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Weed and Pest cautions landowners about Yellow flag iris, Musk thistle

by Wyoming Livestock Roundup

As landowners around the state are preparing for the winter, Wyoming Weed and Pest urges Wyomingites to be vigilant in watching for invasive species on their property.

In particular, Yellow flag iris and musk thistle are at the top of the list that producers may be unfamiliar with at this point.

Yellow flag iris

“This summer, Teton County signed an emergency declaration for Yellow flag iris,” says Wyoming Weed and Pest Coordinator Slade Franklin. “This is plant that we don’t know much about.”

“Yellow flag iris will most likely show up in high water areas,” Franklin says, citing riparian areas and waterways.

Yellow flag iris, also known as paleyellow iris, yellow iris or yellow flag, is included on state weed lists in Connecticut, Massachusetts, Montana, New Hampshire and Washington, and USDA’s Natural Resources Conservation Service also lists the plant as invasive.

“Yellow flag iris displaces native vegetation along stream banks, wetlands, ponds and shorelines,” says the King County Noxious Weed Control Program out of Washington.


Yellow flag iris is a perennial plant that creates dense stands. It can grow to five feet tall and has numerous thick, fleshy rhizomes.

The flowers of the plant are yellow and described as “showy.” They occasionally have brown to purple veins at the base of the petals, and several flowers can occur on each stem.

The plant typically blooms from April to August, and it may remain green all winter in mild years.

The leaves of Yellow flag iris are broad, flat and pointed. They are folded and overlap one another at the base.

“When not in flower or seed, Yellow flag iris can be confused with cattails, which are round at the base and taller than yellow flag iris,” adds King County.


To treat Yellow flag iris, integrated pest management (IPM) is a preferred method. The IPM approach allows selecting from a range of control methods to target the management needs of individual sites, according to King County.

In addition, they mention that individual plants can be effectively dug up. However, in manual control methods, collection of rhizome fragments is important. It is also important to minimize disturbance to avoid creating seed germination opportunities.

King County suggests looking for seedlings starting in late winter and digging up small, isolated patches. Plants will likely be located in high water areas along river and lake shorelines, wetlands, ditches and in wet pastures.

Musk thistle

In addition to Yellow flag iris, Franklin mentions, “Musk thistle has exploded in this state and across the West. Musk thistle is all over the place in Wyoming.”

He also notes that the prevalence of Musk thistle this year was higher than he has seen ever before.

“For people with one or two plants, musk thistle can be effectively dug up or knocked down,” Franklin says. “There are also herbicide treatments.”

Musk thistle is capable of appearing anywhere, and it can be a big problem once it is established.

Looking at the plant

Musk thistle can grow to six feet tall. The upright stems of the plant are winged and can be single, multiple or highly branched.

The rosette leaves of the plant are elliptic to lanceolate and pinnately lobed, with each lobe ending in a spine, according to North Dakota Weed and Pest.

Flowers of Musk thistle are deep rose, violet or purple in color, though occasionally white flowers may be seen. They also feature characteristic brown bracts that resemble a pine cone.

North Dakota Weed and Pest also notes that Musk thistle is an aggressive species that can form extremely dense stands, which reduce the ability of native pasture and rangelands to compete by suppressing desirable growth of plant species. At the same time, plants are not palatable to livestock, making their invasion more aggressive.

“Cattle generally will not graze Musk thistle, and sheep will only consume the plant during the rosette growth stage,” they continue. “However, some studies suggest that cattle, domestic sheep and goats may consume Musk thistle flower and seed heads.”

However, concerns about livestock spreading the seeds of the plant have led to a recommendation against grazing as a control mechanism.

Landowners who suspect the presence of Yellow flag iris, Musk thistle or any other invasive species on their property should contact their county weed and pest office for assistance in determining the most effective treatment strategy.

Saige Albert is managing editor of the Wyoming Livestock Roundup and can be reached at

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