(Editor's Note: This article was originally published on January 10, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
Experience then happened. The raccoons got the goldfish, the water lily came with stringy green algae, which I battle to this day. And the yellow flag iris along the border of the patio quickly grew to the point that I had to fight my way through it even to approach the pond. The patio blocks were being heaved up out of place by the relentlessly increasing mass of the rhizomes. The yellow flag iris had to go.
Still, I was sorry to have to get rid of it. Iris pseudacorus is an attractive ornamental plants well-suited [too well-suited] for the banks of ponds and other wet ground. One alternative name for it is "pale yellow iris," which I wonder at, as its best feature is the strong, clear yellow of its flowers. As many as a dozen blooms can unfurl on a single stem in early summer. The sword-shaped leaves stand erect in a clump, although they do tend to get a bit overgrown and droopy late in the year.
But it is certainly no plant for a tiny 2 x 4 foot pond. "Vigorous" is an understatement for the yellow flag's habit of growth. Given plenty of water, as on the margin of a pond, the plants can grow over six feet in height. And they spread – oh, do they ever spread, with the rhizomes forming a dense, fleshy mat. To dig up mine, I needed a hatchet in addition to a spade and a spading fork, and there were moments I considered the axe, or a chainsaw. Still, I failed at first to remove every shred of the rhizomes, and for a few years I would still find them trying to sprout from under the patio blocks. This is a plant that is truly hard to kill.
It is thus for good and sufficient reason, despite its attractiveness, that Iris pseudacorus is now considered an invasive plant. Because of its extreme vigor and its potential to escape and colonize waterways, it is banned in several states, including Connecticut where it is banned as invasive; Massachusetts (prohibited), New Hampshire (prohibited invasive species) , and Oregon (quarantined); others have listed it as a noxious weed.
This is a non-native plant in North America, one that outcompetes and displaces native species, in particular the common cattail, Typha latiofolia, and the native irises known as "blue flag," such as Iris versicolor and Iris giganticaerulea, the Giant Blue Flag. The dense rhizomic mats of the yellow flag exclude all other species. In streams, they collect sediments and reduce water flow, sometimes altering the course of the stream. Once established, it is almost impossible to eradicate. Fragments of the rhizome readily take root, and it also spreads as the corky seeds are carried downstream. And of course herbicidal control has the potential to injure fish and other aquatic creatures.
What, then, is the gardener to do when smitten by the seductive lure of yellow iris? First, there are alternatives. Hybridizers have produced yellow cultivars of the wetland-loving Louisiana iris. There are also yellows among the Siberian irises. And if color is not the critical issue, there are also the blue flag and Laevigata irises, that will do just as well in ponds. I am quite happy with the blue Iris pallida 'Variegata' that I planted along the border of my patio to replace the yellow flags, although they are not suitable as submerged pond plants.
For areas where they are legal, though, some gardeners may still want to use the yellow flags. There are two sets of considerations here. First, it is important to grow these plants in such a way that they are not likely to spread into the wider world from your garden or pond. Any yellow flag iris in my patio pond (yes, I still have a pot in the pond) is not likely to escape. But there is a larger retention pond on the edge of my property that regularly overflows in the spring, with the water flowing into a ditch, thence through a culvert into a nearby creek, from which it becomes part of the Mississippi watershed. There is a clear risk that seeds from any yellow flag planted there might be washed to some other location where they would not be wanted, so this type of location is best avoided. If the plants are already present and can not be removed, the gardener should at least be sure to deadhead the flowers just after they bloom, to make sure they do not set seeds.
It is also important, when hacking back the plants when they overgrow their bounds [as they will] to dispose of the excess plant material in a manner that will not allow it to take root and spread. Composting and composting landfills are not safe, as the rhizomes of Iris pseudacorus are almost impossible to kill.
The other consideration for gardeners is: how will this plant behave in my own garden or pond? You should be aware, as I was not, how large this iris is and how invasively it will spread, its potential to overwhelm and overrun most other nearby plantings. You should also prepared for the fact that you may never get rid of it once it is well-established.
But there are ways to minimize the aggressiveness of the yellow flag. While it is most rampant in or near water, it will also grow in a dry location (that "impossible to kill" factor), but less vigorously and to a smaller size. To keep it within bounds, you should practice preventative hacking-back while it is still small enough to control, taking care to dispose of the discarded rhizomes responsibly, as above. Gardeners have sometimes attempted to confine it in a container, but you should be aware that in a contest between the yellow flag and the container, the iris will always eventually win unless it is regularly divided.
When it comes to the yellow flag iris, dividing is the only way to conquer.