Grasslands and open forest. On Vancouver Island ssp. roemeri occurs on south-facing, grass balds up to about 500 m elevation and at lower elevations in open, often rocky outcrop areas in the Pseudotsuga menziesii or Quercus garryana forests (Pavlick 1983). The species occurs on moderately dry to moist prairies, savannahs, meadows, and grassy openings within woods. While drought tolerant with extensive roots, it is found on somewhat more mesic (moist) sites such as the edges of grassy balds (USDA 2008).
Shallow and or strongly drained soils on stony and rocky sites; characteristic of Mor humus forms (Klinka et al 1989). It prefers moderately acid to slightly alkaline, fine to medium textured mineral soils (USDA 2008).
Nitrogen-poor soils (Klinka et al., 1989).
As an upland grass it requires good soil drainage and does not tolerate winter soil saturation or flooding (USDA 2008).
Shade-tolerant (Klinka et al., 1989). The species generally grows in full sun but will tolerate partial shade near forest edges and oaks (USDA 2008).
Late successional to climax. Less common in open forests.
Major component of three sub-communities: oak-Idaho fescue; oak-Idaho fescue-Cerastium arvense; and oak-Idaho fescue-Trifolium microcephalum.