A yellow iris soaks up the morning dew in wetlands at Allegany State Park in Salamanca, N.Y. While beautiful, the yellow iris an invasive species that can spread quickly, forming large clumps that crowd out native vegetation and alter ecosystems. Also known as the yellow flag iris, it thrives along shorelines and in wetlands and roadside ditches, and can be difficult to control once established, according to Washington state’s Noxious Weed Control Board.