The emergence of three artificial cohorts of Galium aparine, G. spurium and G. tricornutum with different accessions was studied over two years under the climatic conditions of north-eastern Spain. Seeds were sown in November (first cohort), January (second cohort) and February (third cohort). Higher and lower emergence percentages were obtained in the first and third cohorts, when fresher winter and dry spring occurred. However, this tendency changed when the spring was wet and emergence of third cohorts was more than that of the first. Results suggest that low temperatures in winter break the dormancy of these species, that soil moisture promotes the germination and emergence of these weeds and that emergence is highly influenced by dormancy levels during winter and spring. Lack of rain does not allow spring germination, hence, application of herbicides to control spring cohorts is not necessary.