Photoperiod, the duration of the daily light phase, is a key seasonal signal that shapes vegetative growth in plants. While its role in controlling the transition to flowering has been extensively characterised, the mechanisms through which photoperiod modulates processes such as hypocotyl elongation, leaf expansion, root growth, and carbon allocation during the vegetative phase are only now coming into focus. Understanding these mechanisms is critical, as the vegetative phase establishes the architecture and resources needed for reproduction. Recent work has revealed that plants possess distinct daylength-measurement mechanisms for flowering and vegetative growth, with twilight-length sensing as a complementary mechanism. The circadian clock translates these signals into daily growth rhythms by restricting growth-promoting factors to defined time windows that widen or narrow depending on daylength, contributing to different growth outcomes between short and long photoperiods. Photoperiod further shapes the daily carbon budget, with opposing energy-sensing pathways dynamically adjusting growth to match carbon availability across the day-night cycle. In this review, we examine how photoperiod regulates vegetative growth in Arabidopsis thaliana, from daylength-measurement systems through clock-controlled transcriptional cascades that drive organ elongation and competitive shade avoidance responses, to carbon economy and stress tolerance. We consider how photoperiod modulates responses to temperature extremes, drought, and salinity, and survey emerging evidence from crops and trees. Finally, we discuss chronoculture, propose experimental recommendations, and highlight outstanding questions for the field.
