Determining the duration of the juvenile phase of Coreopsis grandiflora (Hogg ex Sweet.), Gaillardia x grandiflora (Van Houtte), Heuchera sanguinea (Engelm.) and Rudbeckia fulgida (Ait.)

The juvenile phase of Coreopsis grandiflora 'Sunray', Gaillardia x grandiflora 'Goblin', Heuchera sanguinea 'Bressingham', and Rudbeckia fulgida 'Goldsturm' was characterized by exposing plants with 0 to 20 nodes to cold treatments at 5 degrees C. After 0, 10, or 15 weeks cold treatment, plants were grown at constant 20 degrees C under a 9 h photoperiod with 4 h night-interruption lighting (22:00 to 02:00 h; long days) or under a 9 h photoperiod (short days), Based on flowering percentage and time to flower, we concluded that the juvenile phase of Coreopsis, Gaillardia, Heuchera, and Rudbeckia ended when they had approximately 8, 16, 19, and 10 nodes, respectively. Coreopsis required cold treatments to flower. A cold treatment was not required for flowering of Gaillardia and Rudbeckia; however, such treatment improved their flowering percentage and greatly accelerated flowering. Increasing cold duration from 10 to 15 weeks did not influence time to flower of Coreopsis, Gaillardia, and Rudbeckia; however, the increase enhanced flowering percentage of Coreopsis and Gaillardia and hastened flowering of Heuchera. Heuchera was a day-neutral plant after ten weeks cold treatment, Rudbeckia was an obligate long-day plant, and Gaillardia and Coreopsis were quantitative long-day plants. (C) 1998 Elsevier Science B.V.