This experiment was performed to test the hypothesis that tuber formation in potato is inhibited by short-term increases in root-zone temperature. Micro-propagated potato cv. Norland plantlets were grown in recirculating nutrient film culture under daylight fluorescent lamps at 350 μmol·m–2·s–1 PPF with at 20/16°C thermocycle at 1200 μmol·mol–1 CO2 under inductive (12-hr light/12-hr dark) or non-inductive (12-hr light/12-hr dark with a 15-min light break 6 hr into the cycle) photoperiods for 42 days. Root-zone treatments consisted of continuous 18°C, continuous 24°C, 18°C with a 24°C cycle between 14 and 21 DAP (prior to tuber initiation), and 18°C with a 24°C cycle between 21 and 28 DAP (during the period of tuber initiation). The root-zone temperature was maintained with a recirculating, temperature-controlled, heat-exchange coil submerged in each nutrient solution. Warm root-zone temperatures did not inhibit tuber formation under an inductive photoperiod. The non-inductive photoperiod resulted in a 65% reduction in tuber biomass compared to the inductive photoperiod. Continuous 24°C and exposure to 24°C prior to tuber initiation reduced tuber formation an additional 40% under the non-inductive photoperiod. Both continuous and transient 24°C root-zone temperatures increased biomass partitioning to root/stolons compared to the 18°C treatment under both photoperiods. Total plant biomass was highest in plants exposed to continuous 24°C under both photoperiods. Results suggest that transient episodes of warm (24°C) root-zone temperature do not inhibit tuber formation in potato under inductive photoperiods. However, transient episodes of warm (24°C) root-zone temperatures did interact with stage of development under the non-inductive photoperiod.