Carnegiea gigantea, Giant Saguaro

Southwest Desert Flora

Home to the plants of the Sonoran, Chihuahuan and Mojave Deserts

Cirsium ochrocentrum, Yellowspine Thistle

Centaurea melitensis, Maltese Star-thistle

Maltese Star-thistle has a yellow flower atop a bulbous floral-head of spines. The “flowers” are all discoid. Plants bloom from April or May to July. Centaurea melitensis Maltese Star-thistle flower stems are long and the bracts surrounding the flowers are spine tipped with the central spine the longest. These bracts or phyllaries are may be purplish toward the base and often cobwebby or later becoming smooth or glabrous. Centaurea melitensis Maltese Star-thistle is an annual or biennial that grows up to 3 feet or less. It is often referenced as a Knapweed as most other species in the genus Centaurea. Centaurea melitensis Maltese Star-thistle is a forb with erect stiff stems, usually 1 stem and openly branched. Note in the photo that the stems are winged, gray and hairy and resin-dotted. Centaurea melitensis Maltese Star-thistle leaves or green to grayish-green, alternate and with fine hairs thinly tomentose. The leaves are oblong or oblanceolate and the leaf margins are smooth to lobed. Centaurea melitensis Maltese Star-thistle is also called Napa Thistle, Tocalote or Tocolote. It grows in elevations up to 4,500 feet more or less and is found in various habitats, lower and upper deserts, pinyon-juniper communities, chaparral and grasslands, disturbed areas, roadsides, agriculture areas and open woodland. Centaurea melitensis

Scientific Name: Centaurea melitensis
Common Name: Maltese Star-thistle
Also called: Malta Thistle, Maltese Star Thistle, Malta Starthistle, Maltese Star-thistle, Napa Thistle, Spotted Knapweed, Tocalote, Tocolote (Spanish: Cardo)
Family: Asteraceae, Sunflower Family
Synonyms: ( )
Status: Introduced; naturalized, European.
Duration: Annual or biennial.
Size: 3 feet more or less.
Growth Form: Forb/herb; stems erect, stiff, mostly 1 stem, large plants openly branched, stems winged; gray-hairy, resin-dotted.
Leaves: Green, grayish green; alternate; decurrent on the stem; fine hairs, thinly tomentose, scabrous and puberulent basal rosette, pinnately divided, oblong or oblanceolate; margins entire to lobed, lower leaves usually dried by bloom period; upper leaves variable, linear or elliptical.
Flower Color: Yellow; 1 to many spiny discoid flowers, peduncled or sessile in axils; corollas ovoid, yellow and somewhat cobwebby or becoming glabrous, straw colored; phyllaries spine tipped, central spine longest, slender and recurved; terminal spine or prickle of the phyllary normally purplish toward the base, less than 1 cm long; fruit is an achene, fine tan or white pappus bristles.
Flowering Season: April or May to July.
Elevation: Up to 4,500 feet more or less.

Habitat Preferences: Various habitats, lower and upper deserts, pinyon-juniper communities, chaparral and grasslands, disturbed areas, roadsides, agriculture areas and open woodland.

Recorded Range: Primarily a western invasive species in North America quickly spreading east of the Mississippi River extending currently to Massachusetts. Invasive also in Hawaii. In central, south northwest and northeast Arizona.

North America & US County Distribution Map for Centaurea melitensis.

U.S. Weed Information: Centaurea melitensis can be weedy or invasive as noted by the USDA, Natural Resources Conservation Service. It is considered a weed by the following; Cal-IPC, California Invasive Plant Council.
Plants included here may become weedy or invasive.

Invasive/Noxious Weed Information: Centaurea melitensis is listed by the USDA, Natural Resources Conservation Service as a Noxious Weed in: Nevada, Malta thistle is a Noxious weed and in New Mexico, Malta Starthistle is a Class B noxious weed.
Plants included here are invasive or noxious.

Wetland Indicator: No data available.
Threatened/Endangered Information: No data available.

Genus Information: In North America there are 34 species and 38 accepted taxa overall for Centaurea. Worldwide, The Plant List includes 734 accepted species names and a further 1,150 scientific names of infraspecific rank for the genus Centaurea. Members of the genus Centaurea are commonly known as Knapweeds.

In the Southwestern United States: Arizona and New Mexico each have 8 species of Centaurea, California has 17 species, Nevada has 5 species, Texas has 4 species, Utah has 10 species. All data is approximate and subject to taxonomic changes.

Comments: Maltese Star-thistle is an aggressive weed which was introduced to North America (California) in the 18th century. It is a native of the Mediterranean region of Europe and Africa. It continues to spread across North America where it is already listed as a noxious weed in many states. The numerous common names of a plant is often a clue as to its economic significant or importance across its range, either positive or notoriously; such is the case for the Maltese Star-thistle.

Special Value to Native Bees; European Honey Bees (Apis mellifera) are important pollinators of Maltese Star-thistle. Bumblebees (Bombus) are also important floral visitors.

Also see in Southwest Desert Flora; Yellow Star-thistle, Centaurea solstitialis.

The genus Centaurea is derived from an accessory form of centaurëum in the Herbarium of Pseudo-Apuleius, and from Ancient Greek (kentaúreion, "several plants related to Centaurea"), from (kéntauros, “centaur”) (due to the mythological discovery of its medicinal properties by Chiron the Centaur).

The species epithet melitensis is derived from the Latin word "Melita" (adj Maltese) meaning Malta a reference to the the Mediterranean region of Europe and Africa where it is native.

Maltese Thistle has been used as a kidney aid by southwestern United States indigenous peoples.

  • Mahuna Drug, Kidney Aid; Plant used for the kidneys.

  • See complete listing of ethno-botanical uses at Native American Ethnobotany, University of Michigan, Dearborn.

    Date Profile Completed: 8/13/2012; Updated, 07/25/2015, updated 10/13/2017
    Arizona Flora, Kearney, Thomas H., Peebles, Robert H., 1960, University of California Press, Berkley and Los Angeles
    U.S. Dept. of Agriculture, Natural Resources Conservation Service on-line database and USGS ITIS search - (accessed 10/13/2017)
    The Plant List (2013). Version 1.1. Published on the Internet; (accessed 10/13/2017).
    David J. Keil 2017. Centaurea melitensis, in Jepson Flora Project (eds.) Jepson eFlora,, accessed on October 13, 2017.
    David J. Keil, Jörg Ochsmann, FNA | Family List | FNA Vol. 19, 20 and 21 | Asteraceae | 18. Centaurea melitensis Linnaeus, Sp. Pl. 2: 917. 1753. Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 16+ vols. New York and Oxford.
    Michael L. Charters; California Plant Names: Latin and Greek Meanings and Derivations; A Dictionary of Botanical and Biographical Etymology; (accessed 10/13/2017).
    SEINet synonyms, scientific names, geographic locations, general information, (accessed 10/13/2017).