Abstract

Seeds of Rhododendron catawbiense Michx. and Rhododendron maximum L. were germinated at 25°C (77°F) and 25°/15°C (77°/59°F) with daily photoperiods of 0, 1/2, 1/2 twice daily, 1, 2, 4, 8, 12, or 24 hr. After 30 days at 25°C (77°F), germination of R. catawbiense in darkness was 5% but increased to 64% at 25°/15°C (77°/59°F). At both temperatures, gernlination >95% was attained by day 15 for photoperiods of 1/2 to 12 hr. Regardless of temperature, seeds of R. maximum required light for germination. At 25°C (77°F), increasing photoperiods increased germination with 79 and 81% germination occurring by day 21 for the 12- and 24-hr photoperiods, respectively. The alternating temperature of 25°/15°C (77°/59°F) enhanced germination when light was limiting. At this temperature, germination of 92 to 97% was reached by day 21 for photoperiods ≥4 hr.