This study examines how phenological indicators, which track the life cycles of plants and animals, could change from 2025–2100 as simulated in a regional climate model over the contiguous U.S. Chilling units quantify the presence of cooler weather that can benefit plants prior to their growing season. They are projected to decrease in the southern U.S., possibly inhibiting agricultural production. Spring onset is projected to occur earlier in the year, advancing by 1–4 days on average over each future decade. Risk of false springs (damaging hard freezes after spring onset) increases in the western U.S. Our findings highlight the need to understand effects of climate change during transitional seasons, which can impact agriculture and ecosystems.