Pageau, D. and Lajeunesse, J. 2011. Effect of seeding date on oilseed flax grown in a cool climate. Can. J. Plant Sci. 91: 29-35. In Quebec, the growing season is relatively short and most crops are sown early in the spring. However, flax (Linum usitatissimum L.) production is recent in Quebec and the effect of time of seeding on flax productivity has not yet been studied. The objective of this project was to determine the effect of four different sowing dates on the productivity of oilseed flax grown in a cool climate in Quebec. Four planting dates (mid-May to mid-June) were evaluated for 4 yr (2004, 2006, 2007 and 2008) with four cultivars (AC Emerson, AC McDuff, CDC Bethune and Lightning). During 2 yr, delays in seeding reduced grain yields. Moreover, in 2006 and 2007, flax seeded at the last planting date (mid-June) did not reach maturity. Compared with the earliest seeding date, a 2-wk delay in sowing reduced grain yields by 34 to 42% in 2006 and by 25 to 51% in 2007. Late sowing also reduced the oil content and 1000-grain weight of flax. These results indicate that oilseed flax should be sown early (11-18 May) in regions where the climate is cool.