Flowering of Newly Introduced Herbaceous Perennial Ornamentals in Response to Photoperiod and Low-Temperature Treatments

Each year, a few hundred new ornamental plant varieties are introduced to the floriculture industry by plant breeders. Some of these introductions are herbaceous perennial crops with improved flowering characteristics for the consumer and earlier, more uniform flowering for the producer. Each year, we perform experiments with a variety of new perennial plant introductions focused on determining flowering responses to photoperiod and low-temperature (LT) treatments, and we report responses of fifteen recent introductions. Propagules were exposed to 0 to 10 weeks at 5 degrees C and then grown in a controlled greenhouse environment at 20 degrees C under a 9-hour short-day photoperiod (SD), a 9-hour day with 7 hours of day-extension lighting from incandescent bulbs (long day; LD), or a 16-hour day consisting of the natural photoperiod with supplemental lighting from high-pressure sodium lamps (LD+HPS). An LT treatment was required to induce uniform flowering in Heuchera 'Hollywood' but not in Heuchera 'Fire Chief'. Coreopsis 'Citrine' and Salvia nemorosa 'Sensation White' flowered faster following an LT treatment, which could be from earlier flower initiation or development of initiated flowers during the LT period. Photoperiod did not influence flowering of Agastache 'Cotton Candy' and the two Heuchera cultivars while Echinacea 'Secret Passion', Gaillardia xgrandiflora 'Commotion Frenzy' and 'Gallo Dark Bicolor', Leucanthemum 'Victorian Secret', Salvia 'Sensation White', and Sedum 'Cloud Walker' and 'Mr. Goodbud' flowered more rapidly under LD (a facultative LD response). Coreopsis 'Citrine', Coreopsis verticillata 'Sienna Sunset', Nepeta 'Blue Dragon', Polemonium 'Snow and Sapphires', and Sedum 'Black Beauty' required LD for flowering. Nearly all of the cultivars had a more floriferous response when grown under the higher light levels of the LD+HPS treatment compared to under LD. Under the most inductive conditions, flowering time at 20 degrees C ranged from 4 weeks or less (Heuchera 'Fire Chief') to 10 weeks or longer (Sedum 'Black Beauty' and 'Mr. Goodbud').