AbstractTwo phytochrome‐dependent processes are compared inAnagallis arvensis, an absolute long‐day plant: flowering and internode elongation. The effectiveness of red or far red radiation is different according to the length of the treatment and its position within the photoperiodic cycle: for both flowering and internode elongation, 15 h treatments with wavelengths above 700 nm are promotive, whilst wavelengths below 700 nm are inhibitory. In contrast, night breaks of 3 h red light (λ<700 nm), given after the middle of the dark period, are promotive for flowering and inhibitory for internode elongation. These results are discussed in context with data reported in the literature on photoperiodic control of long‐day