ABSTRACT
Cosmos is a genus within the family Asteraceae and tribe Helianthiae. Most of the 25 species are restricted in distribution to the highlands of Mexico and Central America. Achenes are linear, commonly beaked, and the beak terminates in 2 to 4 retrorsely barbed awns. Both C. bipinnatus Cav. variety ‘Sensation’ and C. sulphureus Cav. are widely cultivated in temperate regions and valued as ornamentals. Natural flowering in Cosmos occurs under conditions of relatively LD in late summer. The photoperiod for optimal flowering in C. bipinnatus is less than 14 hr. At day lengths longer than 14 hr flower development is delayed and flower buds appear irregularly. As facultative SDP, most C. bipinnatus will initiate flowers under LD but this will take longer than under SD. From the description of apical ontogeny, it is apparent that the age of the plant, as measured by the number of leaves or stage of apical development, affects the flowering response.
