Perennial grasses have received increased attention in the last few decades for their potential to mitigate global energy and climate change challenges, however, basic aspects of their physiology and aging remain poorly understood. In addition, research on perennial grasses is underpinned by conventional field experiments whose designs still neglect important aspects of plant-age dynamics in these long-lived species. We used a REpeated PLAnting Year (REPLAY), also known as staggered-start, experimental design to study Miscanthus × giganteus nitrogen (N) needs and age-related changes in physiology and phenology. We conducted a REPLAY experiment with three planting years and five N rates during the M. × giganteus yield-building phase at three sites across Iowa, USA. Miscanthus × giganteus reached peak yields (~27.1 Mg dry matter (DM) ha-1) after the third year, and N fertilization effects changed with establishment conditions, but not with plant age (Chapter 2). To make stand comparisons on a common time scale rather than thermal time or calendar days, I developed a unified morphological development scale (Chapter 3). Older M. × giganteus stands usually started development earlier and developed further in a given growing season. Nitrogen fertilization did not change phenology in one-year-old stands and mainly affected emergence and senescence in older (two- and three-year-old) stands (Chapter 4). Older stands also had lower photosynthetic rates and leaf N contents, suggesting potential N dilution in older, larger plants (Chapter 5). However, N fertilization increased leaf N in older stands up to juveniles’ levels but did not compensate for the age-related photosynthetic decline, suggesting that N was not a limiting factor for carbon assimilation. These results could be a consequence of life strategy shifts from a competitor strategy that prioritizes resource acquisition in juvenile stands to a stress-tolerant strategy that prioritizes nutrient remobilization and winter survival in older stands. Further research is needed to study other potential hypotheses.