Switchgrass (Panicum virgatum) occurs naturally from southern Canada to Central America. It exhibits extensive ecoclinal variation across this latitudinal gradient. Our objective was to determine the photoperiodic responses of three switchgrass cultivars with range in origin from 37 degrees N (Cave-In-Rock) to 46 degrees N (Dacotah). Five plants from each cultivar were excavated from the field near Pierre, SD during May, August, and November 2006 and May 2007.Under natural daylength in the field in central and eastern SD, all three cultivars completed floral initiation prior to June 21. Duration-of-light treatments in the greenhouse were ambient and 24-hour (daylength extension with low irradiance fluorescent bulbs). All three cultivars responded similarly to photoperiod extension with increased rate and magnitude of floral development. These cultivars could be categorized as having long-day responses to both floral initiation and development.