Tree growth and leaf phenology are both affected by global warming. Mountain ecosystems are of paramount importance for studying phenological and growth responses of trees to the gradual variation of temperature. However, the relationship between growing season length and tree growth has been little studied at the level of individual trees. Here, we investigated the relationships between leaf phenology, growing degree-days and radial growth of sessile oaks growing in nine populations along an elevational gradient of 1500 m in the French Pyrenees. In each population, leaf unfolding in spring and leaf coloration in autumn were monitored between 2005 and 2015 on 25-30 trees having contrasting spring phenology (i.e. early vs. late flushing trees). These trees were cored in 2013 to analyse annual tree-ring widths. While trees displayed consistent phenological ranks for both leaf unfolding and leaf coloration within their population over the years, the growing period length decreased with increasing elevation, from about 210 days at the lowest elevation (131 m a.s.l.) to 140 days at the highest elevation (1630 m a.s.l.). For a given year, individual leaf coloration dates correlated positively with leaf unfolding dates at lower elevations, but negatively at higher elevations. Radial growth was positively correlated with growing degree-days at higher elevations, but negatively correlated at lower elevations, likely because higher temperatures are often associated with severe droughts in the lowlands of this region. No clear relationship was found between growing period length and radial growth of oaks within their population. This indicates that climatic conditions during the growing period have a more important impact on the secondary growth of sessile oaks than the growing period length. Our findings suggest that the lengthening of the growing period of trees in response to global warming does not necessarily lead to higher radial growth and productivity.