Key message <p>Field experiments in South Korea show that warming alters the seedling phenology of <i>Pinus densiflora&#xa0;</i>Siebold and Zucc.. Spring needle unfolding was accelerated by spring warming and influenced by elevated daily maximum temperatures. Cumulative high-temperature exposure strongly accelerated lammas shoot development, highlighting extreme heat as a key driver of climate impacts on lammas shoot occurrence and needle unfolding.</p> Context <p>Climate warming is reshaping plant phenology worldwide; however, the specific effects of seasonal and extreme temperature accumulation on seedling development remain poorly understood.</p> Aims <p>We investigated the phenological responses of <i>Pinus densiflora</i> Siebold and Zucc. seedlings to seasonal warming in an open-field experiment in South Korea.</p> Methods <p>One hundred eight 1-year-old seedlings were subjected to four treatments: spring–fall warming (W_SF), summer warming (W_S), consistent warming (W), and control (C). We assessed spring needle unfolding and lammas shoot development, including occurrence and needle elongation stages.</p> Results <p>Elevated spring temperatures advanced spring needle unfolding in W_SF and W with 80% and 86% of seedlings reaching needle elongation over 1&#xa0;cm in May. First lammas shoots commenced development in July, with occurrence rates exceeding 95% across warming treatments by October. Rates were consistently higher in W_S and W than in W_SF and C, though W_SF also increased occurrence by late autumn. Second lammas shoots emerged only in September, with greater frequency and more advanced stages in W_S and W until November. Cumulative extreme heat exerted the greatest influence on both first and second lammas shoot development.</p> Conclusion <p>These findings indicate that extreme heat accumulation, rather than elevated mean temperatures alone, is the key determinant of altered lammas shoot formation in <i>P. densiflora</i>.</p>