Catherine M. Whitman

Controlled Flowering of Herbaceous Perennial Plants

We have determined the juvenility, cold (vernalization), photoperiod, and cultural requirements necessary to flower many herbaceous perennials on specific dates and at specified sizes. We identified the long-day (LD) requirement by conducting critical photoperiod experiments in which critical photoperiod is defined as the photoperiod that elicits a complete, rapid, uniform flowering response in a population. The LD requirement for all herbaceous perennials tested can be met by either a continual photoperiod of 16 hours or more, or by four hours of light as an interruption in the middle of the night. Nightinterruption lighting for two hours is adequate for rapid, uniform flowering in some species. Incandescent, cool-white fluorescent, metal halide, and high-pressure sodium lamps effectively promote an LD response; differences among the irradiance levels required for flower induction are horticulturally insignificant. To maintain vegetative growth on quantitative and qualitative LD species, photoperiods should be 10 hours or fewer.