Participants in the USA National Phenology Network’s (USA-NPN) Nature’s Notebook (NN) program select and observe species from a large list of plant and animal species. This national effort to collect standardized ground observations of the phenological phases—or observable life cycle stages—of species by researchers, natural resources managers, students, and volunteers, supports a wide range of scientific applications and management decisions routinely made by citizens and professionals. High-quality data is vital to this effort and this guide is meant to acclimate participants to information referenced within the Nature’s Notebook program. The botanical information covered in this document will help observers make reliable plant observations. Along with basic botany, the phenology vocabulary used here is meant to complement the names used for the Nature’s Notebook plant phenophases (those words are quoted in “italicized red type”). Botanical terminology is defined at the end of this primer; those terms are in italicized green type the first time they are used on a page.