Yellow Flag Iris

Iris pseudacorus

Class C Noxious Weed

General

Native to Europe, Great Britain, North Africa and the Mediterranean region.
Toxic to humans and livestock. Sickens livestock if ingested and will cause vomiting and diarrhea in humans. Contact with its resins can cause skin irritation in humans.
Forms dense single-species stands that outcompete and displace native wetland plant communities and alter successional trajectories. Thick rhizomatous mats can prevent the germination and seedling growth of other native plant species.
Planted around the world as a popular garden and ornamental plant. Still, widely available and sold as an ornamental, it continues to escape into new wildland areas.
It is the only yellow iris found in Washington’s wet areas, but when not in flower may be confused with our native common cattail (Typha latifolia).

Identification

Robust, clumping, aquatic, herbaceous perennial; grows 2 to 3 feet tall along shorelines in shallow water.
Basal leaves are broad, flat, sword-shape and stalkless. Leaves are arranged with sheathing and overlapping leaf bases, in a fan-like fashion. The leaves are a glaucous green color.
Flowers are borne on tall erect peduncles, each may have one to several large, showy flowers. The round stalks stand out from the flat sword shaped basal leaves.
Flowers are cream to bright yellow, with brownish purple mottled markings. Flowers have 3 downward pointing sepals and 3 upward pointing petals. Petals are shorter than sepals; sepals are 3 to 8 cm long. Flowers occur in late spring to early summer.
Fruits are large glossy green capsules up to 8 cm long, somewhat 3 angled. Seed pods or capsules are densely packed with flattened, brown, disc-shaped seeds, about 7mm across.

Photos

Habitat

Prefers part shade or full sun exposure. Commonly found in very shallow water or mud.
Tolerates drying and anoxic sediment and is tolerant of some salinity.
Commonly found in variety of low-lying wetland habits. Grows in wet meadows, swamps, saltmarshes, river and stream banks, lake and pond edges, brackish marshlands, and floodplain riparian areas.

Reproduction and Spread

Spreads by seeds and rhizomes. Water is the primary dispersal agent for both seeds and rhizome fragments.
Yellow flag dies back in harsh winter conditions, but the rhizomes will overwinter. In spring, the long leaves and flowers stalks regrow from the rhizomes.
Flowers are pollinated by bumble-bees and long-tongued flies.
Yellow flag seeds are buoyant for at least 7 months, indicating that they can be dispersed long distances by water. On average a single plant is estimated to produce 221 seeds per year.
During floods rhizomes are washed away and deposited downstream where they can sprout and form new plant colonies.
Plants spread rhizomatously and grow tightly bunched together. Up to several hundred flowering plants may be connected through rhizomes.

Control

Prevent new infestations or begin control efforts while populations are still small and manageable.
Make sure to protect skin when removing yellow flag as contact with the resins can cause skin irritation.
Pulling and digging may be effective on small isolated populations. Carefully remove all of the plant and rhizomes, fragments left in soil will grow to form new plants. This method is time and labor intensive.
Pulling or cutting plants may provide control, if repeated every year for several years to weaken and eventually kill the plant.
Dead-heading (removing the flowers and/or fruits) from plants every year can prevent seed development and dispersal, but will not kill the plant.
Monitor all control work for any regrowth from seeds or resprouts. Survey upstream locations for potential seed sources that could re-infest the control area.

Herbicide Control

Since yellow flag frequently grows in or adjacent to water, be sure to use an aquatic-labeled herbicide and surfactant, and obtain an NPDES permit from WSDA; or contact your county noxious weed control board.
Larger populations can be controlled by either foliar herbicide application or a combination of cutting and herbicide application.
Cutting-Herbicide Combination: Cut leaves and stems, leaving basal leaves about 4 to 6 inches above the ground. Place cut leaves around base of plant to shield native vegetation. Spray or use a dripless wick to apply herbicide to remaining foliage.
Herbicides containing the following active ingredients are effective on yellow flag iris:
- Glyphosate (Non-selective, no residual soil activity)
  - Foliar application, apply a 5 to 8% solution.
  - Wick/wiper application, apply a 25% solution.
  - For both methods, apply directly to uncut foliage or immediately apply herbicide to freshly cut leaf and stem surfaces.
  - Apply early in growing season, pre-bloom. Later applications can be effective but dead head flowers or seed pods.
- Imazapyr (Non-selective, residual soil activity)
  - Foliar application, apply a 0.75% to 1% solution.
  - Apply directly to uncut foliage or immediately apply herbicide to freshly cut leaf and stem surfaces.
  - Apply pre-bloom or in the fall. Dead head flowers or seed pods if present