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Weed Guide

Some of the Most Common Noxious Weeds in Montana

Dalmation Toadflax

Dalmation Toadflax Photo: Joseph M. DiTomaso,
University of California-Davis,

Dalmation toadflax is a short-lived, herbaceous perennial in the figwort family that grows up to four feet tall. Stems are somewhat woody at the base and often branch in the upper portion. Leaves and stems are waxy, leaves are heart-shaped and one to three inches long with clasping bases. Flowers are bright yellow with orange markings. Dalmation toadflax infests 174,000 acres in Montana. Though common in some areas, others remain uninfested. It is critical to fish and wildlife habitat to control this noxious weed.


Spotted Knapweed

Spotted Knapweed Photographer: John Cardina,
Ohio State University,

Spotted knapweed is an herbaceous biennial or perennial plant that invades open areas throughout most of the U.S. Its name is derived from the black margins of the flower bract tips which give the flower heads a spotted look. A basal rosette of deeply lobed leaves is produced the first year. Rosette leaves may grow to eight inches long. Flowering stems grow up to four feet in height and branch. Flowering occurs in the mid to late summer. Flowers are purple to pink on small flower heads. Spotted knapweed infests 2.7 million acres in Montana and is common in many areas though there are uninfested areas. It is critical to fish and wildlife habitat to control this noxious weed.


Leafy Spurge

Leafy Spurge Photo: USDA APHIS PPQ Archive,

Leafy spurge is an erect, perennial, herbaceous plant that grows from two to three feet tall. Leaves are oval, smooth and one to four inches long. The plant produces a milky sap if the stem is broken or a leaf is removed. The stem is smooth and bluish-green. Flowering occurs in the late summer or summer when yellow flowers develop in clusters at the apex of the plant. Large infestations give the landscape a yellowish tinge due to the yellow bracts. This plant can completely overtake large areas of land and displace native vegetation. It infests 797,000 acres in Montana, though many areas remain uninfested. It is critical to fish and wildlife habitat to control this noxious weed.



Houndstongue Photo: Mary Ellen (Mel) Harte,

Houndstongue is a biennial plant that is eight to 48 inches tall. First year's growth is strictly vegetative with flowering in the second year. In the early summer flowers appear, by late summer or fall burrs appear that reseed by first attaching like Velcro to clothing, animal hair and other objects. Flowers are reddish-purple with a five-lobed corolla. Houndstongue is found throughout western Montana and is increasing in eastern areas of the state to infest nearly 400,000 acres.


Recently Introduced Noxious Weeds

Contact your County Weed District immediately if you find these weeds. Note the location carefully or mark it in some way if possible by GPS or colored tape.

Yellow Starthistle

Yellow Starthistle Photo: Steve Dewey,

Yellow starthistle is an annual that can grow from ¼ inch to three feet high. Leaves near the base are dissected, two to six inches long and usually absent when the plant flowers. Flowering occurs from June to October when yellow flowers appear in heads at the tips of the branches. Several plants were discovered south of Dillon in 2009 and eradicated. Yearly monitoring will be necessary. During winter 2010, a 10-acre infestation was discovered near Columbus.


Rush Skeletonweed

Rush Skeletonweed Photo: Gary L. Piper,
Washington State University,

Rush skeletonweed is a perennial forb that can grow up to four feet tall. Course-looking, multiple stems appear leafless due to inconspicuous leaves and arise from a basal rosette of sharply lobed leaves. The lower four to six inches of stem is covered with coarse brown hairs. Both stem and leaves produce a milky sap when broken. Flowers are small and yellow and develop in the mid-summer to fall. Healthy plants can produce up to 20,000 seeds. Currently distribution of this highly invasive plant has been limited to northwestern Montana.


Tansy Ragwort

Tansy Ragwort Photo: Richard Old,
XID Services Inc.,

Tansy ragwort is a biennial to short-lived perennial herb native to Eurasia. Plants can reach one to four feet in height. Flowering occurs in summer to early fall, when yellow, daisy-like flowers develop. Fruit are small and have a persistent ring of white hairs attached. Distribution at this time is mostly limited to northwestern Montana but the plant is making its way south and east.


Dyer’s Woad

Dyer’s Woad Photo: Steve Dewey,
Utah State University,

Dyer’s woad is a biennial plant that can grow from one to four feet tall. First year’s growth is represented by a rosette of hairy, bluish-green leaves. Yellow flowers with four petals appear in small clusters at the top of the stems in spring to late summer. Seed pods are flattened and hang from short stalks at the ends of these stems. Small infestations have been found in Montana in several areas and were eradicated. Areas to watch are near Dillon, Butte, Missoula, Bozeman and Yellowstone National Park. Continuous monitoring is necessary.