How to Grow and Care for Northern Blue Flag

Northern blue flag plant with purple and yellow flowers surrounded by sword-like leaves closeup

The Spruce / Adrienne Legault

In This Article

Northern blue flag (Iris versicolor) is a member of the iris family more often seen in the wild growing in wetlands and along shorelines than in home gardens. This is a shame because homeowners would find this plant easy and attractive to grow, especially along the margins of water features.

It is a clump-forming plant with bluish-green, sword-shaped leaves. Its stalks each bear three to five violet-blue flowers with purple veining and a central yellow and white patch. The flowers span up to four inches (10.2 centimeters) in diameter, making them an eye-catching addition to any garden.

Botanical Name Iris versicolor
Common Name Northern blue flag, blue flag, harlequin blue flag, blue flag iris, large blue iris, larger blue flag, purple iris
Plant Type Herbaceous perennial
Mature Size 2 to 3 feet in height and spread
Sun Exposure Full sun to part shade
Soil Type Rich and moist
Soil pH Less than 6.8 (acidic)
Bloom Time May to July
Flower Color Violet-blue
Hardiness Zones 3 to 9, USA
Native Area Eastern North America

How to Care for Northern Blue Flag

Northern blue flag can fit nicely into a low-maintenance landscaping plan. Given the right growing conditions—primarily sunshine, moisture, and rich soil—it should not require much care on your part.

Because it is a wetland species, northern blue flag can thrive in wet areas of your property. Use it in a rain garden as a natural way to soak up water in low-lying areas or to add interest along a pond or other water feature. Most animals, such as deer, tend to avoid eating this plant, yet its showy flowers will bring pollinators to your property. It is specifically known to attract hummingbirds.

Northern blue flag plant with purple striped flowers surrounded by sword-like leaves

The Spruce / Adrienne Legault

Northern blue flag flower with purple striped and yellow petals with leaves from above

The Spruce / Adrienne Legault

Northern blue flag flower with purple stripes and yellow petals closeup

The Spruce / Adrienne Legault

Light

Northern blue flag grows best in full sun to partial shade. However, the plant might not flower if it doesn't receive enough sun.

Soil

It craves a loamy soil that is rich in organic matter. But the plant can grow in other soil types when given enough moisture.

Water

This type of iris is considered a marginal aquatic plant, meaning it grows around the edges of water rather than in deep water. It can tolerate standing in as much as six inches (15.2 centimeters) of water, and it can survive being completely submerged for a short period, such as in a flood. It also can tolerate dry spells, though it would prefer to remain consistently moist. Provide a shallow layer of mulch around the plant to retain moisture if necessary, and give it a good watering if the soil dries out.

Temperature and Humidity

This plant does well in the range of conditions in USDA hardiness zones 3 to 9. It's partial to humidity, which helps to retain soil moisture. And though northern blue flag readily grows in climates that experience chilly winters, a young plant can benefit from some cold-weather protection. Blanket the area around the plant for the wintertime with a layer of mulch, hay, leaves, or even just additional soil. Also, in the fall, cut back the plant to one inch above the crown—especially any diseased, dying, or dead parts—so it can more efficiently use its energy.

Fertilizer

Northern blue flag likes rich soil. So for best results, add some compost around it each spring to provide nutrition. Compost is the ideal fertilizer because it's organic, it's essentially free (if you have a compost bin), and it won't burn your plants the way chemical fertilizers can.

Propagating Northern Blue Flag

This plant spreads by self-seeding and creeping rhizomes to form colonies. Although some herbalists use them in medicine, the rhizomes are toxic to people. So wear gloves if you will be dividing this perennial to propagate it. Spring division is usually recommended or you can wait until after the plant is done flowering in the fall.

Varieties of Flag Irises

Several types of flag iris plant are typically found in low-lying, wet areas. Besides the northern blue flag, here are two other common varieties:

  • Southern blue flag (Iris virginica): Predictably, southern blue flag, or the Virginia iris, is less cold-hardy than northern blue flag, growing only in USDA hardiness zones 5 to 9. But both perennials are indigenous to eastern North America. The two plants share many similar traits, though the flowers of southern blue flag are frequently a lighter violet-blue.
  • Yellow flag (Iris pseudacorus): Also known as the yellow iris and water flag, yellow flag grows wild across North America, with the exception of the Rocky Mountains. But the plant is actually native to Europe, northwest Africa, and western Asia. In fact, yellow flag is seen as an ecological threat in North America because its prolific growth can out-compete native species. Still, many gardeners prefer to use it as an ornamental pond plant due to its bright yellow flowers.
Southern blue flag iris
Southern blue flag iris Judy Darby / Getty Images
Yellow iris
Yellow iris JazzLove / Getty Images 
Article Sources
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  1. White, Sarah A., and Terasa Lott. “Rain Garden Plants: Iris Versicolor and Iris Virginica.” Clemson.Edu, 26 Jan. 2017, https://hgic.clemson.edu/factsheet/rain-garden-plants-iris-versicolor-and-iris-virginica/.