Carnegiea gigantea, Giant Saguaro

Southwest Desert Flora

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

Cirsium ochrocentrum, Yellowspine Thistle

Laennecia coulteri, Coulter's Horseweed

Laennecia coulteri, has beautiful, but small white and yellow flowers; Coulter's Horseweed Laennecia coulteri, showing a large cluster of beautiful white and yellow flowers; Coulter's Horseweed Coulter's Horseweed has very small white flowers with yellow centers. This species, with its sap releasing glands appears to attract Aphid insects. Aphids are sap sucking insects. Laennecia coulteri Coulter's Horseweed has a raceme inflorescence and both male and female flowers. A careful examination shows a couple of Aphids likely enjoying the sappy extruding’s of the flowers glands. Laennecia coulteri Coulter's Horseweed has green leaves, both from a basal rosette as well as along the upper stems. Leave are sessile and often clasping to the stems in an alternate fashion. Laennecia coulteri Coulter's Horseweed has many clustered stem leaves that are oblong and with irregular dentition. Note that toothy notches are mostly around the base of the leaf. Laennecia coulteri Coulter's Horseweed blooms from April to October and year around in some places. Elevation preferences are sea level to about 5,000 feet or more. Plants are common in fields, stream banks and river bottoms and in disturbed areas. Laennecia coulteri

Scientific Name: Laennecia coulteri
Common Name: Coulter's Horseweed

Also Called: Annual Horsetail, Conyza, Coulter's Woolwort

Family: Asteraceae, Sunflower Family

Synonyms: (Conyza coulteri, Conyza coulteri var. virgata, Erigeron discoideus, Eschenbachia coulteri)

Status: Native

Duration: Annual

Size: 4 inches to 3 feet (10-100 cm) or more 5 feet (150 cm)

Growth Form: Forb/herb; plants short or tall, multiply stems, glandular and covered with fine white sticky hairs (arachnoid), similar to a spider's web.

Leaves: Green; leaves both basal and cauline; cauline leaves alternate and mostly without a leaf stalk (sessile) and others clasping to stem; mature leaves oblong, irregular dentition mostly around the leaf base; leaves with minute somewhat bristly hairs and with long soft, straight hairs (hirtellous and (villous).

Flower Color: White with yellow center; small floral heads; inflorescence a raceme; pistillate florets 60 to 100 per head; disk florets 5 to 20 per head; fruit a cypsela with a pappus of white straight stiff hairs (bristles).

Flowering Season: April to October or throughout the year

Elevation: Sea level to 6,000 feet (0-1,800m)

Habitat Preferences: Fields, plains, stream banks and river bottoms, roadsides and other disturbed sites; soils; clay or sandy and often alkaline soils.

Recorded Range: Coulter's Horseweed is found in the southwestern United States and Mexico; in the United States it is found in AZ, CA, CO, NM, NV, TX, UT and in the northern parts of Mexico and in Baja California.

North America & US County Distribution Map for Laennecia coulteri.

North America species range map for Coulter's Horseweed, Laennecia coulteri:
North American range map courtesy of Virginia Tech, Dept. of Forest Resources & Environmental Conservation

North America species range map for Coulter's Horseweed, Laennecia coulteri: Click image for full size map.
Click image for full size map

U.S. Weed Information: Unknown
Invasive/Noxious Weed Information: Unknown
Threatened/Endangered Information: Unknown

Wetland Indicator: In North America Laennecia coulteri has the following wetland designations:

  • Arid West, FAC
  • Atlantic and Gulf Coastal Plain, FACW;
  • Eastern Mountains and Piedmont, FAC;
  • Great Plains, FACW;
  • Western Mountains, Valleys, and Coast, FAC;

  • FACW = Facultative Wetland, usually occur in wetlands, but may occur in non-wetlands
    FAC = Facultative, occur in wetlands and non-wetlands.

    Genus Information: In North America there are 6 species and 6 accepted taxa overall for Laennecia. Worldwide, The Plant List includes 13 accepted species names and a further 6 scientific names of infraspecific rank for Laennecia.

    The genus Laennecia was published by Alexandre Henri Gabriel de Cassinni, (1781-1832) in 1822.

    In the Southwestern United States: Arizona and Texas each have 4 species of Laennecia, California, Nevada and Utah each have 1 species and New Mexico has 3 species. All data approximate and subject to revision.

    Comments: Coulter's Horseweed somewhat resembles Canadian Horseweed Conyza canadensis which has ray florets and Coulter's Horseweed does not.

    In Southwest Desert Flora also see: Canadian Horseweed, Conyza canadensis.

    Importance to Wildlife, Birds and Livestock
    Seeds of Laennecia coulteri may likely be eaten by birds and small mammals.

    Special Value to Native bees, Butterflies and Insects
    Laennecia coulteri flowers are visited by butterflies, bees and other small insects.

    The genus Laennecia (Laenne'cia:) is named for the French physician René Théophile Hyacinthe Laennec (1781-1826), a professor of clinical medicine.

    The genus Laennecia was published by Alexandre Henri Gabriel de Cassinni, (1781-1832) in 1822.

    The specific epithet, coulteri (coul'teri:) is named in honor of Thomas Coulter, (1793-1843).

    Note this species name recognition should not be confused with John Merle Coulter (1851-1928) who graduated at Hanover College, Indiana, and was botanist to the United States geological survey of the territories in the Rocky mountain system. That Dr. Coulter was honored with the species Erigeron coulteri.


    Date Profile Completed: 05/25/2020
    Kearney, Thomas H., Peebles, Robert H., 1960, Arizona Flora, University of California Press, Berkley and Los Angeles, California; as Conyza coulteri.
    U.S. Dept. of Agriculture, Natural Resources Conservation Service on-line database and USGS ITIS search - (accessed 04/24/2020)
    The Plant List (2013). Version 1.1. Published on the Internet; (accessed 04/24/2020).
    Native Plant Information Network, NPIN (2013). Published on the Internet; (accessed 04/24/2020). Lady Bird Johnson Wildflower Center at The University of Texas, Austin, TX.
    Bruce G. Baldwin, adapted from Strother (2006) 2012, Laennecia coulteri, in Jepson Flora Project (eds.) Jepson eFlora, /eflora/eflora_display.php?tid=3698, accessed on February 17, 2020.
    John L. Strother, FNA | Family List | FNA Vol. 20 | Asteraceae | Laennecia; 6. Laënnecia coulteri (A. Gray) G. L. Nesom, Phytologia. 68: 217. 1990.; Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 16+ vols. New York and Oxford.
    Kleinman, Dr. Russ,Vascular Plants of the Gila Wilderness, Presented in Association with the Western New Mexico University Department of Natural Sciences; Laennecia coulteri, Coulter Horseweed, Coulter's Woolwort) (accessed 02/17/2020).
    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 02/17/2020)