Significance We report a landscape-scale experiment with controlled initial and boundary conditions to reveal incipient dune growth under natural action of wind. We measured growth rate of dunes of different wavelengths for 42 mo in a desert plot of 7,500 m 2 . We identify an early phase in which a clear maximum growth rate occurs for a 15-m dune spacing. Successful comparison with dune instability theory allows us to quantify the size-selection mechanism leading to the emergence of periodic dune patterns, which can be directly related to flow and sand transport properties. An experiment of this type and scale is unprecedented. Its results boost confidence in the existing theory, confirming its application to a variety of planetary landscapes from repeated aerial/orbital imagery.