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Yellow flag iris is an aquatic plant that grows as leafy clumps up to 2m tall. The sword-like leaves emerge in fans from a reddish-purplish base. From October to December it produces pale-yellow to golden-orange flowers that are up to 12cm in diameter and are followed by seed capsules containing many brown, flattened, three-sided, disc-like seeds.
Yellow flag iris grows in still and slow-flowing water bodies and wetlands and it can invade flood-prone pasture. In other countries it has been recorded in salt marshes.
Yellow flag iris is a fast-growing and fast-spreading invasive plant that can out compete other plants, forming almost impenetrable thickets as it over-tops and replaces native species. It can also grow out across the water, forming floating mats that are strong enough to support the weight of a person. It is poisonous to grazing animals and is tolerant of saline conditions, frost, flooding, drought, physical damage and many soil types.
Yellow flag iris reproduces from both rhizomes (roots) and seeds. Each seed capsule contains many viable seeds but the longevity of yellow flag iris seeds in the seed bank is unknown. It has been found to facilitate its own spread by contributing to soil compaction and hardpan development, which prevents other desired species from establishing on a site.