Some switchgrass (Panicum virgatum L.) cultivars originating in the northern USA show limited late-summer and fall growth when grown in more southerly locations despite adequate temperature and moisture. Our objective was to determine the effects of low-light photoperiod extension on the dry matter yield of switchgrass cultivars originating from contrasting latitudes. Seedlings of the four cultivars (Cave-in-Rock, 'Caddo', 'Kanlow', and 'Alamo') were grown for 100 d in a greenhouse in winter under ambient (11.2-12.2 h) and extended (18 h) photoperiods. Photoperiod extension was with 7 mumol m(-2) s(-1) photosynthetic photon flux density. Cultivars responded differently to photoperiod extension (P < 0.05). Large increases in dry matter yield at extended photoperiods were observed in Cave-in-Rock (+98%) and Caddo (+129%). The dry matter yield of Kanlow increased by 31%, whereas dry matter yield of the southernmost cultivar Alamo was not affected by photoperiod extension. Yield increases for Cave-in-Rock and Caddo were associated with increased tiller number and weight. A reversal of dormancy for some cultivars with a photoperiodic signal verified that dormancy was not simply a result of a low radiant input. (C) 2003 Elsevier Ltd. All rights reserved.