Cool Night Temperatures Stimulate Floral Initiation in Tall Bearded Iris

Iris germanica (tall bearded iris), have long been used in the cut flower industry.  In 2008 iris stems accounted for $19.9 million of the $403 million reported of the 15 state total.  This could be increased if reblooming (remontant) iris could be scheduled to bloom over an extended season.  In 2009, studies were conducted to examine the low temperature effects on flower initiation in reblooming (‘Immortality’) and spring blooming (‘Arctic Fox’) tall bearded iris.  Twelve rhizomes per treatment, per cultivar, were potted in Sunshine Mix 1 in 15cm containers and grown in a >25C (day/night) greenhouse for 2 weeks until well rooted.  Plants were treated with 0, 3, 6, 9, 12, 15, 18, or 21 cool nights (<20C) before being returned to a warm greenhouse (>25C).  Six plants were destructively harvested from each treatment, by extracting the meristems to examine floral initiation histologically at 0, 3, or 6 days after completing cool night temperatures.  The six remaining plants were grown until either flowering or the termination of the study on 11 December 2009.  Data collected included date of first floret opening, number of florets, stem length, and leaf number.  Reblooming iris ‘Immortality’ and spring blooming ‘Arctic Fox’ initiated floral meristems after receiving a minimum of 9 nights below 20C with days above 25C.  Only ‘Immortatility’ developed flowers to anthesis, however, there was no significant effect of number of cool nights on floral development.  Conversely, there were significantly greater number of florets and longer stem lengths in the 21 cool night treatment compared to the 3, 6, or 15 cool night treatments.  When comparing leaf number, the 21 cool night treatment had greater number of leaves per plant compared to the 3 and 6 cool night treatment.  Although there was no correlation between floral development and number of cool nights in ‘Immortality’, the trend in differences between treatments stayed in the same order regardless of independent variable (number of florets, stem length, and leaf number) suggesting there may be a narrow window where the number of cool nights governs floral initiation and development.