Abstract Non-structural carbohydrates (NSCs) are vital for plant growth, with their levels influenced by light intensity and seasonal changes. However, research on how varying light conditions due to forest density and seasons affect carbon allocation in new twigs and old branches is scarce. Vitex negundo var. heterophylla is a leading shrub species in the warm temperate zone’s shrub layer. In this study, we conducted a detailed sampling of V. negundo var. heterophylla branches, differentiating new twigs and old branches across phenological stages under three densities of Robinia pseudoacacia forests. Our sampling schedule was as follows: March (dormant period), May (sprouting period), July (leaf spreading period), September (flowering and fruiting period), and December (deciduous period). The results showed that the seasonal patterns of carbon allocation in the new twigs and old branches were largely in harmony. The starch concentration in the old branches under the high density was significantly lower than in the other two densities during the growing season, but the NSC concentration in December remained at a high level and did not significantly decrease. These indicated even though the light environment was unfavorable to understory V. negundo var. heterophylla during the growing season, cold tolerance in December was not inhibited. And the concentrations of soluble sugars and starch in the new twigs were typically higher than those found in the old branches. This dynamic suggests a strategic prioritization of resources to fuel the growth and development of the plant during the current year. Findings from this study not only contribute to our understanding of carbon allocation strategies in V. negundo var. heterophylla but also provide critical insights for managing and predicting the resilience of warm temperate shrub ecosystems to environmental change.