Invasive Species: Silybum marianum, Blessed Milkthistle
Blessed milkthistle is an invasive annual/biennial herb native to the Mediterranean region. Plants range from 2-6 ft. (0.6-1.8 m) in height, depending on soil moisture. Leaves have shiny, green upper surfaces and are noticeably variegated with white markings. Basal leaves are 6-28 in. (15-70 cm) long with spiny margins. Much smaller cauline (stem) leaves clasp the stem. Flowering occurs from April to July, when purple thistle flower heads develop at the apex of the stems. Flower heads are 0.8-2.5 in. (2-6 cm) in diameter at the base and are enclosed in a projecting, spiny involucre. Fruit are black and brown achenes that are approximately 1/3 in. (6-8 mm) long, slightly flattened with a ring of bristles at the apex. Plants invade roadsides, ditches, disturbed areas, and fertile fields. Milk thistle can be poisonous to livestock, especially during periods of wilt from drought, mowing, or herbicide treatment.
What are invasive species and why should we be concerned about them?
Taxonomy: Scientific and Common Names for This Species
Asterales > Asteraceae > Silybum marianum (L.) Gaertn.
Synonym(s): blessed milk-thistle, milk thistle, spotted thistle, variegated thistle
Silybum marianum – USDA PLANTS Profile
Distribution Maps
Blessed milkthistle – The reported distribution of this invasive species across the United States (Source: Invasive Plant Atlas of the United States)
Up-to-the-minute distribution maps and why they are important
Reporting This Invasive Species
What is the best way and place to report the occurrence of an invasive species?
How to report an invasive species sighting to EDDMapS – Early Detection & Distribution Mapping System
EDDMapS – Report an invasive species to EDDMapS.
Cooperative Extension Offices – Find your local Cooperative Extension office on this map provided by USDA.
How to Identify
This invasive species can be identified by looking for the characteristics described in the paragraphs that follow.
Plant
Blessed milkthistle is an annual/biennial herb that can range from 2-6 ft. (0.6-1.8 m) in height, depending on soil moisture.
 |
 |
| Jan Samanek, State Phytosanitary Administration, bugwood.org | Eric Coombs, Oregon Department of Agriculture, bugwood.org |
Foliage
Leaves have shiny, green upper surfaces and are noticeably variegated with white markings. Basal leaves are 6-28 in. (15-70 cm) long with spiny margins. Much smaller cauline (stem) leaves clasp the stem.
 |
 |
| Forest & Kim Starr, Starr environmental, bugwood.org | Joseph M. DiTomaso, University of California-Davis, bugwood.org |
Flower
Flowering occurs from April to July, when purple thistle flower heads develop at the apex of the stems. Flower heads are 0.8-2.5 in. (2-6 cm) in diameter at the base and are enclosed in a projecting, spiny involucre.
 |
 |
| Jan Samanek, State Phytosanitary Administration, bugwood.org | John M. Randall, The Nature Conservancy, bugwood.org |
Fruit
Fruit are black and brown achenes that are approximately 1/3 in. (6-8 mm) long, slightly flattened with a ring of bristles at the apex.
 |
 |
| Joseph M. DiTomaso, University of California-Davis, bugwood.org | D. Walters and C. Southwick, CPHST, bugwood.org |
No native species in this genus
– Images at invasive.org
| bugwood.org | bugwood.org |
– Images at invasive.org
| bugwood.org | bugwood.org |
Additional Images for Blessed Milkthistle
Blessed milkthistle – Images at Invasive.org
Additional Information, Biology, Control and Management Resources
Control and management recommendations vary according to individual circumstances. Location, habitat, weather, and a variety of other conditions are factors that help determine the best treatment choice. To find the safest and most effective treatment for your situation, consult your state’s land-grant institution. If you will use chemicals as part of the control process, always refer to the product label.
United States Land-Grant University System – Find your land-grant university’s college of agriculture, Cooperative Extension office, or other related partner on this map provided by USDA.
Element Stewardship Abstract – The Nature Conservancy
Plant Profiles – Cal-IPC (California Invasive Plant Council)
Noxious weeds – King County, Washington
Noxious Weed Control – Oregon DOA Plant Division