Yellow flag iris is an excellent example of a plant that is benign, even beneficial, in its native habitat of Northern Europe, where harsh winters keep it in check, but is a worrisome nuisance when it travels abroad. Sadly, it can now be found cheaply through online distributors like Craigslist at low cost because it is so easy to cultivate. And in the time honored tradition of sharing successful garden plants with your neighbors, people are literally giving it away! But when yellow flag iris escapes the confinement of the garden and makes its way into the open waterways of the Western US, trouble is not far behind.
In its native Scotland, yellow flag iris provides an important food source and nesting site for the endangered Corn Crake. However, here in Oregon, it crowds out higher quality native food sources such as sedges and rushes, which are important as forage for our native wildlife. In this way, it reduces the ability of wetlands to support domestic and migratory waterfowl. And unlike in its home climate, yellow flag iris has no trouble surviving the milder winters of the Pacific northwest because of its unusual food storage system. Instead of storing sugars as starches in the root like other plants (think of potatoes and other starchy tubers), this energy is stored as fructan, allowing the plant to metabolize under very low oxygen conditions and protecting against freezing in the winter.
As it escapes urban gardens and ponds, yellow flag iris plant has migrated along stream corridors to invade riparian areas, open water features such a lakes and wetlands, and irrigation ditches. It can spread by both seed and rhizome. The seed pod contains disk-like seeds that, because of their cork-like nature, can often be seen floating long distances in large mats in the fall and winter to new locations, spreading the invasion. This can result in severely restricted water flow in narrow channels like irrigation canals, stormwater basins and flood control ditches due to heavy clumping, negating any potential environmental benefits and thus eliminating from the roster of restoration plants.
Look-alikes and Better Choices
Responsible gardeners have a number of choices in the iris family as well as yellow blossoming plants and other wetland plants that can replicate the appearance of yellow flag iris throughout the season.
• Gardeners seeking the classic look of cultivated irises can select from a wide variety of other non-invasive cultivars, allowing for a spectacular array of color and variegation options. Native Oregon or Douglas irises have a lovely royal purple blossom and is adapted to our regional environment, allowing for a high rate of plant survival.
• For gardeners seeking the distinctive yellow hue to add a variety of color to their landscape, several native options abound. Rocky mountain iris lacks the larger sized blossom of the cultivated varieties, but mimics their bright yellow color. Like yellow flag iris, native skunk cabbage thrives in standing water environments with a bloom that closely resembles a yellow-hued peace lily in shape.
• Yellow flag iris can also resemble cattail late in the season when the bloom has faded and is replaced by the seed (an oblong tripartite capsule between 2-4 inches long appears quite similar at a distance to the classic seed pod of the cattail).
Removal of yellow flag iris usually requires large excavation equipment or use of herbicides once the population has taken over. Moreover, removal may have to occur more than once since the plant may easily re-establish by rhizome fragments left over. As with most invasive species, catching yellow flag iris during its establishment before growth explodes is critical.
If you encounter this invader, you can take immediate action as well as report it. To remove the immediate culprit, pull plant the before it goes to seed, being sure to remove the entire root. All captured plants should be bagged and thrown into the municipal waste bin rather than composted or mulched in order to prevent re-sprouting.