The yellow flag iris (Iris pseudacorus) is not native to North America. It is considered an invasive species. Though very pretty, it can take over a wetland and obliterate every thing else. Stanley Park is making attempts to control the spread of this plant, but I assume it plays a role in treating storm water runoff from the causeway that passes through the park on the eastern side of the Lost Lagoon. The irises can absorb heavy metals through their roots.