Recruitment Biology of Cleavers (Galium spp.) Populations in Western Canada

Abstract Two species of cleavers, Galium aparine L. and Galium spurium L., are known to inhabit croplands in western Canada. The latter is the more abundant of the two species. An increased abundance of these species over the most recent decades warrants a more comprehensive understanding of their developmental phenology and the cause of this increase. This study aimed to identify the base temperature and emergence characteristics of Galium spp. across different populations from western Canada. A thermal gradient plate experiment was conducted using five G. spurium populations collected from various Saskatchewan (SK) and Alberta (AB) locations. One known G. aparine reference sample was also included. A common garden experiment was conducted using the six Galium spp. populations to determine emergence characteristics. The base germination temperature identified was 2 C for all populations of G. spurium and 4 C for G. aparine. The median germination temperature for G. aparine was 8.34 C, whereas G. spurium had a similar median germination temperature of 6.5 C. Despite similar germination characteristics, the field emergence study revealed differences between populations' initiation of emergence (150 to 250 growing degree days [GDD]) and time to 50% emergence (275 to 470 GDD) in spring. Highly variable emergence among years and populations within the year (200 to 600 GDD in 2013 and 100 to 200 GDD in 2014) were observed during fall, probably due to differences in moisture availability. Cumulative emergence among populations in fall was very low (1% to 9%) compared with spring emergence (2% to 17%). Overall, this study provides evidence for a low base temperature and differences in emergence periodicity among populations, both of which may be significant factors contributing to the seasonal success of this species.