Rocky Mountain Iris

Rocky Mountain iris (*Iris missouriensis*), also called western blue flag or Missouri flag, is a rhizomatous perennial with narrow, arching leaves and pale lilac to whitish flowers veined in lilac-purple to violet-blue. It forms clumps, and published references list mature height and spread ranges reaching about 20 inches. ## Identification The [USDA NRCS Western blue flag guide](https://plants.usda.gov/DocumentLibrary/plantguide/pdf/pg_irmi.pdf) describes linear leaves that may be purplish and flowering stems 8–20 inches tall. The [Royal Horticultural Society profile](https://www.rhs.org.uk/plants/9318/iris-missouriensis/details) describes pale-lavender standards and paler falls with violet-blue veins; its listed mature height and spread each range from about 4 to 20 inches. The USDA guide also records regional common names including blue flag, flag lily, water flag and wild iris. ## Growing Conditions Plant in loam that drains well, with neutral to slightly acid pH. Full sun or partial shade are listed; the USDA guide also notes that native irises grow in shade, while interior climates may need afternoon shade and ample water. Water while establishing, then avoid frequent summer watering; the guide says this species is intolerant of frequent summer water. It suits flower borders and beds, and the guide notes that iris flowers attract hummingbirds and insects. ## Maintenance Divide rhizomes when clumps need renewal. The RHS advises division from midsummer through early autumn, while the USDA guide describes fall or winter division. Remove dying foliage in autumn and cut old flower stems after bloom. The RHS identifies iris as harmful if eaten, so keep roots and rhizomes away from children and pets.