Ishikawa M., Gusta L. V., Freezing and heat tolerance of Opuntia cacti native to the Canadian prairie provinces, Canadian Journal of Botany, 1996, 74(12):1890-1895, doi: 10.1139/b96-226
Seasonal studies of material from Saskatchewan examine cladode freezing, subsequent necrosis and regeneration. They compare hardened winter cladodes with young growth.