Effect of daylength on rosette prevention in Elatum hybrids of Delphinium

"Seedlings of many Elatum hybrids of Delphinium form rosettes when they are grown from late autumn to winter and this rosette formation causes serious problems of retarded bolting and malformed flower spikes. This study was conducted to determine the effects of the photoperiod on bolting and flowering in Elatum hybrids of Delphinium at different temperatures and to identify desirable methods for rosette prevention. Bolting was greatly stimulated by exposing seedlings to a photoperiod longer than 18 hours at a daily minimum night temperature of 15degC. The stimulating effect of light break was less, similar to that of a 16-hour photoperiod. Under photoperiod of 15 hours, some seedlings remained not to bolt when the day time temperature was prevailing (average temperature of 20-30degC) and the night time temperature was kept at 20degC. Earlier bolting resulted in earlier flowering with a decreased number of leaves to the first floret and fewer florets. These results suggest that exposing seedlings to a photoperiod longer than 18 hours before lowering the night time temperature to 20degC was effective in preventing them from forming rosettes in Delphinium forcing."