Heterosis for growth traits was investigated on 2-yrold seedlings from related intra- and inter-specific progenies collected in hybridisation seed orchards combining Larix decidua (EL) and L. kaempferi (JL). Following taxa identification, seedlings phenology and growth were observed over a complete growing season in two experiments. Start, cessation, duration and rates of growth for the different taxa were explored in relation to hybrid vigour. Variable levels of heterosis were observed, depending on the genetic background of the parental species, the species over which superiority was evaluated, environmental conditions and the traits considered. While significant and consistent differences have been observed between hybrids and their related EL progenies for phenological growth traits, a better timing of the growth rhythm over the growing season together with a greater growth efficiency allowed for a consistently positive heterosis over EL. The situation appeared more complex with JL as heterosis could be either positive or negative. Besides the high variation in heterosis levels, the way heterosis for height built up over the growing season seemed also to be variable according to genetic backgrounds: cumulative constant superiority of the hybrid for growth rhythms along the growing season or greater growth rates during discrete periods of time.