Detailed knowledge of below-ground growth is poor relative to that of growth above ground and limits our understanding of whole-plant responses to the environment. Contador et al. use minirhizotron techniques to examine below-ground growth in field-grown walnut (Juglans regia) and find that root production follows a unimodal curve across the season, with the decline in root production corresponding to increased soil temperature, as well as to the period of major carbohydrate allocation to reproduction. Root vertical distribution appears to have greater plasticity than timing of root production in this system, with temperature and/or carbohydrate competition constraining the timing of root growth.