Technical Abstract: Upland cultivars of switchgrass (Panicum virgatum L.), such as ‘Cave-in-Rock’, are most frequently recommended for the northeastern USA. Lowland ecotypes typically originate from more southerly locations and have coarser stems than upland ecotypes. Long-term data on the persistence and yield of lowland ecotypes are limited for the northeastern USA. In this study, I report the yields and tiller densities of synthetic populations of lowland germplasm compared with Cave-in-Rock after 20 years of management. In 1989, as part of a germplasm evaluation trial, six synthetic switchgrass populations (NCI-16, NCII-16, NCIII-16, NCII-4, NCIII-4, and NCII-8; progenitors of the recently released cultivars BoMaster and Peformer) along with Cave-in-Rock were established in replicated small-plots at Rock Spring, PA. At the end of the germplasm evaluation in 1994, the plots were maintained with reduced management until 2008. In May 2008, the plots were reactivated and N fertilizer was applied at 100 kg ha**-1. Plant height, tiller density, and biomass yield were determined in November of 2008 and 2009. There were no significant differences among switchgrass populations and Cave-in-Rock for plant height, tiller density, or biomass yield. Although the synthetic populations are mainly adapted to the southeastern USA, these data suggest that under bioenergy crop management (one harvest annually) plant material developed from these populations should persist as far north as central Pennsylvania.