How to Plant and Grow Yellow Flag Iris

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. 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.

Yellow Flag Iris Care

Here are the main care requirements for growing:

• Prefers high humidity
• Needs constant moisture
• Does well in partial to full sun

Light

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.

Soil

Yellow flag iris grows in wetlands in the wild, so it performs best if you can provide it with boggy soil in your landscape.

Water

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.

Fertilizer

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.

Pruning

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.

How to Propagate Yellow Flag Iris

Yellow flag iris spreads by fast-growing underground rhizomes that make it easy to divide. Here's how:

1. After blooming season ends, use a spade to dig up a clump of yellow flag irises.
2. 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.
3. Trim off the top half of the leaves.
4. 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.
5. Water well and keep the soil moist.

How to Grow Yellow Flag Iris From Seed

Yellow flag iris can also be grown from seed. Here's how:

1. Harvest the orange-brown seeds when the seed pods ripen and open in the fall.
2. Soak them in warm water for 24 hours before planting.
3. Sow the seeds on moist, rich garden soil immediately for the best chance of germination the following spring.
4. Cover the seeds lightly with about 1/8 inch of soil.
5. 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.

Overwintering

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.

Potting Yellow Flag Iris

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.

Common Pests

If you notice leaves of your yellow flag iris turning brown, this could be due to iris borer caterpillars. These pests will eat holes in the leaves, which may eventually cause the leaves to die.

How to Get Yellow Flag Iris to Bloom

Bloom Months

Yellow flag iris blooms in the spring and summer months.

What Do Yellow Flag Iris Flowers Look Like?

Yellow flag iris bears showy flowers consisting of 3 upright standard petals and 3 pendant petals. It has striking, sword-shaped leaves that are a greenish-gray color. 

How to Encourage More 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.

Deadheading

After a flower dies, deadhead it, an action that sometimes prompts a second bloom.

Common Growing Problems

Slowed Growth

If you notice your plant has slowed in its growth, it may need water. If yellow flag iris is deprived of moisture, it grows less vigorously.

Yellowing Leaves

Too little light can cause leaves to turn yellow. If light levels look accurate, yellow leaves could also be due to overwatering or poor drainage.