Previous laboratory studies in species of the Laminariales had revealed that both the onset of growth in early winter and the summer drop in growth rate are controlled by the annual course of daylength synchronizing endogenous, circannual clocks within the thallus. Moreover, it is known for some laminarian species that cultivation in the laboratory at constant short-days (SD) leads to arythmic, continuous growth activity of the blade throughout the year. Such a prolonged SD treatment was now performed for the first time in outdoor-cultivated Laminaria digitata. Field-grown sporophytes were collected in May from the sea near Helgoland (North Sea) and cultivated on the island of Sylt (North Sea) in temperature-controlled outdoor tanks (300 l) at 10°C for 1.5 years either at constant 8 h light per day provided by an automatic blind on top of the tank, or at ambient daylength. At constant SD, the growth rate stayed at a steady-state level of approximately0.4 cm day-1 from the end of June until mid-October, while at ambient daylength growth rate declined steadily after June to half of the rate in SD in October. Growth became light-limited between October and February at both conditions, and in the second year, from July onwards there was again the tendency of higher growth rates at constant SD than at ambient daylength. Further work is required to find out whether SD treatment in summer would also prevent the summer drop of growth rate in other perennial seaweed species, e.g. commercially valuable red algae. This would increase the chance of more constant biomass production throughout the year, and decrease the danger of the cultivated perennial algae being overgrown by annual epiphytes in summer.