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Yellow Flag Iris: Weed of the Month

Bright, beautiful irises are invasive in Oregon


by: COURTESY PHOTO: TUALATIN SOIL AND WATER CONSERVATION DISTRICT - This bright beauty may look harmless, but it takes over streams and wetland areas quickly.Yellow Flag Iris doesn’t look like a weed — it’s not prickly like a thistle, it doesn’t vine up your plants like morning glories, it isn’t ugly like common groundsel. But it is considered invasive.

In their efforts to encourage Oregonians to remove this invasive flower, report it and choose plants wisely to replace this bright yellow beauty, the Tualatin Soil and Water Conservation District staff have deemed

Yellow Flag Iris their weed of the month. Tualatin SWCD has provided a few facts to help locals identify this iris and help control it:

Yellow Flag Iris has become a popular ornamental with dramatic bright yellow blooms that doesn’t take a lot of maintenance in moist areas like natural and artificial wetlands and ponds, but it can take over quickly, destroying natives such as sedges and rushes that feed native wildlife, reducing food and habitat sources for domestic and migratory waterfowl in wetlands.

In narrow channels such as irrigation canals, stormwater basins and flood-control ditches, the tendency of this iris to clump heavily restricts water flow, eliminating the plant’s potential to act as a restoration plant. by: COURTESY PHOTO: TUALATIN SOIL AND WATER CONSERVATION DISTRICT - Native Oregon and Douglas irises are good options to replace Yellow Flag Iris.

A hardy wetland plant that reaches about four feet tall with long, flat sword-like leaves and disk-like seeds that can float down long distances to new locations, it doesn’t take long for this iris to spread. The wetter the environment, the quicker the growth — this plant actually prefers standing water.

A native to northern Europe where harsh winters keep it in check, the Yellow Flag Iris thrives in the Pacific Northwest’s milder winters. Instead of storing sugars as starches in the root like other plants, this iris stores its energy as fructan, allowing the plant to metabolize under very low oxygen conditions and protecting it against winter freezing.

Yellow Flag Iris spreads by both its rhizomes and seeds, making it even more difficult to eradicate. Pull out these plants before they go to seed and be sure to remove the entire root. Bag remnants and throw them away; don’t compost or mulch Yellow Flag Iris parts.

The Tualatin SWCD encourages Oregonians to choose other plants, even though Yellow Flag Iris is beautiful, often inexpensive or free, and easy to maintain.

SWCD staff recommends native Oregon or Douglas irises that have royal purple blossoms and are easy to care for because they’re adapted to this region. For gardeners who are seeking the bright pop of yellow, try Rocky Mountain Iris, which also has yellow blooms. Native Skunk Cabbage also thrives in standing water and its blooms resemble the shape of a yellow-hued peace lily. Yellow Flag Iris resembles cattail late in the season when the blooms have been replaced by seed.

Report sightings of Yellow Flag Iris to the Oregon Invasive Species hotline at oregoninvasivehotline.org or call the Tualatin SWCD at 503-648-3174, Ext. 5. Catching this iris early in its establishment is critical.




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