The Yellow Flag Iris: Beautiful, Interesting, and Invasive

yellow iris two

A yellow flag iris in May

Yellow flag irises produce beautiful flowers. I love the sunny field of yellow that a group of the plants creates when all of them are in bloom at the same time. The plants grow in marshes and beside or in streams, lakes, and ponds. The yellow flag is an introduced species in British Columbia and grows in the wild. It’s sometimes planted deliberately in gardens. Unfortunately, it’s invasive in many areas.

Yellow flag iris belongs to the iris family, or the Iridaceae, as its name suggests. It’s often called yellow iris or yellow flag. The plant is native to Europe, Asia, and northern Africa. It’s perennial and herbaceous. The underground parts persist during the winter and produce new shoots in the spring. In some areas, the leaves also persist over the winter. In others, they die. The plant forms dense stands in favourable habitats. These habitats include drainage ditches and irrigation canals as well as streams, ponds, lakes, and wetlands.

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Yellow flag iris beside Trout Lake in Vancouver

The scientific name of the plant is Iris pseudacorus. The first or genus name is said to come from Iris, who was the Ancient Greek goddess of the rainbow. She was also a messenger for the gods. The second or species name is derived from Acorus, which is the genus name of the sweet flag. Like the yellow flag, this plant belongs to the monocot group of flowering plants. The species name indicates that the yellow flag bears some similarity to Acorus.

Every year, I admire the lovely flowers of the plant beside Trout Lake in Vancouver, which is where I took my photos that are shown in this article. It’s a small lake that is located in John Hendry Park. The park staff are probably controlling how far the plant spreads. In an unsupervised or larger area, this is much harder to do. I photographed the plants in May, which was early in their blooming season. The carpet of yellow flowers becomes denser as the season progresses.

Yellow flags were introduced to North America in the 1800s or 1900s. Different sources give different dates. They were admired as ornamental plants in garden ponds and still are. Today they are a problem in other parts of Canada besides British Columbia. They aren’t described as invasive in every part of North America, however.

Like other iris flowers, yellow flags have six petals (though this is not their botanically correct name): three upright petals known as standards and three drooping ones known as falls. The falls of yellow flag are larger than the standards. They also have brown or purple lines or flecks, which the standards lack. The fruit is an elongated capsule that is glossy green and ribbed. It bears two rows of flat and brown seeds.

Yellow Flag fruit

The fruit of a yellow flag iris (Photo by WikimediaImages, CC0 public domain license)

Technically, only the standards of the flowers are petals. The falls are really sepals. In most plants, the sepals are green and are located under the flower. When the petals and sepals of a flower look almost the same or are of the same colour, as in yellow flags, they are known as tepals. The flowers of yellow flag are pollinated by insects, including bees.

The leaves fan out from the base of the plant. Each leaf is long, narrow, and flat. It has a pointed tip and a raised midrib. The leaves clasp or wrap around the stem. Some of them curve so that their tip points downward.


A view of the tepals of the iris (Photo by AnRo0002, public domain license)

Yellow flags reproduce not only by their seeds but also by their thick rhizomes. Rhizomes are underground and horizontal stems. The rhizomes produce new shoots that emerge from the ground. According to the Invasive Species Council of BC, sometimes several hundred yellow flags are connected to one another. If a piece of a rhizome breaks off and drifts downstream, it’s sometimes able to form roots and generate new plants.

A dense growth of yellow flags can crowd out plants like sedges, rushes, and cattails, where some birds create their nests. In some habitats, the colony of irises is almost impenetrable. It prevents the growth of plants that animals eat. It also fills the space normally used by fish and amphibians. The irises themselves and the sediment that they trap can block the passage of water, which can cause a problem in drainage ditches and other channels meant for water flow.

yellow flag iris in pond

The plants by a pond (Photo by anpe, CC0 public domain license)

The plants are poisonous due to the presence of a chemical called irisin, which is sometimes called iridin. The substance is found in other iris species as well. Yellow flag irises can cause gastrointestinal problems if ingested and may irritate the skin when handled. The plants have caused digestive problems in livestock that have eaten them.

The seriousness of yellow flag’s effects on humans and animals is uncertain. The effects may depend in part on the quantity of plant material that’s eaten and individual sensitivity.  It certainly seems wise to remove the plant from areas that animals explore and to make sure that children don’t eat any part of the plant. It would also be wise to seek veterinary or medical advice if the plant is eaten. People should wear gloves when removing yellow flag from an area.

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Trout Lake and yellow flag plants

Despite the problems that the irises can cause, in some places they are sold as ornamental plants. I imagine that they look very attractive when in bloom around a garden pond, but I’ve read multiple reports from people complaining that they spread from where they’re planted and are hard to remove.

Interestingly, yellow flags have been deliberately planted in one type of habitat outside of gardens. It’s been discovered that when the irises are planted in wastewater treatment plants, they remove heavy metals from the water. Heavy metals are toxic to humans.

The plant is the only iris in North America that produces completely yellow flowers (apart from the brown or purple markings), so it’s easy to identify when it’s in bloom. Unfortunately. it’s not so easy to identify when no flowers are present. It would be a good idea for people concerned about a plant that is growing on land important to them to investigate all of the clues need to distinguish the yellow flag from other local species.

It’s very important to distinguish between yellow flags and cattails, which sometimes grow in the same habitat. Their leaves are quite similar. Once the plants are in flower, there’s no problem distinguishing cattails from irises. When both plants are young and before they flower, there might be. Telling them apart is important when foraging for food because some people eat parts of cattail plants. This wouldn’t be safe if yellow flags were collected and eaten instead. Foragers need to do a lot of research before collecting food from the wild.

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Trout Lake in John Hendry Park

It’s certainly important to take precautions with the yellow flag iris with respect to protecting the environment, our animals, and ourselves. I think it’s a beautiful plant when it’s in flower, though. I’m always happy to see it bloom, at least in places where it appears to be kept under control. It’s a shame that it’s invasive in some places.


  • Iris pseudacaurus PDF from the Government of Alberta
  • Information about the yellow flag from the University of Florida
  • Yellow flag iris problems in British Columbia from the Invasive Species Council of BC (ISC)
  • Facts about yellow flags from the SSISC (Sea to Sky Invasive Species Council)
  • Iris pseudacorus poisoning information from the Canadian Government



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