Sciegienka, J. K., Keren, E. N. and Menalled, F. D. 2011. Impact of root fragment dimension, weight, burial depth, and water regime on Cirsium arvense emergence and growth. Can. J. Plant Sci. 91: 1027–1036. Cirsium arvense is an aggressive, introduced, perennial invasive weed that flourishes in a wide variety of environments including conventional and organic agricultural systems as well as disturbed non-crop habitats. Even though much research has been conducted on the chemical, biological, and cultural management of C. arvense, less information is available on how pre-emergence factors affect its reproductive biology and growth. This research assessed the combined impact of root fragment size, root fragment biomass, burial depth, and water regime (a proxy of water availability) on C. arvenseemergence and growth in fallow conditions. In field experiments, root burial depth was the most important factor determining C. arvenseemergence and growth, with roots at the 10-cm depth having the greatest average emergence (51.2±2.0% in 2007 and 43.5±7.2% in 2008; mean±SEM) compared to roots at the 2 cm (8.9±7.4% in 2007 and 38.1±8.3% in 2008) or 20 cm (12.8±4.0% in 2007 and 17.6±2.7% in 2008) depth. In greenhouse experiments, water regime was the overriding variable determining C. arvense emergence as well as above-ground and below-ground biomass. These results could improve decision-aid models and enhance the efficacy of site-specific C. arvense management practices.