Purple Starthistle
Centaurea calcitrapa
Class A Noxious Weed
This plant is new to the state and has been designated a Class A Noxious Weed by the New Mexico Department of Agriculture.
Toxic to Horses
This plant has been documented to sicken or injure horses.
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Children & Pets
This plant may pose a particular risk to unsupervised children and animals due to its toxicity or potential for injury.
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Agricultural Threat
This plant threatens our state's food security and economy by reducing agriculture yields and/or degrading soil resources.
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QUICK FACTS
- Purple starthistle can grow up to 3 feet tall and has long, sharp spines that can injure people, livestock, and wildlife. These spines can lead to puncture wounds or infections, seriously threatening the health and safety of individuals who come in contact with them.
- Originally from Eurasia, purple starthistle spread to other continents, including North and South America, New Zealand, and Australia, where it has become a noxious, globally recognized weed. It disrupts agriculture by crowding crops and making land unsuitable for grazing.
- Purple starthistle is toxic to some animals, particularly horses. Its spines deter livestock from grazing in infested areas. The plant also outcompetes native plants, reducing biodiversity and negatively impacting ecosystems.
1. Overview
A world-famous nuisance.
Around the globe, purple starthistle is known as a determined and disruptive weed. The plant grows up to 3 feet tall and possesses uniquely long and sharp spines, which can cause significant injury. Likely spread via contaminated seed, purple starthistle escaped its native range and is now listed as noxious in North and South America, New Zealand, and Australia. This invader threatens food production by deterring desirable crops and cattle and endangers the health and safety of any person or creature that comes in contact with it.
History of Purple Starthistle
Purple Starthistle was introduced to North America in the 1800s, likely as a contaminant in imported crop seeds. Initially confined to coastal regions, it has since spread inland, particularly in California, Oregon, and other western states. It gets the name calcitrapa from the ancient instrument of war, the caltrop. These four-pointed iron jacks were used to impede cavalry advances and resemble the flower heads of purple starthistle. [1,2,5,6]
Historically, Purple Starthistle was tolerated in some agricultural settings due to its minimal impact in its native range. However, in the arid western United States, it behaves aggressively, forming dense, thorny monocultures that reduce biodiversity and degrade pastureland. In heavily infested areas, the plant can render fields unusable for grazing and suppress the establishment of native plants. [3,4]
The name “centaurea” is thought to have derived from Chiron, the wise and skilled centaur from Greek mythology known for his healing abilities. [8]
2. ID Guide
What does it look like?
Purple starthistle is a multi-branched bushy forb that can act as an annual, biennial, or short-lived perennial. This weed closely resembles spotted and diffuse knapweed, other noxious weeds commonly found in the West. Purple starthistle can be differentiated by the sharp spines at the tips of the flower bracts. Knapweeds lack these sharp spines. [1,2,3,6]
Having trouble identifying a weed? Contact us.
Key Features
- Plant: Appears as only a basal rosette until stems are ready to bolt. The stems are usually found in groups of one to several and often form rounded mounds. They are covered in short, dense, tangled hairs. When fully mature, purple starthistle can grow to be 4 feet tall. [1,2,7]
- Roots: This weed has a thick, fleshy taproot, which can grow long enough to sequester deeper moisture from surrounding plants. [1]
- Leaves: The dark green basal leaves are short-lived and have petioles (leaf stems) and hairy surfaces. These leaves are lanceolate with deep lobes and toothed margins, growing approximately 9.8 inches long by 1.6–2 inches wide. Stem leaves appear much more narrow (linear) and deeply divided (less so closer to branch tips). They are rough to the touch and have no stems. [2,6]
- Flowers: The cluster of stems terminates in roundish heads, each about 0.6-1 inches in size. The outer part of the flower heads is covered in green or straw-colored bracts, each tipped with a sharp, yellow spine with 2-6 shorter spines at the base of the flower head. Topping each head is a vibrant purple thistle-like flower with 25-40 florets. Flowers appear from June through November. [2,3]
- Seeds: Fruits, each containing a single smooth, hairless seed, are dry and do not open at maturity. These fruits (achenes) are usually white and streaked with brown. Each seed is 3-4 mm long and about 2mm wide. [2,3]
First year rosette; purple starthistle will store energy through the winter so it can bolt the following spring.
1354007-SMPT
The outer part of the flower heads is covered in green or straw-colored bracts, each tipped with a sharp, yellow spine.
1354005-SMPT
The stems are usually found in groups of one to several and often form rounded mounds. They are covered in short, dense, tangled hairs.
CECA1
The narrow, dark-green basal leaves are short-lived and have petioles (leaf stems) and hairy surfaces.
original (3)
Topping each head is a vibrant purple thistle-like flower with 25-40 florets. Flowers appear in June through November.
original
Fruits, each containing a single smooth, hairless seed, are dry and do not open at maturity. These fruits (achenes) are usually white and streaked with brown.
3. Infestation Basics
“It (C. calcitrapa) is entirely without value and exceedingly troublesome if neglected.”
-Alfred J. Ewart, 1909
Impacts
Where does it grow, and how does it spread?
Purple starthistle is most commonly found in recently or frequently disturbed soils such as pastures, agricultural fields, or roadsides. While it prefers alluvial soils with higher moisture levels, it is drought-tolerant and thrives in various soil types. [1,2,8]
Most purple starthistle plants will act as biennials, storing energy in the first year’s rosette stage and bolting in the second growing season to produce flowers. Depending on growing conditions, the plant may complete its entire lifecycle in one year in ideal areas. It may take several years to flower and die in less favorable environments. [7]
This weed prefers full sun and is not shade-tolerant. Open areas with full sun are more at risk for infestations. Because of its preference for moisture, it may be found in riparian areas or along irrigation ditches. It is more likely to be found in temperate to subtropical regions as opposed to colder climates with long winters. Regions with 28-35 inches of rainfall annually create ideal moisture levels for purple starthistle. [1,2]
Purple starthistle can sprout from seeds or leftover root crowns. Germination can occur in a wide variety of growing conditions, but seedlings usually emerge after the first fall rains. Seedlings will die off if the environmental conditions after germination are too dry. In drought years, seed viability is generally lower. [1,4]
Seeds lack the necessary pappus to travel via wind, and most seeds will fall at the base of the parent plant. To facilitate long-range dispersal, the entire plant will break off at the base and travel as a tumbleweed. Seeds typically stay attached until the weed comes to rest. Another means of spreading is by way of its sharp spines, which stick themselves into the clothes and fur of passing humans and animals. The spiny flower heads can become attached to vehicles and equipment, inadvertently spreading the weed to new areas. The seeds may also float and occasionally be deposited by streams. [2,8]
Because seeds lack a pappus, long-distance dispersal is difficult, and most seeds fall and germinate near the base of the parent plant. Seeds can spread via water or when the entire plant breaks off at the base to become a tumbleweed. Many seeds remain within the dried flower heads and become attached to fur and clothing by the head’s sharp spines. Each plant can produce upwards of 50 flower heads. Seed viability is low, with most only surviving 1-3 years. [6,7]
Key takeaway: Purple starthistle thrives in disturbed, sunlit areas with moist soils, mainly in temperate to subtropical regions. It spreads via tumbleweeds, spiny flower heads, or water.
Why is it so invasive?
These invasive plants possess extremely sharp spines, which can deter livestock from grazing in certain areas. Hay made in infested areas has severely reduced quality and palatability. Wildlife may avoid infested areas, reducing habitat size and forage availability. Because purple starthistle is avoided by livestock, other plants will be grazed first, creating more space for the weed to invade. [1]
The sharp spines of purple starthistle can cause harm to passing animals as well as people. This issue can affect public parks and other tourist areas, as well as endanger pets and children on private properties. Dense stands of the plant exclude desirable crops and native plants. Spines may cause grievous injury to the eyes and mouth of livestock. The plant possesses toxins that are harmful to animals, but as cattle do not typically consume it, this is of little concern to ranchers. [1,2,7]
In places not well represented by native plant communities, purple starthistle is more likely to invade. This is a problem, especially in wildlife areas with open woodlands or grasslands. Purple starthistle will usually not become established in areas with other dense vegetation. [2]
Horses may graze on young purple starthistle rosettes, which may cause “chewing disease” due to toxins found in the plant. [7]
Key takeaway: Purple starthistle’s sharp spines and toxic properties threaten livestock, wildlife, and native plants by reducing native grazing areas and poisoning or injuring animals.
Common risk factors for invasion
- Soil disturbance: Purple starthistle loves to invade areas where the soil has been disturbed, such as construction sites, tilled fields, or overgrazed pastures. This disruption weakens existing vegetation and allows the weed to become established. Minimizing unnecessary soil disturbance can help reduce the risk of invasion.
- Open spaces: The weed favors open, sunny environments with little canopy cover, such as pastures, cultivated fields, and south-facing slopes. These conditions allow purple starthistle to establish quickly and outcompete other plants. Maintaining healthy, dense vegetation in these areas can help limit its spread.
- Moisture availability: While purple starthistle has adapted to a wide range of soil and water conditions, it generally requires adequate moisture to establish and spread. Irrigated lands and areas with consistent rainfall or acequias are particularly vulnerable.
Health and Safety
Purple starthistle can physically injure children, workers, livestock, and wildlife, causing puncture wounds or infections. The obstruction of firebreaks and pathways by large weed infestations can exacerbate fire risks in dry regions, threatening property and personal safety. Horses are particularly endangered as the plant’s consumption can cause “chewing disease.” [7]
Wildlife Habitat
The continued spread of purple starthistle harms wildlife as well as agriculture. Areas taken over by this thorny plant will likely not be grazed by ungulates. This can disrupt historically grazed areas and alter nutrient forage patterns. While the dense brambles caused by plant stands may offer concealment and protection to rabbits, many larger species will not be able to find shelter in infested areas due to the sharp spines. [2,8]
Agriculture and Food Security
Farmers and ranchers face increased costs of managing this weed, including herbicide expenses, additional labor, and mechanical control measures. Dense stands of purple starthistle inhibit and crowd out the growth of crops. Loss of crops in this manner, combined with the loss of associated rangeland for cattle, can severely disrupt food production and cause substantial economic loss in agricultural settings. [2]
Livestock
This weed is bitter and unpalatable, avoided by most creatures except in its early rosette stage. Its bitterness, combined with the sharp thorns that develop later on, can completely deter livestock from infested areas. Purple starthistle is likely toxic to domestic animals, particularly horses, but poisonings are not common among grazers due to the plant’s unpalatability. [2,8]
4. Management Strategies
Purple starthistle is the most vulnerable during its early stages after germinating in winter or spring. Herbicides can be effective at this stage when used in conjunction with cultivation. Isolated plants should be dug out immediately, removing the root crown to avoid resprouting. Mowing will not kill plants completely but may reduce seed production if timed correctly. Burning and grazing are not viable options as these methods can leave behind the root crown to re-sprout. [1,2]
DO’s
- Encourage the growth of competitive native plants. Purple starthistle tends to thrive in disturbed soils, so maintaining healthy, dense vegetation can help outcompete the weed.
- Mow or cut plants before they set seed. This prevents new plants from germinating and helps limit their reproductive success.
- Combine different management techniques, such as mechanical removal (digging or pulling), herbicides, and seeding desirable plants.
DON’Ts
- Avoid disturbing the soil unnecessarily, as tilling or excessive digging can spread purple starthistle seeds and help them germinate. Soil disturbance can create ideal conditions for its spread.
- Allow livestock to overgraze, disturbing the soil and reducing competition from native or desirable plants, facilitating the weed’s growth.
- Don’t wait until purple starthistle has become a widespread issue. Addressing small plants and early infestations is far more manageable.
** The following information is provided courtesy of the UC Weed Research and Information Center. The Taos Soil and Water Conservation District does not endorse the use of any particular product, brand, or application thereof. **
Purple starthistle can be managed without chemicals by combining manual removal, mowing before seed set, targeted grazing, and promoting competitive vegetation.
| Hand-pulling | Hand pulling, grubbing, or digging can be used to control small infestations. These techniques must be repeated several times a year. Purple starthistle populations were sharply reduced after 3 years of hand grubbing. |
| Mowing | Mowing is not effective at killing plants but can reduce seed production if timed at full bloom. Regrowth occurs from root crowns when the tops are removed. |
Conventional grazing by sheep or cattle will not control purple starthistle and in fact can promote it because grazing animals usually avoid this plant and selectively feed on species that would otherwise compete with it.
Burning is not considered an effective tool for control.
In California where purple starthistle is a common pasture weed, fertility management is occasionally used as a management tool
While there are many biological control programs for other species of Centaurea, there is no biological control program for purple or Iberian starthistle.
The following specific use information is based on published papers or reports by researchers and land managers. These are products known to provide effective control. Those that do not provide sufficient control have been omitted from the table. Other trade names may be available, and other compounds are also labeled for this weed. For foliar applications, use low pressure and a coarse spray pattern to reduce spray drift damage to non-target species. Directions for use may vary between brands; see label before use. Herbicides are listed by mode of action and then alphabetically. The order of herbicide listing is not reflective of the order of efficacy or preference.
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2,4-D Several names |
Rate: 1 to 2 qt product/acre (0.95 to 1.9 lb a.e./acre) Timing: Postemergence in spring when plants are still in rosettes but before flower stems elongate. Treat rapidly growing plants. Thoroughly covering foliage enhances control. Remarks: Generally requires repeat applications. It is not considered as effective as other growth regulator herbicides for season-long control. 2,4-D is broadleaf-selective and has no soil activity. Do not apply ester formulation when outside temperatures exceed 80°F. Amine forms are as effective as ester forms for small rosettes, and amine forms reduce the chance of off-target movement from volatility. |
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Aminocyclopyrachlor + chlorsulfuron Perspective |
Rate: 4.75 to 8 oz product (Perspective)/acre Timing: Postemergence and preemergence. Postemergence applications are most effective when applied to plants from the seedling to the mid-rosette stage. Remarks: Aminocyclopyrachlor gives control of many members of the genus Centaurea, including purple starthistle. Its effect is similar to aminopyralid. Perspective provides broad-spectrum control of many broadleaf species. Although generally safe to grasses, it may suppress or injure certain annual and perennial grass species. Do not treat in the root zone of desirable trees and shrubs. Do not apply more than 11 oz product/acre per year. At this high rate, cool-season grasses will be damaged, including bluebunch wheatgrass. Not yet labeled for grazing lands. Add an adjuvant to the spray solution. This product is not approved for use in California and some counties of Colorado (San Luis Valley). |
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Aminopyralid Milestone |
Rate: 4 to 7 oz product/acre (1 to 1.75 oz a.e./acre). These rates are used for control of diffuse and spotted knapweed, and are expected to provide similar control of purple and Iberian starthistle Timing: Postemergence to rapidly growing plants in fall, or in spring from rosette to bolting stages. Late winter to early spring applications provide residual control of germinating seedlings. Remarks: A non-ionic surfactant (0.25 to 0.5% v/v of spray solution) enhances control under adverse environmental conditions. Aminopyralid is a broadleaf herbicide. |
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Clopyralid Transline |
Rate: 0.67 to 1.33 pt product/acre (4 to 8 oz a.e./acre) Timing: Postemergence to starthistle rosettes but before flower stem elongates. Plants should be rapidly growing at time of treatment. Remarks: Apply in 10 to 40 gal/acre of water. |
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Clopyralid + 2,4-D Curtail |
Rate: 2 to 5 qt Curtail/acre Timing: Postemergence after most rosettes emerge but before flower stem elongates. Remarks: Use higher rates for fallow and Conservation Reserve Program (CRP) applications. With CRP applications, use in established grass only. Apply in enough total spray volume to ensure good coverage. Add a non-ionic surfactant. |
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Dicamba Banvel, Clarity |
Rate: 2 to 4 pt product/acre (1 to 2 lb a.e./acre). Use higher rate for older plants or dense stands. Timing: Postemergence when plants are still in rosettes but before flower stems elongate. Plants should be rapidly growing at time of treatment. Remarks: Dicamba is a broadleaf-selective herbicide often combined with other active ingredients. It is not typically used alone to control starthistles. |
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Picloram Tordon 22K |
Rate: 1 to 2 pt product/acre (4 to 8 oz a.e./acre) Timing: Preemergence and postemergence. With postemergence application, optimally treat at rosette to mid-bolting stage (before flowering to prevent current year seed production), or fall rosette stage. Apply when plants are growing rapidly. Remarks: Picloram gives a broader spectrum of control than aminopyralid, aminocyclopyrachlor, and clopyralid, and has much longer soil residual activity. Most broadleaf plants are susceptible. It will not damage perennial grasses at the suggested rate. Treatment made in bud stage may not prevent seed production in the year of application. Do not apply near trees. Tordon 22K is a restricted-use herbicide. Picloram is not registered for use in California. |
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Glyphosate Roundup, Accord XRT II, and others |
Rate: Broadcast foliar treatment: 3 qt product (Roundup ProMax)/acre (3.375 lb a.e./acre). Spot treatment: 1.5% v/v solution Timing: Postemergence to rapidly growing plants when most plants are at bud stage. Remarks: Glyphosate will only provide control during the year of application; it has no soil activity and will not kill seeds or inhibit germination the following season. Glyphosate is nonselective. To achieve selectivity, it can be applied using a wiper or spot treatment to control current year’s plants. |
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Sulfometuron Oust and others |
Rate: 3 to 5 oz product/acre (2.25 to 3.75 oz a.i./acre) Timing: Preemergence or early postemergence, before or during the rainy season when weeds are germinating and actively growing. Remarks: Treated soil should be left undisturbed to reduce the potential movement of the herbicide by soil erosion due to wind or water. Treatment of powdery, dry soil or light, sandy soil when there is little likelihood of rainfall soon after treatment may result in off target movement when soil particles are moved by wind or water. |
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Photosynthetic inhibitors |
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Hexazinone Velpar L |
Rate: 1 to 2.5 gal product/acre (2 to 5 lb a.i./acre) Timing: Preemergence or postemergence when weeds are germinating or actively growing. Remarks: Centaurea control is only registered for uncultivated non-agricultural areas (such as rights- of-way), uncultivated agricultural areas (non-crop producing which includes uses such as farmyards and barrier strips), and industrial sites. Use lower rate on coarse-textured soils (sand to sandy loam). Use the higher rate on fine-textured soils (clay loam to clay) and on soils high in organic matter. High rates of hexazinone can create bare ground, so only use high rates in spot treatments. |
5. Citations
[1] Anderson, Lars W., et al. Weed Control in Natural Areas in the Western United States. Davis, California, University Of California Weed Research And Information Center, 2013.
[2] Centre for Invasive Species Solutions. “Centaurea Calcitrapa L.” Weeds.org.au, Jan. 2024, weeds.org.au/profiles/star-thistle-purple/.
[3] NPS. “Exotic Species: Purple Starthistle (U.S. National Park Service).” Www.nps.gov, 29 Apr. 2016, www.nps.gov/articles/purple-starthistle.htm.
[4] USFS. Field Guide to Noxious and Invasive Weeds Known to Occur or Are Potentially Occurring on the Apache-Sitgreaves National Forests. 2008.
[5] von Mueller, Ferdinand. Illustrated Description of Thistles, Etc., Included within the Provisions of the Thistle Act of 1890. Robt. S. Brain, Government Ptr., 1893.
[6] Victoria State Government. “Star Thistle.” Agriculture Victoria, Department of Energy, Environment and Climate Action, 2024, agriculture.vic.gov.au/biosecurity/weeds/weeds-information/star-thistle. Accessed 21 Feb. 2025.
[7] California Invasive Plant Council. “IPCW Plant Report – Centaurea Calcitrapa.” Cal-Ipc.org, 2025, www.cal-ipc.org/resources/library/publications/ipcw/report26/. Accessed 21 Feb. 2025.
[8] Darlington, William. American Weeds and Useful Plants. A.O. Moore, 1859.
[9] Culpeper, Nicholas. Culpeper’s Complete Herbal: A Book of Natural Remedies for Ancient Ills. 1653. Ware, Herts., Wordsworth Reference, 2007.
[10] Cropper, John. “The Geographical Distribution of Anopheles and Malarial Fever in Upper Palestine.” Journal of Hygiene, vol. 2, no. 1, 1 Jan. 1902, pp. 47–57, https://doi.org/10.1017/s0022172400001686.
[11] Dimkić, Ivica, et al. “New Perspectives of Purple Starthistle (Centaurea Calcitrapa) Leaf Extracts: Phytochemical Analysis, Cytotoxicity and Antimicrobial Activity.” AMB Express, vol. 10, no. 1, 12 Oct. 2020, https://doi.org/10.1186/s13568-020-01120-5.
Malcolm Storey, BioImages – Virtual Field-Guide, Bugwood.org
Fun Facts
In Southern Italy, young shoots of purple starthistle are consumed in various dishes. Traditionally, it has been used to treat eye conditions, fever, jaundice, and digestive and skin disorders. Nicholas Culpeper wrote in his 1653 book Culpeper’s Complete Herbal:
“The seed of this Star Thistle made into powder and drank in wine provokes urine, and helps to break the stone and drives it forth. The root in powder and given in wine and drank is good against the plague and pestilence.” [9,11]
Joseph M. DiTomaso, University of California – Davis, Bugwood.org
Barry Rice, sarracenia.com, Bugwood.org
Fun Facts
In recent studies, different plant extracts have shown bioactivity, with potent antioxidant properties and notable cytotoxicity against cancer cell lines. The methanol extract also shows significant antibacterial activity against pathogens, including Bacillus, Pseudomonas, and Staphylococcus. [11]
Barry Rice, sarracenia.com, Bugwood.org
Fun Facts
At the start of the 20th century, the narrow spines of purple starthistles were commonly used as entomological pins (for mounting deceased insects for study or collection). These spines were used as a perfectly adequate substitute when scientists abroad ran out of traditional pins. [10]
Steve Dewey, Utah State University, Bugwood.org