Field experiments reporting the relative growth rate (RGR) patterns in plants are scarce. In this study, 22 herbaceous species (20 Aegilops species, Amblyopyrum muticum and Triticum aestivum) were grown under field conditions to assess their RGR, and to find out if the differences in RGR amongst species were explained by morphological or physiological traits. Plants were cultivated during two months, and five harvests (every 11-19 days) were carried out. Factors explaining between-species differences in RGR varied, depending on whether short (13-19 days) or longer periods (62 days) were considered. RGR for short periods (4 growth periods of 13-19 days each) showed a positive correlation with net assimilation rate (NAR), but there was no significant correlation with leaf area ratio (LAR) (with the exception of the first growth period). In contrast. when growth was investigated over two months, RGR was positively correlated with morphological traits (LAR, and specific leaf area, SLA), but not with physiological traits (NAR). A possible explanation for these contrasting results is that during short growth periods, NAR exhibited strong variations possibly caused by the variable field conditions, and, consequently NAR mainly determined RGR. In contrast, during a longer growth period (62 days) the importance of NAR was not apparent (there was no significant correlation between RGR and NAR), while allocation traits, such as LAR and SLA, became most relevant.