The Alstroemeria cultivars Diamond, King Cardinal and Libelle were grown for 18 months under five lighting regimes with, and without, soil cooling. The aim was to optimize the daily investment of light energy from artificial sources with respect to photoperiod and photosynthetic fluence rates and to elucidate possible links between reactions to photoperiod and root-zone temperature. The more photons (photosynthetically active radiation, PAR) that were supplied to the plants per day (8, 11 and 13molm - 2 ), the higher was the total production of flowering stems. The total yield from regimes with 13molm - 2 day - 1 was higher when the light was spread over 20 and 16h compared to 12h. In treatments with soil cooling, the plants flowered continuously under all combinations of photoperiods and photosynthetic fluence rates, and the summer and autumn recession in flower production that occurred for non-cooled `King Cardinal' and `Diamond' was the same under all lighting regimes. It is concluded that it might be more cost-effective to spread the daily investment of light over 20 rather than 16 or 12h when the total energy budget and CO 2 costs are taken into consideration.