Yellow Starthistle

Pest Notes: Introduction

Yellow starthistle, Centaurea solstitialis, is an invasive plant in the sunflower family, Asteraceae. It is native to Eurasia and was introduced to California around 1850 by way of South America. Through the early 1900’s, yellow starthistle became established along river levees, alfalfa fields, and neighboring roadsides. It is now common in many areas including roadsides, urban and rural lots, parks, recreational areas, home landscapes, rangelands, natural areas, and grasslands.

Yellow starthistle is considered one of the worst weeds in the western United States and is estimated to infest over 14 million acres in California. It is highly competitive and often develops impenetrable stands, displaces desirable vegetation, uses deep soil moisture, outcompetes desirable forages, eliminates access to recreational areas, and causes a fatal disease in horses.

Identification and Biology

Yellow starthistle is a gray-green to blue-green plant with a deep, vigorous taproot. It produces bright, thistlelike yellow flowers with sharp spines surrounding the base. Yellow starthistle grows to heights varying from 6 inches to 6.5 feet tall. The stems of mature plants are rigid, spreading, and typically branching from the base in open areas. Stems and leaves are covered with loose, cottony wool, giving them a whitish appearance. Stems appear winged due to leaf bases that extend beyond the nodes. Leaves at the base of the plant are 2 to 3 inches long and deeply lobed. Upper leaves are short (0.5 to 1 inch long) and narrow with few lobes. Other closely related non-native species that look similar include Malta starthistle or tocalote (Centaurea melitensis) and Sicilian starthistle (Centaurea sulphurea).

Yellow starthistle is a long-lived winter annual, occasionally biennial, that is usually found below 7,000 feet elevation in dry, sunny areas where average annual rainfall is between 10 and 60 inches. Large numbers of seedlings germinate after the first significant autumn rain, and smaller flushes can occur through winter and spring with adequate irrigation, precipitation, or soil moisture. After germinating, the plant puts most of its resources into root growth. By late spring, roots can extend over three feet into the soil, although the portion above ground is a relatively small basal rosette (circular arrangement of leaves), measuring 3 to 8 inches in diameter. This allows yellow starthistle to out-compete 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, and why it can regrow after top removal from mowing or grazing.

The competitive ability of yellow starthistle also depends on sunlight at the soil surface during the seedling and rosette stages of development. Yellow starthistle thrives at high light intensity and does poorly in shady conditions. Sunny conditions often occur in home landscapes, parks, outdoor recreational areas, roadsides, disturbed sites, grasslands, and on south-facing slopes at higher elevations.

Yellow starthistle is dependent on seed production for population growth and spread. Flowers develop two seed types. The outer portion of the flower produces a dull, dark brown seed that is often speckled with tan and lacks any pappus (modified calyx made up of a tuft of hairs). The inner flower produces seeds that are glossy gray or tan to mottled cream-colored and tan, with slender white pappus bristles. Seeds are quite heavy and unlike other plants in the sunflower family that can have windblown seeds, most seeds fall near the parent plant. Seed output can be as high at 300,000 seeds per square foot, with about 95% of the seed being viable soon after dispersal. While seeds can survive for up to 10 years in the soil under ideal conditions, few seeds survive beyond 4 years, and most seeds germinate within 1 year of dispersal.

Impact

Yellow starthistle is a highly invasive plant that can have significant impacts on ecosystems, agriculture, recreation, and local economics. Yellow starthistle outcompetes desirable plants for resources, leading to reduced plant diversity. This can disrupt local ecosystems and reduce habitat availability for native wildlife. Yellow starthistle can create a high risk of wildfires. Its dry, fibrous stems and leaves are highly flammable, and its dense growth can facilitate the spread of fires. Its deep taproot allows it to access groundwater, which can reduce water availability for other plants and potentially impact local water tables, especially in arid and semi-arid regions. By forming dense stands, yellow starthistle can create a monotypic stand that offers little value to many wildlife species. This loss of habitat complexity can lead to decreased populations of insects, birds, and other animals that rely on diverse plant communities.

During flowering, the spines can make areas difficult to access and injure the eyes, nose, and mouths of animals. Additionally, it reduces forage availability and contaminates hay and grain crops. Horses should never be allowed to eat yellow starthistle. The plant is toxic, causing a neurological disease called nigropallidal encephalomalacia or “chewing disease.” Continued feeding results in brain lesions and ulcers in the mouth. There is no known treatment for horses that have been poisoned. In most cases, the animals die from starvation or dehydration.

Yellow starthistle can reduce the visual appeal of natural landscapes, recreational areas and home landscapes. Its dominance in areas such as parks, trails, and scenic vistas can diminish the enjoyment and attractiveness of these spaces for visitors. Invasive species like yellow starthistle can also affect outdoor recreational activities by creating impenetrable or unusable terrain, particularly in areas meant for hiking, camping, or horseback riding.

While yellow starthistle can negatively affect some wildlife, it does produce nectar that can be attractive to certain pollinators. The plant is considered an important late season food source for honey bees in California and is an important source of pollen and nectar for honey production. Honey bees play an important role in the pollination of starthistle and have been reported to account for 50% of seed set. When honey bees are excluded from visiting starthistle, the average seedhead weight significantly declines. Like yellow starthistle, honey bees are also not native to North America and were originally imported from Europe in the 17th century. This may imply that honey bees and starthistle may act as invasive mutualists, increasing the survivorship of each other.

Yellow starthistle’s ability to thrive in a range of environmental conditions might make it particularly resilient to climate change. Changes in temperature and precipitation patterns could potentially expand its range and exacerbate its impacts.

Management

Control of yellow starthistle cannot be accomplished with a single treatment or in a single year. Effective management requires control of the current population, suppression of seed production, and monitoring the area to control any new plants from becoming established. Establishing competitive, desirable vegetation will help to suppress new outbreaks.

Identify isolated or incipient populations and treat those first. For larger populations, start on the perimeter and work inwards. Neighbors can work together by controlling populations along fencelines, roadsides, disturbed sites and other areas prone to establishment. Reduce human, livestock, pet, vehicle and equipment movement through infested areas during flowering to prevent further seed dispersal. Some seeds can be retained in the dry flower head at the end of the season, so care must be taken to not spread flower heads to uninfected areas.

Prevention

Yellow starthistle proliferates along roadsides and disturbed sites. Invasion by this weed may be increased with disturbances created by landscaping with contaminated equipment, road building and maintenance, grading or other types of soil movement or disturbance. Seeds are often spread by vehicles, ATV’s, bicycles, transportation of livestock, contaminated soil, birds, infested hay, and by hitchhiking on clothing or hiking boots.

Be sure to inspect vehicles and motorcycles and clean any seeds that may be lodged in the undercarriage. Clean all equipment after use in an infested area. Check shoes and clothing for seeds after walking through infested areas. Survey roadsides, construction sites, home landscapes, and disturbed areas for the presence of this weed and immediately control new infestations to prevent seed production and subsequent spread of yellow starthistle. Monitor areas where birds might nest or perch on a fence line or wire. If mechanical tools such as string trimmers or mowers are used to control yellow starthistle, be sure to clean the equipment after use.

In some areas, addressing the impacts of yellow starthistle can require a coordinated effort from land managers, residents, researchers, and the community to mitigate its effects and prevent further spread.

Education about yellow starthistle and its impacts is crucial for prevention and management. Raising awareness about how to identify and report the plant can help in early detection and control efforts. Community involvement in monitoring and managing invasive species can enhance control efforts. Volunteer programs and crowd-sourced science projects can aid in tracking the spread and effectiveness of management strategies.

Biological Control

The United States Department of Agriculture (USDA) and the California Department of Food and Agriculture (CDFA) are working to control yellow starthistle using biological agents. Six insects and one rust fungus have established in California, with the the yellow starthistle bud weevil (Bangasternus orientalis), the yellow starthistle hairy weevil (Eustenopus villosus), and the yellow starthistle gall fly (Urophora sirunaseva) being widespread. The false peacock fly (Chaetorellia succinea), though not officially released, is widespread and specifically targets yellow starthistle. In 2019, the rosette weevil (Ceratapion basicorne) was approved for release, feeding on the plant's rosettes. The yellow starthistle rust (Puccinea jaceae var. solstitialis) has been released, but establishment rates have been low.

All these agents are host-specific and do not harm crops or native plants. Currently, the hairy weevil and false peacock fly are the most effective at reducing seed production by about 50%. However, even at 50% reduction, total seed counts per plant remain high. A combination of control methods is recommended for better results. For more extensive information about biological control agents for yellow starthistle, see the WeedCUT page on biological control of yellow starthistle.

Mechanical Control

Hand pulling can be an extremely effective technique for controlling yellow starthistle. Although labor intensive, the cost can be minimal if using your own labor or with volunteers. Manual removal is most effective for controlling small patches. It can also be an important strategy in tough-to-access locations or rocky areas where mowers or string trimmers can’t access. Hand pulling can be done any time prior to seed set. Pulling early in the season allows more desirable vegetation to  establish; however proper identification is more difficult. Pulling during the bolting stage is effective as plants are easy to identify, the stems give something to grab onto, and the soil is still moist.

Cultivation can be an effective treatment for managing starthistle. Plants begin emerging with fall rains and continue throughout the rainy season. A single cultivation after the rainy season when soils are starting to dry effectively controls yellow starthistle. This treatment must be made after the last rains but before seeds are produced. If cultivation is carried out too early (e.g., before the last rains) seeds will continue to germinate and another cultivation will be needed to control each new flush of seedlings. For small infestations, a scuffle or stirrup hoe can be highly effective on small seedlings. Frequent hoeing throughout the germination period when plants are small is effective. These tools can be used with minimal soil disturbance and be selective depending on the scale and density of infestation. For late season cultivation in larger areas, equipment such as a rotary tiller, disc, or plough may be needed. These cause more soil disturbance and are non-selective. Cultivation leaves bare ground which is highly susceptible to reinvasion. Although highly effective at killing starthistle, cultivation should be integrated with other control strategies including competitive planting, mulching, herbicides, or other land uses that are not suitable for starthistle establishment.

Mowing or String Trimming can be highly effective at managing yellow starthistle. If using a standard rotary mower, it is important to understand the growth pattern of yellow starthistle. When starthistle is growing amongst grass or other vegetation, the competition for light often results in the plants having a higher branching pattern, meaning there are few leaves at the base of the plant. In open areas without competition, yellow starthistle will look bushier and have a lower branching pattern, with more leaves occurring near the base. Mowing with rotary mowers is only effective on high branching plants as these plants are less inclined to resprout. For low branching plants, regrowth following mowing is more common. The best time to mow is the early flowering stage (just as the yellow blooms begin to appear). Mowing in earlier growth stages results in increased light penetration and rapid regrowth of the weed. Repeated mowing of plants too early in their life cycles (rosette or bolting stages) or when branches are below the mowing height will not prevent seed production, as flowers will develop below the mower cutting height. Plants with a high branching pattern are easier to control, as regrowth will be greatly reduced. Even plants with this growth pattern must be mowed in the late spiny or early flowering stage to be successful. Even when mowed at the optimal stage, yellow starthistle plants will often resprout and multiple mowings will be needed.

In general, mowing is most effective when soil moisture is low, and no irrigation or rainfall follows mowing. A string trimmer may be used in small areas. Severing the plant at ground level using a string trimmer gives the greatest level of control. This strategy can be implemented at any time of the season and on any growth form. If using a string trimmer on low branching plants, or rosettes, you must severe the plant at ground level or regrowth may occur. If using a string trimmer on high branching plants, you can leave a short stalk, like as with a mower.

Extreme caution must be used mowing late in the season when vegetation is dry. Fire starts from mowing can be reduced by avoiding areas where rocks or other obstacles exist. Be sure to mow early in the morning when humidity is high, and dew is present. Don’t mow at midday, during high wind events, or when fire risk is high. All equipment should be equipped with a spark arrester. Be sure to have water and a fire extinguisher readily available and take all necessary precautions.

Cultural Control

Cultural control strategies use techniques to manipulate the environment such as mulching, irrigation, burning, solarization and grazing to improve desirable species and reduce weeds.

Grazing is effective in reducing yellow starthistle seed production when properly timed. The best time to graze is in late spring/early summer during bolting. Sheep, goats, and cattle eat yellow starthistle before spines form on the plant. Goats will eat starthistle even in the spiny stage. Intensive grazing in late May and June using large numbers of animals for a short duration can reduce plant height, canopy size, and seed production. After intensively grazing, rest the area for up to a month and graze again if needed.

Conversely, grazing in late winter or early spring allows animals to feed on young grasses with an erect growth form, causing little damage to starthistle rosettes which can result in an increase in starthistle cover. To avoid overgrazing, do not allow animals to graze more than half the available grass forage during that time. Grazing more than this will reduce grass recovery rate and the ability of the grass to shade out yellow starthistle.

Grazing can be an option even for those without animals. Contract grazing is an agreement with a livestock owner to graze an area to accomplish a specific management objective. Depending on the scale and scope, contractors will often be responsible for setting up portable fencing, hauling water, and providing livestock guarding dogs for the animals' protection. In urban areas, grazing may have some limitations due to size of parcel, access to animals, ability to fence, or public access. Grazing is most often used on larger parcels of land including pastures, parks, recreational areas, and open space. For more information on contract grazing visit matchgraze.com.

Burning can be an effective tool for controlling yellow starthistle and is best performed at the end of the rainy season when flowers first appear. Burning is often conducted through local, state or federal fire authorities. Residents wanting to burn should contact their local fire agencies. In addition to summer burning, starthistle seedlings have been controlled using winter or early spring flaming. This technique is somewhat nonselective, and control is inconsistent. Flaming works on small infestations, though multiple applications may be required as new flushes of seedling germination may occur after treatment. Flaming is often done with a propane torch or flame weeder. Flaming is not intended to ignite plants; the goal is to pass high intensity heat over the leaf surface to cause plant cells to rupture. Plants will change color immediately after treatment, going from dull to glossy green. Caution must be used to only use this equipment when vegetation is green and moisture is high. This treatment works on seedlings and small rosettes and not on bolting plants. Conducting this treatment immediately after a rain event when humidity is high will lessen the risk of fire. Any dry vegetation may catch fire. Have a charged water hose or fire extinguisher readily available in the event of fire.

Solarization is a non-chemical soil treatment that utilizes solar radiation and a thin film of transparent mulch, usually of polyethylene, to heat the soil. Plastic sheets are applied to moist soil during periods of high ambient temperature. The plastic sheets allow the sun's radiant energy to be trapped in the soil, heating the upper levels. Solarization during the hot summer months can increase soil temperature to levels that kill many disease-causing organisms (pathogens), weed seeds, and seedlings. Preparation of the site begins by mowing, disking, rototilling, or turning the soil by hand and then smoothing the soil surface. Remove any large rocks, weeds, or any other objects or debris that will raise or puncture the plastic. Once plastic is laid, seal edges with moist soil and monitor. Solarization can take several months to achieve the desired effects.

Competitive Plantings

Control practices can reduce yellow starthistle populations, but in the absence of competition, starthistle will often reestablish. Effective management requires that desirable plant species be encouraged or planted and managed to prevent yellow starthistle germination or growth. Species used for revegetation will depend on the intended use of that site. Vegetation such as perennial bunchgrasses, shrubs, ground cover, turf, annual grasses and forbs, or wildflowers may be desirable. In these situations, mechanical, cultural, biological, or chemical methods can be used to reduce yellow starthistle while encouraging other plant species. Yellow starthistle does not compete well with maintained turf and groundcovers. Where perennials are used, try to minimize spacing between plants; otherwise, mulch between landscape plants to help inhibit starthistle germination. Perennials are slow to establish and may require alternative strategies such as properly timed mowing, string trimming, hand pulling, hoeing, hand pulling or herbicides to control yellow starthistle during establishment.

For areas where seeding is to be used, eliminate dense stands of yellow starthistle and reseed the area with fast-growing, competitive plant species. Grasses are good options because selective herbicides can be used to control yellow starthistle that is not suppressed by competition. In areas with scattered yellow starthistle infestations, eliminate scattered plants and overseed with a desirable species to provide enough competition to prevent yellow starthistle from reestablishing. In all instances, choose desirable species that are well adapted to the site and not likely to become invasive themselves.

Chemical Control

Most available herbicides used for starthistle control provide postemergence (foliar) activity, while a few give preemergence (soil activity and seedling) control. Be sure to read the pesticide label and ensure the use site is listed. The label will specify the sites where the product is approved for use, such as residential lawns, parks, golf courses, recreation areas, pastures, roadsides, or around ornamental trees in public areas. Unless specifically identified as such, recreation sites include areas used for hiking and other outdoor recreation and do not include parks or playgrounds.

Postemergence Herbicides

Postemergence herbicide treatments generally work best on seedlings and small rosettes. The long germination period of yellow starthistle makes control with a single application almost impossible. Treating too early in the season will provide good control, but more seedlings are likely to germinate following the treatment. Waiting until later in the season to apply a postemergence herbicide allows a greater number of seedlings to be treated. Larger plants will require higher herbicide rates and may not be controlled.

Triclopyr is a growth regulator herbicide with little or no soil activity. It is foliar-absorbed and active on many broadleaf species, but typically does not harm grasses. Triclopyr is formulated in several formulations including amine, ester, and choline. The ester is more sensitive to vapor drift so caution should be observed when using this formulation. Triclopyr is registered for use on pastures, rights-of-way, roadsides, and other non-crop areas. Some products may be used around residential areas, parks, and recreational sites. The best efficacy is achieved when plants are small, and higher rates may be needed for larger plants. Several over-the-counter products are available including Brushtox, Monterey Brush & Vine Control, Roundup Concentrate Poison Ivy Plus Tough Brush Killer, and Brush Killer for Large Property. Several of these products may contain other active ingredients including 2,4-D, glyphosate, and pelargonic acid for increased control. Professional products include Garlon 4 Ultra, Triclopyr 4, Remedy Ultra, Garlon 3A, Tahoe 3A, and Vastlan.

Glyphosate is a non-selective, postemergence herbicide that kills most plants. It has no soil activity and can be used throughout the growing season. Glyphosate is available in hundreds of products both as a concentrated form for professional and resident use, as well as many products with lower concentrations that can be diluted or are in ready-to-use sprayers. Like triclopyr, spraying early in the season will only kill sprouted plants and retreatment may be necessary for future flushes. Glyphosate is effective on all stages of growth including rosette, bolting, spiny and early flowering. Although non-selective, glyphosate can be used to target starthistle and minimize non-target injury by making spot treatments or treating after desirable annual vegetation has died. Glyphosate is registered for use around the home as well as for pastures, rights-of-way, roadsides, recreational sites, and vacant lots. Glyphosate can be used alone, tank mixed, or purchased as a product that contains other active ingredients including diquat, imazapyr, nonanoic acid, and others.

Clopyralid and aminopyralid are selective growth regulator herbicides effective on yellow starthistle both postemergence and preemergence. Generally, both herbicides can be used on pasture, fencerows, parking lots, and some recreational areas. Some clopyralid products can be used on landscapes and ornamental plantings. The most effective timing for aminopyralid application is from December to March, when yellow starthistle is in the seedling to mid-rosette stage; its soil residual should last until the end of the rainy season. Clopyralid has a shorter soil residual and should be applied January to March. For both chemicals, earlier applications (i.e., in fall) may not provide full-season control, and later applications (bolting to early spiny stage) will require higher rates. A single application at the recommended time will provide season-long control. Both clopyralid and aminopyralid are selective on many members of the sunflower family, particularly thistles, but can also injure legumes (Fabaceae), including clovers.

Clippings or animal manure from treated areas should not be used as compost; these herbicides degrade slowly in compost and can be a problem when used as a mulch or fertilizer source in gardens and landscapes.

Clopyralid is available in several professional products including Transline, Clean Slate, Stinger, and Lontrel. It is also available over the counter as Star Thistle Killer.

Aminopyralid is available in several products including Milestone, Malibu, and Whetstone. There are no consumer over-the-counter products that contain aminopyralid. Licensed pest control businesses can be hired to spray these chemicals.

Preemergence Herbicides

Preemergence herbicides must be applied before seeds germinate to be effective. The long germination period of yellow starthistle requires that a preemergence material have a lengthy residual activity or multiple applications will be needed to provide season long control. Make applications just before the beginning of the rainy season as rain will move the material into the soil. Because these materials adhere to soil particles, off-site 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, it is possible to combine a postemergence herbicide (to control sprouted plants) with a preemergence herbicide (to provide residual control of any subsequent germination) for an effective control strategy.

Home gardeners and landscape professionals can use preemergence herbicides dithiopyr, pendimethalin, prodiamine, and trifluralin to limit seedling germination in sites where these materials are permitted. For infestations in pastures, roadsides, parking lots, and other urban landscape sites, aminopyralid and clopyralid provide both postemergence and preemergence control with a single treatment, often providing season-long control when properly timed.

Application Methods

Herbicides can be applied as a broadcast application or spot treatment. Broadcast applications are used to treat existing stands of starthistle with a postemergence herbicide or to treat areas with a preemergence where starthistle is known to exist. Broadcast treatments can be made using a boom, boomless nozzle, or with a single nozzle applied uniformly across an area. Postemergence herbicides may also be applied to individual plants or small patches with a more directed application technique. Spot treatments can be made using a backpack sprayer, ready-to-use sprayer, squirt bottle, or by wicking. Wicking allows you to control starthistle growing above desirable vegetation by using herbicides that might otherwise kill or severely injure desirable plants. This technique is done with a weed wick—a sponge, pad, rope, or roller saturated in a concentrated herbicide solution. The herbicide is wiped or painted onto the weed without touching non-target plants. Applications can be made with a specialized wicking applicator or a disposable foam brush. Concentrated glyphosate applied with a weed wick at a 33% to 50% solution in water is effective at controlling starthistle during bolting and early flowering stage.

Product labels can vary significantly, so be sure to read the product label to ensure the product is registered for your site. For treated areas exposed to livestock, some products may have grazing and haying restrictions.

Integrated Approaches

Each control strategy has its own strengths and limitations. The most effective weed management program uses a combination of methods tailored to the specific site and scale of infestation. When using competitive plants, pretreating the area with herbicide, solarization, or cultivation can help shift the competitive advantage to the desired plants. Following planting, mulch can help suppress yellow starthistle germination. Spot treatment with postemergence herbicide, hand weeding, hoeing, and string trimmers can be used to treat yellow starthistle around desirable plants. Combining preemergence herbicides and mulch are also effective. A long-term commitment of three or more consecutive years will be necessary to deplete the weed seedbank. It is not unusual for a starthistle infestation to seem more vigorous after the first year of treatment. Once the seedbank has been depleted, annual monitoring and continued control of starthistle can be accomplished by either hand-pulling, mowing, or spot spraying.

References

DiTomaso JM, Kyser GB, Pitcairn MJ. 2006. Yellow starthistle management guide. Cal-IPC Publication 2006-03. California Invasive Plant Council: Berkeley, CA. 78 pp.

Pitcairn MJ, Schoenig S, Yacoub R, Gendron J. 2006. Yellow starthistle continues its spread in California. California Agriculture, 60(2).

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