U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
 
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Yellow Starthistle (Centaurea solstitialis)

Sunflower Family (Asteraceae)

Centaurea solstitialis (yellow starthistle).  Charles Turner, USDA Agricultural Research Service, Bugwood.org
Photo: Charles Turner, USDA Agricultural Research Service, Bugwood.org

     Centaurea Solstitialis (yellow starthistle).  Photo courtesy of Peggy Greb, USDA Agricultural Research Service, Bugwood.org     
Photo: Peggy Greb, USDA Agricultural Research Service, Bugwood.org

Centaurea solstitialis (yellow starthistle). Photo courtesy of Steve Dewey, Utah State University, Bugwood.org
Photo: Steve Dewey, Utah State University, Bugwood.org

 

Description/Habitat:  Yellow starthistle (Centaurea solstitialis) is a winter annual native to southern Europe that can grow up to 2m in height. Yellow starthistle was accidentally introduced into the United States in the mid-1800s with contaminated alfalfa seed. It invades woodlands, pastures, roadsides and fields, and can create dense, impenetrable stands that displace native vegetation. As far as our records show, yellow star thistle was discovered on Fort Ord National Monument property within the past few decades, but it’s a reasonable guess that it could have been around much longer.
 
Yellow star thistle is considered one of the most serious rangeland weeds in the western United States and is a huge problem across the state of California, with an estimated infestation of 22 million acres. This has brought on great economic loss to agricultural and grazing practices by reducing the quality and yield of forage in grazing lands, as well as creating infestation issues in agricultural crops.
 
Leaves:  The stems are stiff, wiry, and simple in very small plants, and larger plants are more openly branched from near or above the base of the plant.  Leaf bases extend down the stems and give the stems a winged appearance. At its widest, the stems are usually 5mm wide.
 
The stem leaves are alternate, and are mostly linear to narrow oblong or oblanceolate. The lower stem is typically deeply pinnately lobed. The foliage of yellow starthistle is grayish to bluish green, and is densely covered with fine, white cotton-like hairs that hide most of the stiff, thick hairs. Rosette leaves typically wither by the time the plant is flowering.

Flowers:  Flowering starts in June, when the yellow flowers appear in heads at the tips of the branches. Bracts that subtend the flower head have a small cluster of spines, and a stout central spine 0.4-1” (10-25mm) in length. Large plants may produce as many as 1,000 composite flower heads, which can produce immense amounts of seeds. Grazing and mowing can delay flowering.
 
Seeds:  Each singular flower produces a seed head which contains 30-80 seeds. Large plants can produce almost 75,000 seeds in one season. Yellow starthistle seeds are mostly wind-dispersed, but can travel greater distances when accidently transported in contaminated hay or seed loads, as well as by becoming attached to vehicles, humans, and animals. Approximately 90% of seeds can germinate immediately after being released from the parent plant. Yellow starthistle generally germinates frequently after the first rains in autumn. The seeds need moisture and light to germinate, and can remain viable in the soil for at least 3 years.
 
Flowering Period:  June to December. Plants usually senesce in late summer or early fall, when the flower heads shed their central spines and retain a tightly dense ball of fuzzy gray hairs (chaff) on the receptacle. On heavily infested sites, a dense thatch layer develops.
 
Management:  There are several methods used to control yellow starthistle. The main treatment methods include grazing, prescribed fire, herbicide treatments, and manual removal. In terms of grazing, yellow starthistle can be fatally poisonous to horses when eaten, and its spines will deter various other livestock from grazing. Cattle will eat it before it has spines. Goats and sheep are quite possibly the most tolerant of eating yellow star thistle, but only if there is nothing better to graze upon. The best time to control yellow star thistle with grazing would be during the late rosette stage when plant has bolted, but has not produced spines and flowers. This ensures that the plants will recover slower than when grazing upon them late in the bolting stage when they have almost begun flowering. The overall key is to not graze upon yellow star thistle too early or too late, as the plants will put all of their remaining energy into flower and seed production. Letting goats and sheep forage at the correct time of year, and by grazing the same areas in rotation throughout the active growing season, helps deplete the species and its seedbank. Some plants bolt later than others, so this rotation method catches the late growers and also re-treats previously missed or aggressively active yellow star thistle plants.

A few other effective methods of controlling yellow starthistle include prescribed fire, and herbicide application. Prescribed fire helps reduce monoculture plant cover, stimulates fire-adapted native species to compete with and replace yellow starthistle, and can also aid in depleting the seedbank in the soil. However, managing yellow star thistle the first year after a burn is critical because fire will stimulate germination and the infestation will increase in size. This enhanced germination depletes some of the seedbank, but this calls for active management after a fire.
 
Chemical applications of glyphosate herbicides at a 1-2% solution during the rosette stage (before bolting) have also proven effective. While there has been some spraying of yellow starthistle on Fort Ord National Monument in the past, the vast majority of control has been manually pulling plants. Hand pulling requires removing the plant from below the ground, otherwise yellow star thistle can resprout. Since yellow starthistle blooms for such a long period of time, this requires repeated visits to each site throughout the season, and includes extensive scouting on sometimes very difficult terrain to ensure all plants are found. We have retired a number of our yellow star thistle sites since the plants at these sites have been eliminated. At our other currently active sites, we are seeing a great reduction in plant numbers as a result of our efforts.



Sources:


Cal-IPC Weed Workers Handbook: A Guide to Techniques for Removing Bay Area Invasive Plants. 2004. The Watershed Project and California Invasive Plant Council: 87-89

DiTomaso, J. M. and E. A. Healy. 2007. Weeds of California and Other Western States. Volume 2: Geraniaceae – Zygophyllaceae. University of California Department of Agriculture and Natural Resources: 250-260

DiTomaso, J.M., Guy B. Kyser, and Michael J. Pitcairn. 2006. Yellow Starthistle Management Guide. California Invasive Plant Council.


 
Last updated: 08-20-2012