Here, we tested the alternation of root summer dormancy and winter growth as a critical survival strategy for a long-lived monocotyledon (Restionaceae) adapted to harsh seasonal extremes of Mediterranean southwest Western Australia.• Measurements of growth and the results of comparative studies of the physiology, water content, metabolites, osmotic adjustments, and proteomics of the dormant and growing perennial roots of Lyginia barbata (Restionaceae) were assessed in field-grown plants.• Formation of dormant roots occurred before the onset of summer extremes. They resumed growth (average 2.3 mm d−1) the following winter to eventually reach depths of 2–4 m. Compared with winter-growing roots, summer dormant roots had decreased respiration and protein concentration and c. 70% water content, sustained by sand-sheaths, osmotic adjustment and presumably hydraulic redistribution. Concentrations of compatible solutes (e.g. sucrose and proline) were significantly greater during dormancy, presumably mitigating the effects of heat and drought. Fifteen root proteins showed differential abundance and were correlated with either winter growth or summer dormancy. None matched currently available libraries.• The specific features of the root dormancy strategy of L. barbata revealed in this study are likely to be important to understanding similar behaviour in roots of many long-lived monocotyledons, including overwintering and oversummering crop species.