Changing growing season observed in Canada Budong Qian & Sam Gameda & Xuebin Zhang & Reinder De Jong Received: 4 November 2010 /Accepted: 9 August 2011 # Crown Copyright Canada 2011 Abstract It is theoretically interesting for climate change detection and practically important for agricultural producers to know whether climate change has influenced agroclimatic conditions and, if so, what the potential impacts are. We present analyses on statistical differences in means and variances of agroclimatic indices between three 30-year periods in the 20th century (i.e., 1911–1940, 1941–1970 and 1971–2000). We found many occurrences of statistically significant changes in means between pairs of the three 30-year periods. The findings consistently support agroclimatic trends identified from trend analysis as an earlier growing season start and an earlier end to spring frost (SF), together with an extended growing season, more frost-free days (FFD) and more available heat units were often found in the later 30-year periods as compared to the earlier ones. In addition, this study provides more detailed quantitative information than the trend signals for the practical interests of agricultural applications. Significant changes were detected for SF and FFD at a much larger percentage of stations between the latter two 30-year periods (1941–1970 vs. 1971–2000) as compared to the earlier two periods (1911–1940 vs. 1941–1970). In contrast, changes in variances of the selected agroclimatic indices were less evident than changes in their means, based on the percentage of stations showing significant differences. We also present new climate averages of the selected agroclimatic indices that can be useful for agricultural planning and management. 1 Introduction Earlier studies have shown that the climate is changing in Canada (e.g., Zhang et al. 2000; Bonsal et al. 2001; Zhang et al. 2001; Shabbar and Bonsal 2003; Vincent and Mekis 2006). Climatic Change DOI 10.1007/s10584-011-0220-8 B. Qian (*) : S. Gameda : R. De Jong Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, 960 Carling Ave, Ottawa, ON K1A 0C6, Canada e-mail: Budong.Qian@agr.gc.ca X. Zhang Climate Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, ON M3H 5T4, Canada