Noxious Weed IVM Guide Contents, IVM for Noxious Weeds
Broom, Gorse, Knapweeds, Leafy Spurge, Purple Loosestrife,
Purple Starthistle, Smooth Cordgrass, Tansy Ragwort
IPM Access Key Documents, Home Page
IVM Technical Bulletin
Yellow Starthistle
Gather Background Information


The first step in an IVM program is to gather information on the life cycle and habits of the noxious weed.

Yellow starthistle, Centaurea solstitialis, is a pubescent winter annual, germinating in the fall and overwintering as a rosette. It has a long taproot and stiff, upright stems that branch from the base. Lower leaves are 2 to 3 inches long and deeply lobed. Older leaves are short, 0.5 to 1 inch, and narrow. In the spring, seven or eight lobed leaves emerge to form a basal rosette, which later can have up to 26 leaves. The rosettes tend to grow close to the ground in open places, but grow more upright at high densities (Thomsen et al. 1994).

The small tubular florets produce two types of seed: plumed, light-colored seeds, and plumeless, darker colored seeds. Florets in the center of the head produce seeds with a ring of fine, white, thin bristles (plume). The outer circle of florets produce plumeless seeds. In general, the plants mature by late summer, and by September and October, the plants dry out, lose leaves, and turn to silvery-grey skeletons with white cottony terminal heads. In some places and under certain conditions, yellow starthistle survives over the winter, regrows in the spring, and dries out by early summer (June).

Yellow starthistle poses a serious potential threat to nearly all semi-arid rangeland in the western U.S. due to its ability to colonize and spread rapidly on disturbed soils. It is palatable to livestock and is an important forage plant until it produces sharp spines on the flower head that deter grazing animals, causing poor pasture utilization. It also forms smothering infestations and reduces the pasture production of other forage species through competition. Extensive roots grow much faster and deeper than annual grasses, forming dense monotypic infestations of yellow starthistle.

This plant is toxic to horses in large amounts, causing equine nigropallidal encephalomacia ("chewing disease"). The first signs of poisoning are an inability to eat or drink as the muscles of the lips, face, and tongue become stiff and swollen, giving the horse a fixed expression. Poisoning eventually results in permanent brain damage, and severely affected animals eventually die of thirst and starvation. Sheep, cows and other livestock are not affected (Kingsbury 1964; Cordy 1978).

Yellow starthistle thrives in areas with hot, dry summers and well-drained soils, especially where fire, over-grazing, road construction, or other causes have seriously disturbed the vegetation. It is believed to have originated along the Mediterranean region. It has spread throughout Europe as far as the Asian steppes, but doesn’t persist in cold northerly areas, as a lack of heat appears to be a limiting factor (Thomsen 1996; Prodan 1939; Roché et al. 1986).

Although presumably native to Europe, yellow starthistle was introduced into western U.S. ports as a seed contaminant in imported alfalfa seed from Chile (Gerlach 1998). The earliest specimens collected here were at Oakland, CA in 1869, Vacaville, CA in 1887, and Seattle, WA, in 1898. It is now a widely distributed weed in the western United States, primarily of rangelands but also of alfalfa and cereal grains, orchards, vineyards, roadsides, and recreational lands. It is often spread as a contaminant in crop seed (usually alfalfa), but is also carried in hay or straw or by vehicles, including construction or maintenance equipment, and by birds (Maddox 1981; Roché et al. 1986; Roché and Talbott 1986).

Yellow starthistle is established in 23 of the 48 contiguous states. It is a particular problem in California, where over 10 million acres are infested, and is increasing in Idaho, Oregon, and Washington (Maddox, Mayfield and Poritz 1985; Hastings and DiTomaso 1996; Callihan et al. 1989).

Life Cycle
Yellow starthistle has a very long life cycle for an annual plant, and it sometimes behaves as a biennial. It generally germinates from fall through spring, but may not complete the life cycle until the following fall or winter. 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 (Prather 1995).

Yellow starthistle bolts from May to June, sending up elongated stalks that will produce spiny flower heads. In June and July the flower buds form, and spines appear at the tips of the bracts around the flower bud. Flowering takes place June through August when the bright yellow flowers open. In August, seeds are formed when the leaves and stems begin to turn straw-colored. Seeds mature after the flowerhead fades from a bright yellow to a dull straw color (Thomsen et al. 1994). Flowering and seed production times will vary according to climate.

The plumed, lighter colored seeds disperse quickly after maturity, and the darker, plumeless seeds persist in the flowerhead until windy weather or other disturbances break them up. By December the bracts are usually lost from the flower, and the white cottony base of the head is revealed. Although most yellow starthistle seeds fall within 2 feet of the parent plant (Roché 1992), the plumed seeds can disperse 5 feet by wind, and many seeds disperse over longer distances in fur, feathers, or human activity. In California, human activity probably accounts for most of the seed dispersal. Using contaminated hay or seed, four-wheel-drive vehicles and all terrain vehicles in infested areas all help to establish new populations far from the parent plants. Other contributors include site disturbance by road construction and transport of contaminated soil and gravel (Roché 1991; Callihan et al. 1989; Thomsen et al. 1996).

In a study of starthistle populations in three locations in California, Maddox (1981) reported a range of 700 to 10,000 seeds per plant. Seed output can be as high as 2,700 seeds per square foot, with about 95 percent of the seeds being viable, or as low as seven seeds per flower head under adverse environmental conditions such as drought. Most seeds germinate the following year, but some seeds can last 10 years or more in the soil (Thomsen et al. 1996; Lanini et al. 1995).

Special Challenges to Management
Once it has colonized an area, yellow starthistle can quickly spread and displace other vegetation. Even after control measures have been implemented, yellow star-thistle can quickly re-establish if competitive replacement species are not established. Conventional methods (usual-ly chemical) for yellow starthistle control have often failed on because of lack of competitive vegetation, developed seed banks, high cost of control, size of infestations, and lack of long-term commitment (Coombs et al. 1996).

Site-Specific Questions
Some questions, such as those below, can only be answered on site.

Set Management Objectives

Set Realistic Goals for Your IVM Program
The answers to the following questions can help you set realistic objectives and goals.

Levels of Control
Containment - keeping an established population of the weed from spreading to non-infested areas. This strategy is especially useful when time and money are in short supply or when the infestation is very large. For example, a barrier strip between infested and non-infested areas can be maintained and monitored so that adjacent lands remain weed free. In addition, measures that stop seed production can help prevent further spread of the weed.

Reduction - reducing the area covered by yellow starthistle or reducing its dominance. This strategy can also be used against new or established weeds, but it requires more resources and more time than containment.

The "Bradley Method" - (see Appendix 2), developed in Australia, is a simple yet innovative strategy for natural areas that combines containment and reduction.

Eradication - completely eliminating the weed from the management area.  This strategy usually consumes the greatest amount of time and resources and is applicable mainly to newly-invading weeds that are confined to a limited number of small areas.

Establish Monitoring Programs

When planning a monitoring program, keep in mind the context of your target weed: is it invading or has it already invaded?

Locate and record yellow starthistle infestations on a map. (Chapter 2 of the University of Northern Iowa IVRM Technical Manual contains a detailed discussion on how to map and inventory vegetation - see Bibliography). Note particularly sensitive areas on the map, such as critical habitat for threatened or endangered species, agricultural production areas, or areas subject to frequent disturbance and thus prone to invasion. Update maps at regular intervals.

Focus monitoring efforts on sites where yellow starthistle problems are most likely to occur (see Distribution). Encourage public sighting and reporting through an education or incentive program (see Educate Vegetation Management Personnel and the Public).

Prioritize the sites you will work on. Make a realistic assessment of your weed management resources, keeping in mind the goals of your project and the cost of a follow-up program after any treatments. Without follow-up, your control efforts will be wasted. It is better to thoroughly control a weed at one or two sites than to use up resources to incompletely control the weed at many sites. If the weed is very widespread, try to determine where it poses the most serious economic, social, or environmental problem and concentrate on those areas.

Plan monitoring and treatment efforts to coincide with critical life stages of the weed. To use your resources efficiently, try to include monitoring with other planned activities in the area.

Maintain records of your monitoring activities. Creating standardized forms will make data collection easier and help remind you to gather all the information you need. Forms work best if they include labeled blanks for all pertinent information and allow the user to check or circle rather than having to write words or numbers (See Appendix 3 for some examples of forms).

Include information such as the name(s) of the person(s) collecting the data, the location, and date of monitoring; a qualitative description of the vegetation, such as the names of the plants or types of plants (native vegetation, annual/perennial weeds, trees, etc.) and stage of growth (germinating, flowering, setting seed, etc.); a quantitative description, such as percent cover, plant density, size of the patch, or if possible, the number of plants.

Note special conditions such as unusual weather events and record treatment history, including information on treatment applications (who, when, where, how, cost, difficulties, and successes). This will allow you to evaluate and fine-tune treatments.

Set Treatment Thresholds

Setting treatment thresholds includes prioritizing and balancing treatments with resources. Weeds will be treated when populations increase beyond a predetermined level. This level will largely depend on the characteristics of the site and weed. In some cases the level may be no weeds at all, and in other cases the number of weeds you can tolerate may be much greater.

Considerations for Setting Priorities
What is the size of the weed population? The opportunity for control is related to the infested area. Small patches can be more easily controlled than large infestations.

What is the level of the threat? Is the yellow starthistle population changing? Is it in an area where soils are frequently disturbed? Does it threaten agriculture, pastures, or rangeland? Is it encroaching on critical habitat for a rare, threatened, or endangered species? Is it displacing the best examples of native communities?

What resources are available? Do you have the resources required for carrying out your goal?


With the advent of herbicides, prevention as a weed management technique has often been neglected; however, it is a practical, cost-effective, and extremely important part of noxious weed control.

General Weed Prevention Measures

Revegetation - Follow-up Weed Prevention
Establishing dense, competitive vegetation can help permanently replace weeds. Revegetation is critical in preventing weed infestations in areas where the soil has been disturbed or the vegetation removed. In some situations you may be able to encourage desirable vegetation that is already in place; but because of the aggressive nature of yellow starthistle, it is more likely that you will need to thickly sow seeds of desirable, competitive plants.

Desirable plant species should be chosen depending on the intended use of that site. Before investing in any large-scale revegetation project, preliminary small-scale planting should be tested for their ability to outcompete yellow starthistle at that site. According to Thomsen et al. (1996), usually more than a year will be required to find the species or mix of species most effective, since species that are excellent the first year may be disappointing in subsequent years.

Perennial grasses are slow to establish (most require at least two years to be competitive) and require selective herbicide treatments to prevent yellow starthistle and other weedy vegetation that competes against seedlings. Lanini et al. (1995) recommended using a no-till drill for reseeding, as this allows seeding without turning the soil, and helps to keep deeply buried starthistle seeds from germinating.

By depleting soil moisture and provide additional shade, early-growing perennial grasses can help suppress yellow starthistle. Borman et al. (1991) in southwest Oregon rangeland found that perennial grasses such as Berber orchardgrass (Dactylis glomerata) and Idaho fescue (Festuca idahoensis), which begin growth early, suppressed thistle and other annual weeds more effectively than later growing perennial grasses such as intermediate (Apropyron intermedium) and tall (A. elongatum) wheatgrasses.

In pastures, dense stands of yellow starthistle should be removed and the area reseeded with a fast-growing, competitive forage plants. Dense sowing of tall grass species such as oats or wheat in autumn, after thoroughly tilling and mulching with adequate amounts of proper grass fertilizer (particularly phosphorus) may completely eliminate the weed. Reseeding combined with mulching with wheatstraw produced the best results in one experiment. Dremann recommends covering with wheatstraw at least 1/2 inch deep, into which 50 pounds/acre of oats (Avena sativa) have been mixed (Dremann 1992; Lanini et al. 1995; Dremann 1996).

Thomsen et al. (1996) experimented with reseeding with dry land legumes on pastures, vineyards, and University of California field stations. Subterranean (sub) clovers were chosen as pasture plants because they are palatable, self-seeding, and produce flowers and seeds below the bite of grazing animals. In addition, strong winter and spring growth make them useful as cover crops in vineyards and orchards.

Subclover beds were established by discing a seedbed, applying 100 lb/acre each of phosphorus and sulfur, and sowing 40/lb/acre of a 1:1 mixture of ‘Koala’ and ‘Karridale’ seed. However, both the Koala and Karridale cultivars declined substantially in two years, and the researcher noted that their future choice of subclover cultivars will be different (Thomsen et al. 1997).

From these experiments, Thomsen et al. (1997) found that there were weed control benefits from the subclover plantings and fertilization when used with mowing, and yellow starthistle seed production was reduced to zero in the third year. Subclover performance is enhanced by early mowing because the shading from tall annual grasses is reduced. Some varieties can form a dense network of interwoven stems and leafy canopies that reduce sunlight to yellow starthistle rosettes. However, competition from subclover is insufficient to control yellow starthistle, and late spring or early summer mowings are required.

Thomas (1995) recommends the following treatment for large flats of rangeland in California: disc starthistles in late April (before flowering), finish working the seedbed in the autumn and plant crimson clover at 25 lb/acre, fertilize with phosphorous and sulfur, lightly graze in February, and graze again in the summer. He recommends the following treatment for large flats and rolling hills that can be mowed, disced, or sprayed: disc or mow in late April, mow again in summer, plant a mixture of subterranean and rose clover at 25 lb/acre, fertilize, spot-treat with herbicides, return to normal grazing after the approved waiting period.

Apply Management Methods

No individual method will control yellow starthistle in a single treatment; diligence and persistence will be required over a number of years to subdue this weed. The treatment methods described in this section will help you to design an integrated program that will suit the circumstances of your particular situation.

Biological Controls
Biological control does not aim to eradicate weeds, but to keep them at low, manageable levels. After their introduction, biocontrol agents can take 5 to 10 years to become established and increase to numbers large enough to reduce the density of the target weed. Once established, effective biological controls provide an inexpensive, long-term, and non-toxic means to control weed populations.

Since insects have specific requirements for growing and thriving, it is important to match the insect to the weed management site. Understanding these requirements will help you integrate the insects into other weed control efforts. When you release biocontrols, continue using other control methods on the perimeter of the release site, but avoid using them where they might adversely impact the insect population.

The information provided below is only a summary. For more information consult Biological Control of Weeds in the West (see Bibliography) or contact commercial weed biocontrol insectaries (see Insectaries).

Five natural enemies of yellow starthistle have been imported from Greece and are established as biological control agents: three weevils, Bangasternus orientalis, (brought into the U.S. 1985), Eustenopus villosus (1990), and Larinus curtus (1992); one gall fly, Urophora sirunaeseva (1985); and the peacock fly Chaetorellia australis (1989) (Coombs et al. 1996; Turner et al. 1994a,b; Woods and Popescu 1997).

All five insects attack the flower/seed head, and directly or indirectly reduce seed production, the only means of reproduction. They lay their eggs in or near the flower/seed heads and complete their development within them. They are all highly specific to yellow starthistle and do not attack commercially valuable or native plants. It is still too early to know their efficacy as control agents, but adult feeding damage by the hairy weevil, E. villosus, can be very extensive at release sites three years or older (Villegas 1997).

These beneficial insects do best in areas with warm, dry, summer climates. Following their release, they should be protected 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 can build up to high numbers and spread on their own (Lanini et al. 1995). The peacock fly, C. australis, appears to need close proximity to cornflower (bachelor’s button, Centaurea cyanus), to establish (Villegas et al. 1997).

Bangasternus orientalis and Urophora sirunaseva are available from commercial producers (see Insectaries), or you can contact extension agents or state noxious weed control officers to obtain these and the others. One or more of these organisms may already be established in your area, and simply need encouragement to help provide control. Contact the USDA/ARS Biological Control of Weeds Laboratory for information on Larinus curtus and Chaetorellia australis.

Grazing is successful alone, or in conjunction with other control methods. For it to be effective, the weed manager must move the stock at the appropriate times in order to adjust grazing pressure and be able to confine stock to specific areas. Animals can be confined by temporary electric fencing and/or by an experienced stock manager with well-trained cattle-herding dogs. Because it takes skill and experience to determine stocking rates and to care for the animals properly, it is best to hire a handler with vegetation management experience. Note that since goats will eat almost any plants within a fenced area, exclosures may be needed to protect patches of native or desirable vegetation within the grazed area.

Numerous studies indicate that well-timed, heavy grazing by goats, sheep, and cattle can reduce yellow starthistle seed production as well as biomass. The grazing period should be timed to the bolting stage in late May or June, before spines are on the plant. Grazing earlier, at the rosette stage, favors yellow starthistle development by elimination of competitive plants which do not regrow as quickly. Since most defoliated yellow starthistle will recover from one grazing, it is necessary to bring the animals back one to four times at about two week intervals under rotational grazing. Alternately, grazers can be left on site for two to three months under a continuous grazing regimen (Thomsen et al. 1993).

The plant's crude protein concentration ranges from about 10 percent at the rosette stage to about 12 per- cent from the rosette to bolting stage, and should be sufficient to meet the general maintenance require- ments for most ruminant animals. In addition, because it continues to grow after most other range plants, it provides green forage when other plants are unavailable (Thomsen et al. 1989).

Physical Controls

Manual Removal
Where yellow starthistle infestations are small, manual removal can be an effective way to control this pest. Manual removal can be applied selectively to individual yellow starthistle plants without disturbing other vegetation and can be used on steep or uneven terrain. When beginning a hand removal project, flag the treated areas so they can be identified for follow-up treatment. It is easiest to work in relatively small areas of infestation. When faced with dense and/or extensive stands of yellow starthistle, it is best to divide them into grids (with flags, stakes, etc.) so that workers can thoroughly weed smaller areas before moving onto the next grid. The grid system also facilitates dividing work activities between those pulling and those removing the debris.

Mechanical Removal
Mechanical control involves the use of machinery to remove yellow starthistle from a large area. This method is not selective and can damage desirable vegetation and disturb soil. It is best to use this method only on even terrain with few obstacles. As with manual removal, control efforts should be timed to pre-empt seed production. If plants are removed late in the season (after pollination), contain them in plastic bags to prevent seed dispersal. Mechanical control efforts must always be followed by measures (manual or mechanical control, grazing, or herbicide) to kill shoots from resprouting plants and seedlings sprouting from the seedbank.

According to Biologist Cathy Darling of the San Luis Obisbo County Agriculture Department, mulching with pine chips produced from whole pine trees gave effective suppression of yellow starthistle. The six-inch treatments gave very good overall suppression, and eight inches of mulch was even better (Darling 1996).

Controlled Burns
Although burning is a potentially dangerous and environmentally harmful control practice, it can also be an effective yellow starthistle management tool in some situations. Repeated burns not only kills yellow starthistle, it also significantly reduces the seed bank. The National Park Service has an extensive controlled burn program for various weeds. Consult with the local fire department officials for permit regulations and to plan the logistical details of the burn, including appropriate weather conditions and safety precautions. Do not burn where biological control insects have been released.

Burning should be done at the end of the rainy season, but prior to the formation of viable seeds. According to Thomsen et al. (1996), the best time to burn is probably when plants are in the early flowering stage, prior to seed formation. Yellow starthistle may still be green at this time, so there must be enough dry biomass from other annual plants to carry the fire. If there is insufficient grass to carry the fire, fuel for the burns can be provided by sowing grass seed the winter before a planned burn. Full consumption of the plants by fire is not crucial, as only enough heat to produce foliar scorch and stem girdle is necessary. A flame thrower can be used to scorch patches of accessible plants that do not burn (Hastings and DiTomaso 1996).

Burning is the preferred management technique for yellow starthistle control at Sugarloaf Ridge Park near Santa Rosa, California. A successful burning program for yellow starthistle was conducted for three consecutive summers, producing a 90 percent reduction in relative starthistle cover, increasing perennial grass cover by 300 percent, and reducing the soil seed bank over 99 percent. In addition, the vegetative cover of native species increased two-fold (Hastings and DiTomaso 1996).

Chemical Controls
In IVM programs, herbicides are considered transition tools that enable the manager to suppress weeds and replace them with desirable, competitive vegetation. Thus, it is important to select the least-toxic, low-residual herbicide that is effective against the target weed, and to apply them in a judicious manner.

The following links discuss the primary considerations when using herbicides:  Use Herbicides Properly, Herbicide Information Resources, Criteria for Selecting an Herbicide, and Application Methods.

Proper Timing
Combining a preemergence and postemergence herbicide in the spring can help control yellow starthistle, and also provide some residual control for any new flush of seedlings. Most preemergent herbicides are best applied to the soil before weed emergence. Rainfall or irrigation is required to leach the chemicals to the root zone. The herbicide, along with water, is then translocated to shoots. As with herbicides that are applied to the soil, there is a risk of groundwater contamination or surface runoff. Check the herbicide label since many preemergent herbicides are registered for roadside use, not in pastures, rangelands, and many croplands (DiTomaso 1997).

Post-emergent herbicides are applied directly onto plant leaves and are usually most effective in the seedling and rosette stages prior to bolting. The herbicide is then translocated with nutrients to storage structures such as roots. Many post-emergent herbicides are growth regulators that are effective at the seedling stage but offer less control after bolting (DiTomaso 1997).

Applying herbicide to plants when yellow starthistle is most susceptible (preferably before seeds are produced) is crucial to the effectiveness of the treatment.

Educate Vegetation Management Personnel
and the Public

Evaluate the Vegetation Management Program


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