Montana Weed Control Association

Introduction

Yellow starthistle is found close to the western border of Montana but has not yet successfully infested the majority of the state.  Due to its close proximity to us we need to seriously consider the potential it has to move in and rapidly infest our land.  It is thought that it was introduced to North America from the Mediterranean region, through contaminated seed.  Yellow starthistle is present in 23 states with the largest infestations in California, Idaho, Oregon, and Washington.  There are approximately 12 million acres infested with yellow starthistle in California and over 200,000 acres infested in Idaho.  Until recently only a few yellow starthistle plants had been found within Montana.  Last fall a small infestation was found in Beaverhead County.  A few weeks ago a larger infestation was discovered in Columbus, in Stillwater County.  This infestation is scattered throughout 10 acres.  This spring/summer will tell if the infestation is more widespread than what was found through identifying the standing dead plants remaining from last years growth.

Concerns

The spines that are located below the yellow flowers interfere with grazing, recreation, and wildlife management.  This plant is also toxic to horses.  It causes a chronic and potentially fatal neurological disorder that is commonly called “chewing disease”.  It earned this name due to the loss of the ability to apprehend and chew food.  However, swallowing is unaffected.  The animal will eventually starve to death due to this if it does not die due to inhalation pneumonia first.  There is no specific treatment to date and the irreversible localized necrosis in the brain gives a very poor prognosis.

Identification

Yellow starthistle is a winter annual with a deep taproot (up to depths of 6 feet).  Seeds germinate in the fall and overwinter as rosettes.  Fall germinated seedlings monopolize soil moisture and are highly competitive for nutrients and space.  In the spring, the rosettes will bolt producing branched erect stems with terminal flower heads.  Yellow starthistle is capable of growing in either wet or dry conditions and can respond rapidly to changing conditions.  It will produce large plants with abundant seeds during wet years and small plants with few flowers and seeds in dry years.

The flower heads consist of 20-50 small yellow individual flowers that are tightly clustered appearing as a single flower at the end of each branch.  Radiating out from below the flowers are up to 2 inch long straw colored spines.  The grayish to bluish-green stems are rigid and appear winged due to extending leaf bases.  The stems and leaves are covered in cottony hairs given the plant a whitish appearance.  The plant can be anywhere from 2-6 feet tall.

What can you do?

This plant is a new invader to Montana and therefore requires immediate action and hopefully eradication.  If you have found or think you have found yellow starthistle contact your county weed coordinator before initiating any control.  This plant is a rapid colonizer and germinates quickly under most conditions.  Dense seedbanks and length of seed viability make this plant extremely difficult to control and this is why it is important to locate new infestations and eradicate them prior to seed production.  The seeds can remain viable in the soil for up to 10 years.

Herbicides are the recommended method and most effective means of control when applied before the plants have flowered.  For recommendations on herbicide selection and timing contact the weed district and always follow the label directions.  Persistent hand pulling can be effective, as long as no flowers are allowed to go to seed.

Visit the MWCA Weed ID pages for additional information and pictures of yellow starthistle. If you have any questions about yellow starthistle or any other noxious weeds please contact your local county weed district.

This series of articles was developed by Ravalli County.  If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.