Iris pseudacorus By Matt Young October 11, 2021 11:00 MST Iris pseudacorus – yellow flag iris, Cottonwood Lake, Boulder, Colorado, June, 2015. This plant is an aquatic iris. I took this snapshot in 2015 (and an earlier one in 2009) and did not give it much thought. The plant was not very widespread when I snapped the pictures, but last week I came across a sign warning that the area had been sprayed to control yellow flag iris, cattail, and teasel. Sure enough, the Colorado Department of Agriculture considers yellow flag iris a noxious weed, though it is only on a watchlist and not regulated. I have no idea why cattail is being sprayed, but teasel is very invasive.