The conditions that trigger leaf out or leaf flushing in spring can vary depending on the species or genera of plant. Most herbaceous perennials and some woody plants are triggered to leaf out by warming air or soil temperatures, for example birches ("Betula") and willows ("Salix") will attempt to send out flowers or leaves if there are a few days where ambient air temperatures exceed . This strategy is risky as a renewed burst of cold air may freeze off the new growth. Other woody plants such as oaks, walnuts, and hickories leaf out based on photoperiod, meaning they wait until the day length is long enough. These tend to be plants that have frost-intolerant foliage, so leaf out is held off until late spring when the danger of frost has largely passed.