Low phosphorus availability (low P) often delays flowering and maturity in annual plants, while abiotic stress generally accelerates flowering and maturity. The utility of this response is unknown. We hypothesize that phenological delay in low P is beneficial by permitting more time for phosphorus acquisition and utilization. We grew seven genotypes of Arabidopsis thaliana with contrasting phenology in high and low P. Low P delayed bolting and maturity in all genotypes. Low P decreased root length, but not root-length duration (the integral of root length over time), because phenological delay allowed low-P plants to compensate for shorter root length. Root-length duration was correlated with phosphorus accumulation. Leaf phosphorus duration (the integral of leaf phosphorus over time) was correlated with reproductive biomass, indicating the utility of increased phosphorus utilization. Phenological delays accounted for up to 30% of biomass production when low-P plants were compared to models of plants with no delays. These results support the hypothesis that phenological delay in low P is adaptive and leads to increased phosphorus acquisition and utilization. Because low P conditions are prevalent, understanding the utility of this response could be useful in crop breeding and in predicting plant responses to global climate change.