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, Napa Thistle, Spotted Knapweed, Tocalote, Tocolote (Spanish: Cardo)

Family: Asteraceae, Sunflower Family

Synonyms: ( )

Status: Introduced and naturalized, European.

Duration: Annual or biennial.

Size: 3 feet (100 cm) more or less.

Growth Form: Forb/herb; large plants gray-hairy, covered in dense, soft and sometimes woolly hairs (tomentose); generally 1 stem, openly branched, stems winged and upright more or less (erect), tiny resin-gland-dotted.

Leaves: Green, grayish green; leaves arranged alternately along stem; fine hairs, thinly tomentose; leaf blades linear to oblong; leaf edges or margins smooth and not divided (entire) or toothed (dentate) or pinnately lobed; leaves resin (resinous) dotted.

Flower Color: Yellow; 1 to few disk flowers, flower heads singles or generally in open leafy corymbiform arrays; flowers slender, appear cobwebby or becoming smooth (glabrous); bracts or phyllaries surrounding the flower heads are straw-colored, the lower appendages purplish, these phyllaries are spine-tipped, the slender central spine is the largest; fruit a dull white or light brown cypsela with a pappus of white stiff bristles.

Flowering Season: April or May to July.

Elevation: Up to 5,000 feet (1,500 m) more or less.

Habitat Preferences: Various habitats, lower and upper deserts, pine-oak woodlands, pinyon-juniper communities, chaparral and grasslands; disturbed areas, roadsides, fields and agriculture areas.

Recorded Range: Primarily a western invasive species in North America including British Columbia, spreading now east of the Mississippi River extending currently to Massachusetts. Widely introduced. In central, south northwest and northeast Arizona.

North America & US County Distribution Map for Centaurea melitensis.

North America species range map for Centaurea melitensis:
North American range map courtesy of Virginia Tech, Dept. of Forest Resources & Environmental Conservation

North America species range map for Centaurea melitensis: Click image for full size map
Click image for full size map

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;

  • STATE; Assorted authors. State noxious weed lists for 46 states. State agriculture or natural resource departments.
  • Cal-IPC California Invasive Plant Council. 2006.

  • 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;
  • New Mexico; Malta Starthistle is a Class B noxious weed.

  • Plants included here are invasive or noxious.

    Wetland Indicator: Unknown
    Threatened/Endangered Information: Unknown

    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.

    The genus Centaurea was published by Carl Linnaeus in 1753.

    In the Southwestern United States: Arizona has 8 species of Centaurea, California has 17 species, New Mexico has 8species, Nevada has 6 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 and it is 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.

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

    Importance to Wildlife, Birds and Livestock
    Tiny wind-borne seeds and parts of flowers and foliage of Centaurea melitensis may possibly be eaten by birds and small mammals.

    Special Value to Native Bees, Butterflies and Insects
    European Honey Bees (Apis mellifera) are important pollinators of Maltese Star-thistle. Bumblebees (Bombus) are also important floral visitors. Additionally Centaurea melitensis flowers may be visited by butterflies and other small insects.

    The genus Centaurea (Centaur'ea/Centaur'ium:) is Latin and a reference to the Centaur Chiron who was supposed to have discovered the medicinal uses of a plant in Greece that came to be called Centaury.

    The genus Centaurea was published by Carl Linnaeus in 1753.

    The species epithet melitensis (meliten'sis:) is from the Latin word "Melita" an adjective of 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 06/09/2020
    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 06/03/2020)
    The Plant List (2013). Version 1.1. Published on the Internet; (accessed 06/03/2020).
    David J. Keil 2012, Centaurea melitensis, in Jepson Flora Project (eds.) Jepson eFlora, /eflora/eflora_display.php?tid=1934, accessed on June 03, 2020.
    David J. Keil, Jörg Ochsmann, FNA | Family List | FNA Vol. 19, 20 and 21 | Asteraceae |Centaurea; 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.
    FNA 2006, Kearney and Peebles 1969; Editors; S.Buckley 2010, F.S.Coburn 2015 from SEINet Field Guide, on-line; (accessed 06/03/2020).
    Wikipedia contributors, 'Centaurea melitensis', Wikipedia, The Free Encyclopedia, 15 September 2019, 20:50 UTC, [accessed 3 June 2020]
    Virginia Tech Dendrology; Department of Forest Resources and Environmental Conservation
    SEINet synonyms, scientific names, geographic locations, general information.
    Etymology:Michael L. Charters California Plant Names: Latin and Greek Meanings and Derivations; A Dictionary of Botanical and Biographical Etymology - (accessed 06/03/2020)