Elephantgrass (Pennisetum purpureum Schum.) and energycane (Saccharum spp. interspecific hybrid) are perennial C4 grasses with potential for use as bioenergy feedstocks. Their biomass production has been quantified, but differences in plant morphology and the relationship of morphology with biomass harvested and plant persistence are not well understood. The objective was to quantify monthly changes in morphological characteristics of elephantgrass (cv. Merkeron and breeding line UF1) and energycane (cv. L 79-1002) and relate these changes to biomass accumulation and plant responses to defoliation. All were evaluated monthly during full-season growth or when defoliated once in mid-season. Merkeron and UF1 elephantgrass generally showed similar morphological characteristics. Relative to energycane, elephantgrass had fewer tillers early in the growing season, less seasonal variation in tiller number, greater tiller mass and maximum leaf area index (LAI), and earlier spring development of LAI. Energycane showed slower leaf area development in spring, lower maximum LAI, and shorter period of increasing tiller mass and canopy height during the growing season relative to UF1. Elephantgrass had greater incidence of lodging than energycane when exposed to high wind, likely due to greater elephantgrass tiller mass. Morphological characteristics of tall-growing bioenergy grasses help to explain differences among them in biomass production and plant persistence responses to defoliation.