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. 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.
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.
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.
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.
