Developmental hourglass refers to evolutionarily conserved mid-embryonic phase, as shown by transcriptomic analyses. Although transcriptional reprogramming is widespread, the hourglass concept has been applied primarily to development. Here, we identified a seasonal hourglass in transcriptome dynamics in mature leaves of perennial Arabidopsis: evolutionarily older genes were preferentially expressed in winter, whereas younger genes predominated in summer. Winter activation of endosymbiotic-origin genes supported core metabolism under cold, whereas summer activation of defence genes reflected lineage-specific novelty in biotic interactions. The seasonal hourglass was also evident in epigenome dynamics, indicating chromatin-level control in age-structured expression. Temperature-manipulation experiments demonstrated that temperature alone was sufficient to induce these patterns. These findings suggest that transient transcriptional conservation may represent a general strategy under developmental transitions or environmental constraint.