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

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. From May to July, 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 in diameter, making them an eye-catching addition to any garden. Plant northern blue flag in the late summer to early fall.

Northern blue flag is toxic to humans and pets.

Common Names Northern blue flag, blue flag, harlequin blue flag, blue flag iris, large blue iris, larger blue flag, purple iris
Botanical Name Iris versicolor
Family Iridaceae
Plant Type Perennial, herbaceous
Mature Size 2-3 ft. tall and wide
Sun Exposure Full, partial
Soil Type Moist
Soil pH Acidic
Bloom Time Spring, summer
Flower Color Blue, purple
Hardiness Zones 3-9 (USDA)
Native Area North America
Toxicity Toxic to humans, toxic to pets

Northern Blue Flag Care

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. The soil should be acidic with a pH less than 6.8.

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 4 inches 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. Northern blue flag readily grows in climates that experience chilly winters,

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.

Types of Northern Blue Flag

Besides northern blue flag, two other types of native iris varieties are typically found in wet areas:

  • 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.
  • Western blue flag, or Rocky Mountain iris (Iris missouriensis) grows 12 to 18 inches tall in meadows the higher elevations east of the Cascades, in USDA zones 3 to 8, Like the northern blue flag, it likes to grow in wet conditions. The flowers are a blend of sky blue and white.

Pruning

It is not necessary to prune the plant during the growing season, let the leaves die back naturally. For a neater appearance, you can remove the dead flower heads. After the first frost, prune back the foliage to 1 inch above the crown.

Propagating Northern Blue Flag

Northern blue flag spreads by self-seeding and creeping rhizomes. Dividing the clumps is not just a way of propagating the plant, it also prevents the iris from overcrowding and keeps it healthy and vigorous. As the rhizomes are toxic, wear gloves when handling the plant.

  1. In late summer, cut the foliage back to about half its height.
  2. Lift the clump from the soil with a garden fork. Shake off all loose soil.
  3. Cut the clump into sections using a sharp knife (sterilize the blade beforehand with a 10% bleach solution). Make sure that each section has at least one eye. If there are any rotten or shrived rhizomes, remove and discard them.
  4. Replant each section in moist, rich soil, slightly below the soil surface, with the top exposed and the roots in the ground.

How to Grow Northern Blue Flag From Seed

The best way to grow northern blue flag from seed is starting it outdoors in the late fall to mid-winter so the winter weather will take care of the necessary stratification.

  1. Fill 4-inch pots with damp, sterile potting mix.
  2. Scarify the seeds by rubbing them between two pieces of sand paper to nick the hard seed coat.
  3. Place the seeds in the prepared pots 1/4 inch deep. Keep the pots outdoors where they will get natural precipitation.
  4. In the spring when the temperature warms up, the seeds will germinate. Keep the pots moist at all times and let the seedlings grow to about 2 inches before transplanting them in garden soil.

Potting and Repotting Northern Blue Flag

Northern blue flag is not a good choice for regular container-growing because it needs consistently moist to wet soil, which is difficult to maintain in pots, especially during the summer heat. But you can grow it in containers if you have a pond or water garden. Choose a container with large drainage holes and fill it with rich organic soil. Place it at the edge of your pond or water garden where it stands up to 4 inches deep in water. Make sure that you don't drown the plant by placing the pot on bricks.

When the rhizomes have filled the container, it's time to repot it to a container one size up, or divide the rhizomes.

Overwintering

This is a very hardy native plant that does not need winter protection. However, a young plant can benefit from some cold-weather protection. Blanket the area around the plant for the wintertime with a layer of mulch.

Common Pests & Plant Diseases

The plant might get aphids, iris borer, and iris thrips. Common diseases include blossom or leaf blight, bacterial soft rot, crown rot, rhizome rot, leaf spot, and mosaic virus.

How to Get Northern Blue Flag to Bloom

Failure to bloom is most likely due to lack of sunlight, or improper planting depth. Makes sure the rhizomes are just below the soil surface, if planted to deeply, the iris won't bloom.

Common Problems with Northern Blue Flag

In suitable conditions, this is an easy-going native plant without any major problems.

FAQ
  • Is northern blue flag deer-resistant?

    Deer find it unpalatable so you won't need to worry about them eating it.

  • Is blue flag iris related to yellow flag iris?

    Yellow flag (Iris pseudacorus) grows wild across North America but it is 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 use it as an ornamental pond plant due to its bright yellow flowers.


  • Are blue flag and Siberian iris the same?

    No, they are different species. Siberian iris, as the name indicates, is not native to North America.

Southern blue flag iris
Southern blue flag iris Judy Darby / Getty Images
Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Iris versicolor. North Carolina State University Cooperative Extension.

  2. 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/.