Agriculture

Yellow Starthistle

Yellow starthistle, Centaurea solstitialis, a native of Eurasia, was first recorded in California in 1869. Now common on roadsides, rangeland, hay fields, pastures, and waste areas, it is estimated to infest over 8 million acres in California. The disturbance created by cultivation, poorly timed mowing, road building and maintenance, or grazing favors this rapid colonizer. It forms dense infestations and may produce allelo chemicals that slow the growth of competing species, allowing starthistle to take over large areas of land. It is also poisonous to horses, causing a nervous disorder called "chewing disease" that is fatal once symptoms develop.

Identification

Yellow starthistle is a gray-green to blue-green annual plant with a vigorous taproot. It produces bright, dandelion-like yellow flowers with sharp spines surrounding the base. Yellow starthistle grows to heights varying from 6 inches to 3 feet. The stems of mature plants are rigid, spreading and branching from the base. Stems and leaves are covered with a loose, cottony wool that gives them a whitish appearance. Stems appear winged due to leaf bases that extend beyond the nodes. Basal leaves are 2 to 3 inches long and deeply lobed. Upper leaves are short, 0.5 to 1 inch, narrow, and sharply pointed.

Biology

Yellow starthistle is an annual, usually found below 5,000 feet elevation, in dry, light-intensive areas where average annual rainfall is between 10 and 40 inches. It is able to regrow after top removal from mowing or grazing. Seed output can be as high as 29,000 seeds per square meter, with about 95 percent of the seed being viable. Most seeds germinate the following year, but some can last 10 years or more in the soil.

Yellow starthistle seeds generally germinate from fall through spring, which corresponds to the normal rainy season in California. After germinating, the plant initially allocates most its resources to root growth. By late spring, roots can extend 3 feet or deeper into the soil profile, although the portion above ground is a relatively small basal rosette. This allows yellow starthistle to outcompete shallow-rooted annual species during the drier summer months when moisture availability is limited near the soil surface. It also helps explain why yellow starthistle survives well into the summer, long after other annual species have dried up. The competitive ability of yellow starthistle also depends on light intensity at the soil surface during the seedling and rosette stages of development. Yellow starthistle proliferates at high light intensity and does poorly in low light. Highlight conditions often occur along roadsides, in disturbed sites, and on south-facing slopes. 

Management

Control of yellow starthistle can not be accomplished with a single treatment or in a single year. Effective control requires suppression of the weed combined with establishment of competitive desirable vegetation.

Prevention

Yellow starthistle proliferates along roadsides. The invasion by this weed may be increased by the disturbance created by road building and maintenance. Seeds are often spread by vehicles or with the transportation of livestock. Survey roadsides for the presence of this weed and immediately control new infestations to prevent its seed production and subsequent spread.

Yellow starthistle also can be spread as a contaminant in grass seed, so use certified seed for range or pasture seeding. Seed may also come as a contaminant in all classes of hay, particularly grass hay. Carefully check hay shipments for evidence of yellow starthistle. When feeding hay suspected of containing yellow starthistle, feed bales in one area and periodically check around feeding areas for signs of starthistle seedlings. Livestock that have fed in yellow starthistle-infested areas should not be shipped or pastured in uninfested areas. Like wise, clean all equipment used in yellow starthistle-infested areas before moving it to uninfested areas. Control newly emerged seedlings to prevent establishment. It is important to control new infestations when they are small because spot eradication is least expensive and most effective at this time.

Biological Control

Four natural enemies of yellow starthistle have been imported from Greece and are established as of 1994 in California as biological control agents: the three weevils Bangasternus orientalis, Eustenopus villosus, and Larinus curtus; and the gall fly Urophora sirunaseua. They all attack the flower/seed head, and directly or indirectly reduce seed production, the only means of reproduction and spread of the weed. The insects lay their eggs in, on, or near the flower/seed heads and complete their development within them. They are all highly host specific to yellow starthistle and do not attack commercially valuable crops or native plants.

Following the release of these natural enemies, protect the release area from heavy disturbance (insecticides, soil cultivation, burning, or destruction of yellow starthistle) for several years to give the insects a good chance of establishing. After establishment, the insects are capable of building up to high numbers and spreading on their own. These insects do best in areas with warm, dry, summer climates.

It is too early to know the impact of these natural enemies on yellow starthistle in California. It will likely take a long time to achieve effective biological control. The insects become more numerous and thus more available with each succeeding year. Currently, B. orientalis, E. villosus, and U. sirunaseva are the most numerous and widespread of these insects. Land owners and managers with yellow starthistle problems may contact their County Department of Agriculture about obtaining these biological control insects. These biocontrol agents reduce seed production, slowing the spread of the weed. Biocontrol of established populations is uncertain until impact data are available, but would require years to achieve if it is successful. To control established populations, use biocontrol agents in combination with other control measures.

Cultural Control

Cultivation effectively controls seedlings of yellow starthistle. Be cause yellow starthistle begins emergence with the fall rains, this is the best time to begin cultivation. Repeated cultivations are needed to control each new flush of seedlings. Deep tillage in spring can also control established plants, but generally, larger plants with deep taproots, have a greater chance of surviving tillage. Deep tillage also stores seeds in deeper layers that may surface with future tillage.

Mowing can be used to manage yellow starthistle provided it is well timed and repeated as needed. Mowing early growth stages results in regrowth of the weed and additional mowing will be needed. When mowed frequently, the starthistle may re-grow and flower below the mower cutting height. Preliminary studies indicate that waiting until early flowering to mow (when 2 percent of flowers show yellow color) results in less regrowth of the starthistle than if it is mowed earlier. However, if soil moisture is still adequate, yellow starthistle will re-grow and should be mowed a second time, about 4 to 6 weeks later. Monitor for any surviving starthistle in another 4 to 6 weeks.

To encourage growth of desirable vegetation, let the desirable vegetation set seed before mowing, but be sure to mow well before starthistle is in full flower. In general, mowing is most effective when soil moisture is low and no irrigation or rainfall follows the mowing.

Grazing is effective in reducing yellow starthistle seed production. Sheep, goats or cattle eat yellow starthistle before spines form on the plant. The plant's crude protein concentration is variable but ranges from 28 percent at the rosette stage down to 11 percent at the bud stage, and should be sufficient to meet the general maintenance requirements for most ruminant animals. Yellow starthistle appears to have the ability to sustain animals several weeks beyond annual grass "dry down" when it is abundant. Intensive grazing, using large numbers of animals for short durations, in late May or June can reduce plant height, canopy size, and seed production. Burning is best performed at the end of the rainy season when flowers first appear. Yellow starthistle may still be green at this time and may require some form of desiccation to burn. Most annual vegetation other than yellow starthistle may have dried by this time and serve as a fuel source to allow a successful burn. Burning for two or more years in a row helps suppress yellow starthistle. Burning may not be as successful where perennial grasses are abundant. Do not burn areas where insects have been released for biological control because fire will kill them.

Re-vegetation

Control practices are capable of reducing yellow starthistle populations but in the absence of competition, it will reestablish. Effective yellow starthistle management requires that desirable plant species be planted and managed to prevent yellow starthistle germination or growth. Species choice for re-vegetation will depend on the intended use of that site. Resident vegetation such as bunch grasses or wildflowers may be desirable along roadsides, abandoned pastures, or natural areas. In these situations, cultural, biological, or chemical methods can be used to reduce yellow starthistle, while encouraging other plant species, if possible, with practices such as fertilization. Recent efforts made to reestablish native perennial bunch grasses have been moderately successful. Perennial grasses are slow to establish and may require selective herbicide treatments to assist yellow starthistle control during establishment, but once well established, alternative controls such as grazing, mowing, or burning can be used effectively.

In pastures, eliminate dense stands of yellow starthistle and re-seed the area with a fast-growing, competitive forage species. Although annual legumes work well for this purpose, the lack of selective herbicides makes follow-up treatments difficult. Therefore, grasses fit best as selective herbicides can then be used to control yellow starthistle not controlled by grass competition. In areas with scattered yellow starthistle infestations, make every effort to eliminate the scattered plants in conjunction with overseeding of desirable species to provide enough competition to prevent yellow starthistle from reestablishing.

In all instances, pick desirable species that are well adapted to the site. Species that grow well are the best competitors. 

Chemical Control   

Related Link: Transline Specialty Herbicide

Both post-emergent and pre-emergent herbicides are available to control starthistle along road sides, right-of-ways, and non-crop areas. Only post-emergent herbicides can be used on rangeland and pastures.

Post-emergent Herbicides. Post emergent herbicide treatments generally work best on seedlings. The long germination period of yellow starthistle makes control with a single application almost impossible. A treatment following the first flush of seedlings opens the site up for later flushes. Waiting until later in the rainy season to apply a post-emergent herbicide allows a greater number of seedlings to be treated, but larger plants will require higher herbicide rates and may not be controlled.

• 2,4-D can provide acceptable control of yellow starthistle if it is applied at the proper rate and time. Treating in mid-February during the rosette growth stage provides better control than later applications Amine forms are as effective as ester forms at the small rosette growth stage, so use amine forms to reduce the chance of off-target movement. 
  
  Application rates of 0.5 to 0.75 Ib ai/a will control small rosettes. Applications made later in the season, after bolting has been initiated, require a higher application rate (1.0 Ib ai/a) to achieve equivalent control. 2,4-D is a broadleaf herbicide and will control other broadleaf plants, but generally will not harm grasses. Drift from 2,4-D applications is common, particularly from the ester formulations. Use caution when applying near sensitive vegetation or during windy or high temperature conditions. 2,4-D is a restricted use pesticide, requiring a permit for use.
  
• Dicamba is very effective at con trolling yellow starthistle at rates as low as 0.25 Ib ai/a. When yellow starthistle rosettes are small, about 1 to 1.5 inches across, the 0.25 Ib ai/a rate works well, but higher rates may be needed if plants are larger. Applications made as late as mid-March (rosette to early bolting stage) have provided excel lent control, although earlier treatments are slightly better. 
  
  Dicamba is a broadleaf herbicide that controls many broad leaf plants, but generally will not harm grasses. Drift from dicamba applications is common; use caution when applying near sen sitive vegetation. Dicamba is a restricted use pesticide, requiring a permit for use.
  
• Triclopyr at the rate of 0.5 Ib ai/a provides complete control of yellow starthistle seedlings. Larger plants require rates up to 1.5 Ib ai/a. Triclopyr is a foliar absorbed, broadleaf herbicide that has little or no residual activity. Triclopyr generally will not harm grasses. Triclopyr may also drift to sensitive vegetation, thus caution should be ob served when using this material.

• Cyphosate controls yellow starthistle moderately well at 0.5 Ib ai/a rates. Good coverage, clean water, and actively growing yellow starthistle plants are all essential for adequate con trol. Glyphosate is non-selective and controls most plants. A one percent solution of glyphosate also provides effective control and is used at this concentration for spot treatment of small patches.
  

Pre-emergent Herbicides. Pre-emergence herbicides must be ap plied before seeds germinate to be effective. The long germination period of yellow starthistle requires that a pre-emergent material have a lengthy residual activity. Make applications before a rainfall, which will move the material into the soil. Because these materials adhere to soil particles, offsite movement and possible injury of susceptible plants could occur if the soil is dry and wind occurs before rain. When yellow starthistle plants have already emerged, combine a post-emergent herbicide (to control emerged plants) with a pre-emergent herbicide (to provide residual control of any subsequent germination) for an effective control strategy.

The following pre-emergent herbicides are not labeled for use in pasture or rangeland, but can be useful for yellow starthistle control along rights-of-way, and non-crop areas.

• Atrazine can control yellow starthistle at rates of 1 to 1.5 Ib ai/a. Since atrazine is primarily a root-absorbed chemical, make applications before seedlings emerge. Atrazine does provide some post-emergent activity on many weeds when an oil-based adjuvant is used, but this has not been evaluated for yellow starthistle control. Because of ground and surface water con cerns, this product is a restricted-use herbicide and requires a permit from the county agricultural commissioner for its purchase or use.
• Simazine is effective against yellow starthistle when used at rates of 1.5 Ib ai/a or higher. This material is adsorbed tightly to soil so the chances of leach ing are less than with atrazine. Simazine is root absorbed and, like atrazine, works by blocking photosynthesis.
• Sulfometuron is registered for roadside use and is very effective at controlling yellow starthistle when applied at 1 oz ai/a. Some post-emergence activity occurs with this herbicide, but the best control is achieved when applications are made before weeds emerge. For more information, contact the University of California Cooperative Extension office in your county. See your phone book for addresses and phone numbers. 

Warning on the Use of Chemicals

Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock.

Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens containing fruits and/or vegetables ready to be picked.

Dispose of empty containers carefully. Follow label instructions for disposal. Never reuse the containers. Make sure empty containers are not accessible to children or animals. Never dispose of containers where they may contaminate water supplies or natural waterways. Do not pour down sink or toilet. Consult your county agricultural commissioner for correct ways of disposing of excess pesticides. Never burn pesticide containers.

Written by W. Thomas Lanini, Craig D. Thomsen, Timothy S. Prather, Charles E. Turner, Joseph M. DiTomaso, Michael J. Smith, Clyde L. Elmore, Marc P. Vayssieres, and William A. Williams

Editor: B. Ohlendorf
Technical Editor: M. L. Flint
Production: M. Trulson

To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products that are not mentioned.

Produced by:
IPM Education and Publications
UC Statewide IPM Project
University of California
Davis, CA 95616-8620