Cosmos

Cosmos require exposure to long nights (about 15 hours) to initiate flower buds. Because of nearly 24-hr daylength in early summer, cosmos in interior, Alaska often bloom late in the season or not at all. Considerable difference to dark treatments exists among cultivars of cosmos. The purpose of this project was to identify cultivars of cosmos that will bloom early in the season with and without treatments to lengthen the dark period.

Approach
We grew 21 cultivars of cosmos (Cosmos bipinnatus) seedlings in the greenhouse and treated half with an extended, 15-hr dark period using blackout cloth The treatment occurred daily for two weeks just prior to transplanting. We compared flowering dates and the number of flowers produced by control and treated plants

Progress
Individual cosmos cultivars varied significantly in response to dark treatment. Three cultivars (Daydream, Sweet Dreams and Yellow Garden) did not bloom at all without dark treatment and Campus Apricot produced an average of only 1 bloom in late August. All other cultivars bloomed with or without dark treatment but blooming was delayed as much as 4 weeks in 11 cultivars. All cultivars tested produced more blooms when given the dark treatment except for Versailles Mix. Sensations Mix and Sensations Radiance which were breed to be less sensitive to long night conditions.

Impact
This information is useful for the greenhouse industry and home gardeners in selecting for appropriate cosmos cultivars to grow for season-long color. It will also be useful for growers of field-grown cut flowers who can select early-blooming cultivars.