Quantifying the Cooling Period for Flowering of 'Nellie White' Easter Lily

Abstract

In the commercial development of an innovative process for the production of pathogen-indexed Easter lilies (Lilium longiflorum Thunb.) in less than a year, it became desirable to quantify the cooling period so that the time of flower induction/initiation could be determined. Based upon studies of forcing flowering plants from bulbs, I hypothesized that cooling could be quantified using the cool temperature unit (CTU), defined as 1 °C below 21 °C for 1 hour, and the warm temperature unit (WTU), defined as 1 °C above 21 °C for 1 hour. The purpose of these studies was to determine if the hypothesis was valid. With 'Nellie White' Easter lily, it was determined that cooling could be quantified as hypothesized. The minimum threshold number of CTU that would induce flowering in at least one plant of the test population after exposure to long days was between 1200 and 2400, whereas the threshold number of CTU that would induce flowering in 100% of the population was 3600. The optimum threshold, i.e., the number of CTU that would result in the best market quality, was estimated to be 4800. The studies demonstrate that market quality, pathogen-indexed flowering potted Easter lilies growing continuously from bulblets can be produced in about 1 year, and that the cooling required to sensitize the plants to long days, which is central to the success of the fast production process, is quantifiable.