The objective of this study was to determine depth of soil that should be applied over spoil to ensure adequate production of cool‐season grasses at surface mines in the Northern Great Plains. During a 6‐yr period, data were collected from 15 wedge plots and the response of perennial grass production to increasing soil depth was measured. Production responses were dependent on spoil traits and four spoil types were recognized: generic, sodic, acid, and soil‐like. Soil depth required for maximum production was 50 cm for generic spoil, 71 cm for sodic spoil, indeterminate for acid spoil, and 0 cm for soil‐like spoil. Root penetration into spoil appeared limited to ≤ 10 cm, except for soil‐like spoil where root penetration exceeded 110 cm. Native and introduced grasses responded in a similar manner to increasing soil depth, and precipitation had no significant effect on the depth of soil required for maximum grass production.

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