How to Grow Yellow Flag Iris

Yellow irises with long narrow leaves closeup

The Spruce / Evgeniya Vlasova

Yellow flag iris or "flag" grows wild in wetland areas throughout North America, except for the Rocky Mountain states. It is not a native but naturalizes easily.

Many American gardeners give up on the thought of growing yellow flag iris in the landscape once they learn that it is invasive, and this is too bad.

Not only does it bear attractive flowers, but it also has striking, sword-shaped leaves (1 1/8 inches wide) that are a nice greenish-gray color. The large seed pods that succeed the blossoms are well-suited to being used in dry flower arrangements too.

Its benefits extend beyond the fact that it adds wonderful color to the yard. It is also 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, plus it is deer-resistant.

Luckily, there are ways to counteract its invasive tendencies, thereby allowing you to safely include it in your landscape plan.

Or, alternatively, blue flag iris (Iris versicolor) is a great 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 three feet in height, is violet-blue, and is indigenous to North America.

The yellow flag iris is considered invasive in parts of North America because its rapid spread allows it to out-compete native species. Note that there are other species of yellow irises that are not invasive.

Botanical Name Iris pseudacorus
Common Name Yellow iris, yellow flag, yellow flag iris, water flag
Plant Type Herbaceous perennial
Mature Size 3 feet tall
Sun Exposure Full to partial sun
Soil Type Medium to wet, with average fertility
Soil pH Acidic
Bloom Time May to June
Flower Color Yellow, with bluish-brown veins
Hardiness Zones  5 to 9, USA
Native Area Europe, northwest Africa, and western 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. This is why it is such an invasive plant. 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.

Happily, there are tricks that you can adopt to prevent this from happening. For example, grow it in a container.

If you are not growing yellow flag iris in a water garden, there is a different tactic that you can use to check its spread. This plant prefers constant moisture. If it is deprived of that moisture, it grows less vigorously. Take advantage of this fact to enable yourself to grow the plant without having to worry so much about its spreading (although you should never let its soil dry out completely).

Finally, whether you grow it as a water garden plant or not, remove the seed pods as soon as they form to prevent self-seeding.

Yellow iris flower closeup

The Spruce / Evgeniya Vlasova

Yellow irises and long narrow leaves next to water

The Spruce / Evgeniya Vlasova

Yellow iris stems with buds and long narrow leaves against bright sky

The Spruce / Evgeniya Vlasova

Warning

The yellow flag iris is considered invasive in parts of North America because its rapid spread allows it to out-compete native species. Verify with your local extension office that it's not considered invasive in your area before you plant any.

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. But 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 will perform best if you can provide it with a boggy soil in your landscape.

Water

While it likes a wet soil, yellow flag will survive in a soil that is only moderately moist. But you will have fewer flowers if you grow it under the latter conditions. Do not let the soil dry out.

Fertilizer

Yellow flag iris needs no more than an average degree of fertility in its soil. Fertilize it occasionally with manure tea.

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 six inches.