An understanding of flower induction mechanisms is useful to growers who intend to sell plants in flower on specific dates of the year. At MichiganStateUniversity, we have studied flower induction requirements for over 500 species and cultivars of cold-hardy herbaceous perennials and have attempted to classify each as to their photoperiodic and vernalization requirements. Though many perennials fall easily into a given category, some do not. A few plants fall into rather unique flower induction categories such as facultative long-day plants, intermediate-day plants, and long-day-short-day plants. In this review, issues related to flower induction and its categorization in this diverse group of plants is discussed. Variation in flower induction requirements within species and genera is also discussed.