||In switchgrass (Panicum virgatum L.), maximum yields occur with late flowering genotypes that have an extended duration of vegetative growth. However, little is known about the developmental basis for a long vegetative growth phase in perennial grasses. Studies were carried out on the leaf development of five switchgrass cultivars with varying flowering times (Cave-in-Rock, Caddo, Kanlow, NC2, and Alamo) to provide inputs for crop simulation models and to determine if a long duration of vegetative growth was associated with a reduced rate of leaf appearance or a high final leaf number. The studies were carried out in the field at College Station, TX, (30 degrees 38’N, 96 degrees 20’W), during 1994 and 1995. Thirty tillers emerging in spring were tagged and visible leaves were recorded weekly until panicles emerged. Final leaf numbers were recorded on the flowering tillers which had emerged in spring and after a June defoliation. Tillers of all cultivars emerged in March and panicles emerged during a 2-mo period from May to July. Final leaf number of spring-emerging tillers of all cultivars ranged from nine to 11, whereas summer-emerging tillers powered after seven leaves had appeared. The phyllochron [growing degree days (GDD) between the appearance of two successive leaves] for all cultivars increased with advancing leaf number. High average phyllochron values (GDD base 10 degrees C) were associated with late panicle emergence and a long duration of vegetative growth. Cave-in-Rock, the earliest flowering cultivar, averaged 79 GDD per leaf and required 634 GDD to attain panicle emergence, while Alamo, the latest to power, averaged 152 GDD per leaf and required 1777 GDD to attain panicle emergence. A slow rate of leaf appearance was identified as the primary developmental trait associated with a long duration of vegetative growth in switchgrass.