Technical Abstract: Orchardgrass (Dactylis glomerata L.) is a valuable pasture species in much of temperate humid North America. However, profuse and early flowering in spring creates management problems for graziers and reduces intake of livestock in a management-intensive rotational grazing system. The objectives of this study were estimate environmental stability, genotypic variability, and frequency of non-flowering and sparse-flowering plants in two sparse-flowering orchardgrass populations. Seven cultivars and 299 half-sib families were evaluated for 2 years at five locations between 42º and 47ºN latitude. Populations WO-SF-B and WO-SF-C were later in maturity, produced fewer panicles per plant, and had higher frequencies of sparse-flowering and non-flowering plants than the cultivars. Plants had varying levels of expression of the non-flowering trait, ranging from slightly sensitive (sparse-flowering in one year) to highly sensitive (stable non-flowering across years), with highly sensitive plants found only within populations WO-SF-B and WO-SF-C. The non-flowering trait of orchardgrass appears to be controlled by floral-regulation genes that are turned off by short-day temperatures below a critical threshold. Such a threshold appears to exist for all orchardgrass plants, but is increased in those plants expressing the non-flowering trait.