Cold Treatments Alter the Photoperiodic Flowering Response of Some Herbaceous Perennial Species

Abstract

Many species of herbaceous perennials either require a cold treatment for flowering or exhibit enhanced and more desirable flowering characteristics following a cold period. For some species, the photoperiodic induction of flowering can change following cold treatments, the parameters generally becoming less strict as plants receive longer durations of cold until a saturation duration is reached. To quantify the minimum and saturation durations of cold for flowering under short days (SD) and long days (LD), five species were selected and cooled for 0, 3, 6, 9, 12, or 15 weeks at 5 °C, then forced at 20 °C under 9-h photoperiods without (SD) or with a 4-h night interruption (LD). The effects of cold and photoperiod on flowering varied by species. For example, under SD, 0%, 80%, or 100% of Leucanthemum xsuperbum `Snowcap' flowered after 0, 3, or ≥6 weeks at 5 °C, respectively, and time to flower decreased from 103 to 57 days as the duration of cold increased from 3 to 12 weeks. `Snowcap' cooled for ≥3 weeks, then forced under LD, flowered completely in 45 to 55 days. Flowering percentage of Lavandula angustifolia `Hidcote' reached 100 under LD or SD only when plants were cooled for ≥6 or 15 weeks, respectively. Under LD, flowering percentage of Astilbe chinensis pumila progressively increased from 0 to 100 as cold treatment increased from 0 to 15 weeks; flowering percentage under SD was low regardless of cold treatment. Additional species and flowering characteristics will be discussed.