Fact Sheet No. 1:
IntroductionYellow 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 close to 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 allelochemicals that prevent 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.
Yellow starthistle is an annual, generally germinating from fall through spring, which corresponds to the normal rainy season in California. The plant develops a deep taproot allowing it to proliferate on dry sites or in dry years. The deep taproot extends below the zone of root competition of associated annual species and allows growth and flowering to occur well into the summer, long after other annual species have died and dried up. Yellow starthistle 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 seeds being viable. Most seeds germinate the following year, but some seeds can last 10 years or more in soil.
Control of yellow starthistle cannot 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.
PreventionYellow starthistle proliferates along roadways. the invasion by this weed may be increased by disturbance created by road building and maintenance. Seed 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 buildup. Yellow starthistle also can be spread as a contaminate in grass seed, so use certified seed for range or pasture seeding. Seed may also come as a contaminate 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 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 most effective at this time.
Four natural enemies of yellow starthistle have been imported from Greece and are established as of 1994 in California as biological control agents:
- Three weevils
- Bangasternus orientalis,
- Eustenopus villosus and
- Larinus curtus;
- One gall fly
- Urophora sirunaseva.
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 number and spreading on there 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 biological control. the insects become more numerous and thus more available with each succeeding year.
Currently, B. orientalis 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. Because these control agents for starthistle are seed feeders, they only slow the spread of yellow starthistle by reducing seed production; they do not control established populations. To control this weed, use biological control agents in combination with other control measures.
Cultivation effectively controls seedlings of yellow starthistle. Because yellow starthistle begins emergence with fall rains, this is the best time to begin cultivation. Repeated cultivations are generally 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 regrow 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 regrow 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, but before seeds form. 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 be dried down by this time and serve as a fuel source to allow a successful burn. Do not burn areas where insects have been released for biological control because fire will kill them.
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 regrowth. Species choice for revegetation 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 practice 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, cultural controls such as mowing or burning can be used effectively.
In pastures, eliminate dense stands of yellow starthistle and reseed 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 infestations, make an 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.
Both postemergent and preemergent herbicides are available to control starthistle along roadsides, right-of-ways and noncrop areas. Only postemergent herbicides can be used on rangelands and pastures.
- Postemergent Herbicides
- Preemergent Herbicides
Postemergent herbicide treatments generally work best on seedlings. the long term 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 postemergent 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-Feberary during the rosette growth stage provides better control than later application. 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 lb ai/a will control small rosettes. Applications made later in the season, after bolting has been initiated, require a higher application rate (1.0 lb 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 to use.
Dicamba is vary effective at controlling yellow starthistle at rates as low as 0.25 lb ai/a. When yellow starthistle rosettes are small, about 1 to 1.5 inches across, the 0.25 lb 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 excellent control, although earlier treatments are slightly better. Dicambas is a broadleaf herbicide that controls many broadleaf plants, but generally will not harm grasses. Drift from dicamba applications is common; use caution when applying near sensitive vegetation. Dicamba is a restricted use pesticide, requiring a permit for use.
Triclopyr at the rate of 0.5 lb ai/a provides complete control of yellow starthistle seedlings. Larger plants require rates up to 1.5 lb ai/a. Tricloyr is a foliar-absorbed, broadleaf herbicide that has little or no residual activity. Tricloyr generally will not harm grasses. Triclopyr may also drift to sensitive vegetation, thus caution should be observed when using this material.
Glyphosate controls yellow starthistle moderately well at 0.5 lb ai/a rates. Good coverage, clean water and actively growing yellow starthistle plants are all essential for adequate control. 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.
Preemergence herbicides must be applied before seeds germinate to be effective. The long germination period of yellow starthistle requires that a preemergent 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, off site movement and possible injury of the susceptible plants can occur if the soil is dry and wind occurs before rain.
When yellow starthistle plants have already emerged, combine a postemergent herbicide (to control emerged plants) with a preemergent herbicide (to provide residual control of any subsequent germination) for an effective control strategy. The following preemergent 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 lb 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 concerns, this product is a restrictive-use herbicides and requires a permit from the county agriculture commissioner for its purchase or use.
Simazine is effective against yellow starthistle when used at rates of 1.5 lb ai/a or higher. This material is absorbed tightly to soil so the chances of leaching 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 postemergence activity occurs with this herbicide, but the best control is achieved when applications are made before weeds emerge.