Iris missouriensis
Common Name: Rocky Mountain iris
Type: Herbaceous perennial
Family: Iridaceae
Native Range: Western and central North America
Zone: 3 to 8
Height: 1.00 to 2.00 feet
Spread: 0.75 to 1.00 feet
Bloom Time: May to June
Bloom Description: Violet blue
Sun: Full sun to part shade
Water: Medium to wet
Maintenance: Low
Suggested Use: Naturalize
Flower: Showy
Tolerate: Deer, Wet Soil

Culture

In mild summer climates, best performance typically occurs in medium to wet, well-drained soils in full sun. In hot summer climates, this iris appreciates some afternoon shade. Although considered a wetland plant, it can tolerate some dry soils once established but only after bloom. It needs consistent moisture from spring to 6 weeks after flowering ends in order for plants to bloom well. With clay soils or in areas of high rainfall, plant this iris on slopes (growing end uphill) or in raised beds to promote good drainage and discourage the onset of rhizome rot. Plant rhizomes, depending on location, 12-20” apart from late July through October (late July–early September in areas with cold winters or September-October in areas with mild winters). Plant each rhizome shallowly over a baseball-sized mound of soil with 1/3 of the rhizome above the soil and with the roots horizontally spread to support the plant. Growth comes from the leafy end of the rhizome. If overcrowding occurs over time, lift the clump in late summer (August) with a garden fork, divide and replant. Keep the iris bed free of weeds. This rhizomatous iris propagates readily from plant division in fall or winter.

Noteworthy Characteristics

Iris missouriensis, commonly known as western blue flag and Rocky Mountain iris, is a beardless iris that grows from an irregularly-creeping tuberous rhizome. In the wild, it typically rises to 12-24” tall. It is native to wet meadow and marshy mountainous areas in western North America from North Dakota to British Columbia south to California and New Mexico. It is commonly found in habitats ranging from low valleys to alpine areas up to 9000’ in elevation.

Each plant typically bears 1-4 variegated, violet blue iris flowers (each 2-3” long) per stem. Each flower has 6 perianth segments: (a) three elongated spreading to reflexed falls have a central dark yellow-orange stripe and diverging blue lines on a white background and (b) three erect standards are narrower and lilac-purple to dark blue. Flowers bloom at the top of stout leafless stalks that rise up among dense, flexible, tough, sword-shaped leaves. Leaves typically rise above the flower stems.

Genus named for the Greek goddess of the rainbow.

Specific epithet is in reference to the Missouri River and not to the State of Missouri where this plant is not native. Lewis and Clark first collected this plant along the Missouri River in 1806.

Problems

Leaf spot, root rot, bacterial soft rot, crown rot and mosaic viruses may appear. Watch for slug, snails, whiteflies, aphids and thrips. Iris borers can cause significant problems in areas where they are found.

Garden Uses

Best grouped or massed in sunny areas of perennial beds, borders or foundations.