Adventitious root bud development and assimilate translocation were studied in Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR] grown in nutrient solution in controlled environments using combinations of two photoperiods (PP) (13 and 15 h), three day/night shoot temperatures (ST) (15/5, 25/15, and 30/22 C), and three root temperatures (RT) (10, 20, and 30 C). Total root bud elongation increased with RT and length of PP and was greatest (65 cm/plant) at 25/15 C ST, 15-h PP, and 30 C RT. The number of root buds produced was greatest at 20 C RT (7.3 to 10.3 buds/plant), whereas variations in PP and ST had little effect. Total dry-matter production was greatest (7.2 g/plant) at 15-h PP, 30/22 C ST, and 20 C RT. To study phloem translocation, photoassimilates were labeled in Canada thistle plants by exposing mature leaves to 14CO2. Net assimilate translocation from a source leaf following 24-h temperature acclimation was affected little by RT and ST, but was greater under the 13-h PP than under the 15-h PP. After 7 days of temperature preconditioning, net translocation of 14C-assimilates increased with both RT and ST, but no effects due to PP were noted. With 24-h temperature acclimation, net assimilate accumulation in roots was enhanced by 13-h PP and low ST (15/5 C), whereas RT itself had no effect. In temperature-preconditioned plants, 10 C RT enhanced assimilate accumulation in roots, but ST and PP had no effect.