Pretty plant poses ugly threat to wetlands

Written by Tara Bohnsack for The Observer June 13, 2012 08:19 am

The yellow flag iris is an invasive species that can damage wetlands and cause other problems.
The yellow flag iris is an invasive species that can damage wetlands and cause other problems.

Yellow flag iris, a designated noxious weed in Oregon, can quickly spread from gardens to wetlands 

Yellow flag iris, Iris pseudacorus, is an invasive plant of European and African origin officially designated as a noxious weed in six states, including the state of Oregon. 

Seemingly harmless and eye-catching at first glance, this plant poses a significant threat to surrounding ecosystems. Currently only small populations have established in Eastern Oregon, making it the ideal scenario for successful eradication.

Horticultural trade has spread yellow flag iris throughout the United States.

As a result, it shows up occasionally in flower gardens around the northwest. Locally, yellow flag iris appears to be moving from horticultural sites to areas along streams, lakes and rivers. In its invasive state, yellow flag is found in wetlands, along the edges of ponds, lakes, reservoirs or slow moving streams or rivers and can easily become an aggressive wetland bully.

When it blooms in late spring to early summer it is unmistakable with its showy yellow iris flowers. The flowers look very similar to a garden iris but they are often streaked with brown to purple lines. Each stem may have several flowers that each has three large downward-facing yellow sepals and three upward facing yellow petals.

Its leaves are usually longer in the center and are folded around the stem in a fan-like fashion. The leaves will stay green until harsh winter weather begins. Large plant clumps, sometimes attaining 20 feet in width, are formed from the growth of stout rhizomes (a root-like stem that produces roots below and shoots from the upper surface).

When not flowering, it can be confused with cattail (Typha latifolia), which is round at the base and taller than yellow flag iris.

The best way to distinguish this plant when flowers are not out is to look for the large fruit pod in the summer or the fan-shaped plant-base other times of the year.

Yellow flag iris is a problem for several reasons. It colonizes quickly forming dense patches that displace native plants, alter wildlife habitat and decrease biodiversity. 

It reduces habitat and the resources available for wildlife since very few native wildlife species can use this plant for food or habitat. In the eastern United States, reduction of native sedges and rushes that support waterfowl is associated with yellow flag iris invasion.

Seeds clog up irrigation screens and pipes, and the thick growth slows down flow and blocks access to streams.

Once established, it reproduces by seed and vegetatively through rhizomes. It is very easily spread downstream by floating seeds and broken off pieces of rhizomes.

It can cause gastroenteritis in cattle, pigs and humans, and can also cause skin irritation in humans.

Yellow flag iris can be controlled both by mechanical means and with herbicide. In a natural area or along water, it is even more important to carefully remove every root and rhizome, since the water will help move any remaining rhizome bits to a new location.

One small fragment can start a new mass of plants. When hand pulling or digging, make sure to wear gloves because of the irritating resin.

To use herbicides on yellow flag an aquatic license is required because of its proximity to water.

Anyone suspecting they have yellow flag iris on their property can get further information on identification and management by calling Tri-County Cooperative Weed Management Area at 541-523-2740.

Tara Bohnsack is weed specialist for the Tri-County Weed Management Area.