See also: Herbaceous Perennials
Yellow flag iris (Iris pseudacorus) is a highly adaptable and easy to grow perennial that needs sun to flower at its best. It is hard to resist the impeccable clarity and tone of the flowers. In a world where yellow flowers inhabit the landscape in all seasons, yellow flag iris is one of the finest in the land. It is also considered highly invasive in some parts.
Many gardeners drop the 'iris' from the name and merely refer to Iris pseudacorus as 'yellow flag'. The first time I remember seeing yellow flag iris was at the edge of a small lake: a dense colony eight feet across with those pure yellow flowers held aloft like a king's banner.
Yellow flag iris lives naturally where the soil is perpetually wet. Yellow flag iris does fine in ordinary garden soil as well.*
Yellow flag is easy to grow in reasonably fertile gardens. It prospers with normal rainfall amounts in areas of North America that are not overly arid. I have noticed yellow flag iris will grow smaller (perhaps to 2'-3') outside of boggy and wet areas. It loves boggy areas and can get huge, up to six feet or more tall.
*Before planting yellow flag iris, ponder its ecological impact in your area.
Yellow flag iris is native to Europe and parts of Africa and Asia. Yellow flag is widely naturalized in North America. In some locations yellow flag iris is considered highly invasive. For example in the northeastern United States it is a real problem. Both Connecticut and Massachusetts have banned it. Yellow flag's aggressive nature allows it to outcompete other plants.
What factors indicate a plant may be invasive?
In the case of yellow flag iris, we've established that it prefers bog areas but can survive in garden soil with average moisture. This indicates a wide latitude of growth tolerance. Plants that are easy to grow are highly adaptable to different growing conditions. Conversely, plants that are considered difficult to grow normally have very specific growing conditions.
Rhododendrons have a reputation for being difficult to grow outside of their native mountain habitats. This is partly because rhododendrons require consistently moist soils that are well drained, a pretty tough combination when you think about it. In addition, the cool nighttime temperatures found in the Appalachian Mountains retard the growth of the diseases that plague rhododendrons at lower (and warmer) elevations.
I used the rhododendron as an example of a plant with narrow latitude in growing conditions precisely because it is opposite of yellow flag. Both species like a lot of moisture in the root zone. The difference is while rhododendrons must also have very well drained soil, yellow flag can survive happily in wetlands and other poorly drained environments AND it can survive in areas with much less moisture. Yellow flag is highly adaptable to different growing conditions.
Is yellow flag a danger to ecosystems outside of wet areas? No. But when a plant is introduced to new areas, a warning bell should sound if it can grow well outside of its normal habitat.
Should you plant yellow flag iris in your garden?
There are plants, and yellow flag is one of them, where a responsible gardener should be willing to do some research before choosing to plant a species in their garden. Consult with others...here is some sobering information regarding yellow iris and invasiveness from other gardeners.
From what I can discern (with an admittedly very casual eye) most every nursery in my area (Georgia) offers yellow flag iris. Are we just a bunch of noodle-headed neanderthals in our region or is there another possibility? I suspect the answer lies in the idea that some plants are highly adapted to growing better in some places than others and herein lies the root of the problem.
Living in Georgia, would I think less of a fellow gardener with yellow flag in the landscape? No, and I wouldn't lecture them on the subject either. Reasonably, I can accept that yellow flag may be a problem in one area and not the other.
Thinking you know something and actually knowing something are two different things. Here is a question I myself ponder: If ten gardeners are considering planting a specific species that might be invasive and nine of them decide against planting it because of its potential danger to an ecosystem, but one of them chooses to introduce the species has the one made the decision for all?
It seems this question is at the heart of environmental debate...including discussions about whether climate change is human-induced and dangerous to our future. I don't know if climate change is real. I know this however: most if not all of the people denying climate change exists really don't know, yet they plant the yellow flag and oil refineries for us all.