, and fruits five populations, grown outdoors but sheltered from rain, were used. Plants from Belgium and Sweden showed a high similarity, whereas one Norwegian population differed significantly in nearly all parameters. The same populations were used for a comparison of the sensitivity to the herbicide mecoprop-P. In this study, only slight differences appeared between the five populations. Finally, a deoxyribonucleic acid (DNA) sequence analysis of the internal transcribed spacer (ITS) regions was performed. The entire sequence of the ITS1 and ITS2 and the 5.8S subunit of ribosomal DNA were obtained from 15 populations (12 from Norway and one each from Sweden, Belgium, and Germany). The sequences had a length between 590 and 662 base pairs; intraspecific length variation was observed. Based on six insertions–deletions and 26 nucleotide substitutions, two DNA types could be distinguished. The first type consisted of eight Norwegian populations, whereas the second one contained the other seven populations including all non-Norwegian populations. The sequence alignments were used to build a phylogenetic tree. The results of the morphological comparison mostly corresponded with the results of the ITS sequence analysis. The variation was only to some extent correlated with the geographic distribution of the populations.
Nomenclature: Mecoprop-P; catchweed bedstraw, Galium aparine L. GALAP.