Yellow flag iris (Iris pseudacorus) grows wild in wetland areas throughout North America. It is not a native but naturalizes easily. It is easy to grow, low-maintenance, and useful as an ornamental pond plant. It is valued for its ability to live in wet areas of the landscape where many other plants would perform poorly. It bears attractive flowers and has striking, sword-shaped leaves that are a greenish-gray color. The large seed pods are well-suited to being used in dry flower arrangements.
Many American gardeners give up on the thought of growing yellow flag iris in the landscape when they learn that it is invasive, but there are ways to counteract its invasive tendencies to safely include it in your landscape plan.
All parts of the yellow flag iris are toxic to humans, dogs, cats, and horses. The rhizomes and leaves closest to the ground are the most toxic.
|Common Name||Yellow iris, yellow flag, yellow flag iris, water flag|
|Botanical Name||Iris pseudacorus|
|Plant Type||Herbaceous, Perennial|
|Mature Size||3 ft. tall|
|Sun Exposure||Full, Partial|
|Soil Type||Loamy, clay|
|Bloom Time||Spring, Summer|
|Hardiness Zones||5–9 (USDA)|
|Native Area||Europe, Africa, Asia|
Yellow Flag Iris Care
Yellow flag iris spreads via fast-growing underground rhizomes and by self-seeding. It uses this flexibility to form large colonies under the right conditions. Check with your local extension agent before planting yellow flag iris to determine if it's invasive in your area. If left unchecked, much of your work in caring for yellow flag iris will consist of pulling it out of areas where it does not belong.
There are ways to prevent yellow flag iris from spreading. Growing it in a container is an excellent one. Whether you grow it as a water garden plant or in a container, remove the seed pods as soon as they form to prevent self-seeding.
This plant prefers constant moisture. If it is deprived of that moisture, it grows less vigorously. Take advantage of this fact to grow the plant without worrying much about it spreading (although you should never let its soil dry out completely).
Yellow flag iris is present in 46 states and is considered invasive in 40 of them. In some states, it is regulated; it can be bought, sold, or owned as long as it isn't released into a "free-living" state. Check with your local extension office for any limitations before buying.
The wetter the soil that yellow flag iris is growing in, the more sun it can take. Most garden soils are not as wet as those where yellow flag iris is typically found growing in the wild. For this reason, it is usually better to grow yellow flag iris in partial sun in a garden setting. However, if you are growing it in a water garden, give it full sun.
Yellow flag iris grows in wetlands in the wild, so it performs best if you can provide it with boggy soil in your landscape.
This semi-aquatic plant likes wet soil—it is difficult to overwater it. Yellow flag iris can survive in soil that is only moderately moist, but it produces fewer flowers when you grow it in less-than-wet conditions. Don't let the soil dry out completely.
Temperature and Humidity
Yellow flag iris tolerates temperatures to 0°F, and it grows best in areas with high humidity. Gardeners in arid regions can mist their plants regularly to increase humidity.
Yellow flag iris needs no more than an average degree of fertility in its soil. Fertilize it in late winter or early spring with manure tea. If the iris grows in a garden pond populated with fish, it doesn't need additional fertilizer.
During the growing season, remove bloom-bearing stalks after the flower dies. In the fall, prune any old or dead leaves, cutting the entire plant back to 1 inch for winter.
Propagating Yellow Flag Iris
Yellow flag iris spreads by fast-growing underground rhizomes that make it easy to divide.
- After blooming season ends, use a spade to dig up a clump of yellow flag irises.
- Brush off some of the soil and use your hands or a sharp knife to separate the rhizomes into sections containing rhizomes and healthy leaves.
- Trim off the top half of the leaves.
- Dig a shallow hole in moist, rich soil, and bury the rhizomes only halfway; then press them into the soil. Don't plant them deep.
- Water well and keep the soil moist.
How to Grow Yellow Flag Iris From Seed
Yellow flag iris can also be grown from seed.
- Harvest the orange-brown seeds when the seed pods ripen and open in the fall.
- Soak them in warm water for 24 hours before planting.
- Sow the seeds on moist, rich garden soil immediately for the best chance of germination the following spring.
- Cover the seeds lightly with about 1/8 inch of soil.
- After the seeds germinate, wait several weeks for the seedlings to grow large enough to be transplanted to their permanent garden site.
It can take anywhere from 1 month to 6 months for the seeds to germinate.
After the foliage dies back completely in fall, trim the leaves to 1 inch. Don't apply a layer of mulch over yellow flag iris plants as winter approaches. If the winter is particularly frigid, spread straw around the base of the plant.
How to Get Yellow Flag Iris to Bloom
Little work is required to encourage yellow flag iris to bloom. As long as it has optimum growing conditions, gardeners can expect blooms ranging from rich dark yellow to pale yellow in late spring to early summer. The 3–6 inch blooms are not fragrant, but they are showy in the garden and last for a couple of weeks. After a flower dies, deadhead it, an action that sometimes prompts a second bloom.
Growing Yellow Flag Iris in Containers
To prevent unwanted spread via rhizomes, many gardeners grow this plant in containers. If you are using yellow flag iris in a water garden, sink the container right down into the water (a stone mulch will help keep the soil in the container). Yellow flag iris can be grown in water as deep as 6 inches.
Blue Flag Iris: An Alternative
Blue flag iris (Iris versicolor) is an excellent alternative for American gardeners seeking a native iris to grow in a water garden. It likes the same conditions as the yellow flag iris, grows to be around 3 feet in height, is violet-blue, and is indigenous to North America.
How long does yellow flag iris live?
The rhizomes live for up to 10 years and support new growth for that length of time. If the plant is allowed to, it self-seeds prolifically, introducing many new plants to a garden every year.
How do I stop yellow flag iris from spreading?
The most important thing is to deadhead the flower stalks after the plant blooms and before it produces seed pods. That prevents self-seeding but has no effect on rhizome spread. Pull up stray plants, being careful to get all the pieces of rhizomes—even tiny pieces result in new plants. Be prepared to pull up new plants every year.
Iris pseudacorus. North Carolina State Extension