Influence of temperature and photoperiod on sprouting capacity of Cirsium arvense and Sonchus arvensis root buds. Weed Research.52, 449-457. Summary Timing of treatment is a key to successful and sustainable weed management. For perennial weeds, highly energy-demanding, repeated stubble cultivation or tillage is a common strategy in organic agriculture. The fragmentation of underground material stimulates root buds to grow and, if repeated, significantly reduces the energy and nutrient resources of the roots. However, the effect might differ depending on variation in sprouting over the season and among species. Here, we studied the impact of different photoperiods and temperatures on sprouting capacity of root buds of Cirsium arvense and Sonchus arvensis in a climate chamber experiment. Two populations of each species, collected in northern and southern Sweden, were used. Plants were exposed to long (18 h), short (12 h) or decreasing (from 18 to 8 h) photoperiods in combinations with high (18/12 degrees C for 16/8 h), low (12/6 degrees C), decreasing (from 21 to 5 degrees C in 4 weeks) or constant high (16 degrees C) temperatures. Sprouting capacity was evaluated based on the proportion of sprouting buds from short root fragments. Neither temperature nor photoperiod affected root bud sprouting in C. arvense, while in S. arvensis, photoperiod seemed to regulate sprouting capacity. The proportion of sprouted buds decreased in short photoperiods, especially if combined with high temperature. In northern conditions, this suggests that weeding strategies based on fragmentation of the root system of S. arvensis will have low efficiency if the autumns are warm.