Northern Blue Flag Plant Profile

Iris versicolor

John Winder / Flickr / CC By 2.0 

Northern blue flag (Iris versicolor) is a member of the iris family more often seen in the wild growing throughout 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 4 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 Grow 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 the plant because it is moderately toxic, yet its showy flowers will bring pollinators to your property. It is specifically known as a good plant to attract hummingbirds.

Light

Northern blue flag grows best in full sun to partial shade. If there is too much shade, the plant might fail to flower.

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 6 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 ever begins to dry out.

Temperature and Humidity

The plant is hardy to the range of conditions in its growth zones. 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 new 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, cut back the plant in the fall—especially any diseased, dying, or dead parts—so it can more efficiently use its energy.

Fertilizer

Northern blue flag likes a 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

The plant spreads by self-seeding and extending rhizomes to form colonies. Although some herbalists use them in medicine, these rhizomes are poisonous. 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

There are several types of flag iris plants, all typically found in low-lying, wet areas. Besides the northern blue flag, these 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 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