Root hairs are subcellular protrusions from the root epidermis that are important for the acquisition of immobile nutrients such as phosphorus (P). Genetic variation exists for both root hair length and the plasticity of root hair length in response to P availability, where plasticity manifests as increased root hair length in response to low P availability. Although it is known that long root hairs assist P acquisition, the utility of phenotypic plasticity for this trait is not known. To assess the utility of root hair plasticity for adaptation to low phosphorus availability, we evaluated six recombinant inbred lines of maize (Zea mays L.) with varying root hair lengths and root hair plasticity in a controlled environment and in the field. Genotypes with long root hairs under low P availability had significantly greater plant growth, P uptake, specific P absorption rates and lower metabolic cost-benefit ratios than short-haired genotypes. Root hair length had no direct effect on root respiration. In the controlled environment, plastic genotypes had greater biomass allocation to roots, greater reduction in specific root respiration and greater final biomass accumulation at low phosphorus availability than constitutively long-haired genotypes. In the field study, the growth of plastic and long-haired genotypes were comparable under low P, but both were superior to short-haired genotypes. We propose that root hair plasticity is a component of a broader suite of traits, including plasticity in root respiration, that permit greater root growth and phosphorus acquisition in low P soils.