Tall genotypes of elephantgrass (Pennisetum purpureum Schum.) and energycane (Saccharum spp.) are potential biomass energy plants in subtropical areas because annual dry matter (DM) yields often exceed 40 Mg ha−1. A field study was conducted to characterize crop performance of these C4 bunchgrasses during the 35‐ to 40‐wk warm season in north‐central Florida. Entries included two tall elephantgrasses (PI 300086 and N51), a tall energycane (L79‐1002), and intermediate pearlmillet [Pennisetum glaucum (L.) R. Br.] × elephantgrass hybrid (S41). In 1989 at Gainesville and in 1990 at another location 20 km away, the grasses were mowed on 28 March and fertilized 1 wk later with 200 kg N, 22 kg P, and 83 kg K ha−1. Onetime harvests of above‐ground biomass were made at 28‐d intervals from 16 May through 28 November. Final harvest DM yields (2‐yr means) were 46 Mg ha−1 yr−1 for PI 300086, 47 for N51, 49 for L79‐1002, and 37 for S41. Near‐linear DM accumulation continued for 140 to 196 d at mean crop growth rates ranging from 18 to 27 g m−2 d−1. For the tall bunchgrasses, DM production at any point during the season was accurately predicted from canopy height, since height was linearly related to the square root of DM yield (0.87 ≤ r2 ≤ 0.96). Gross energy yields (2‐yr means) in barrels of crude oil equivalent (158.8 L barrel −1) were 135 ha−1 yr−1 for PI 300086, 139 for N51, 141 for L79‐I002, and 106 for S41. The high‐yielding abilities of the grasses were related to the long linear DM accumulation period.