Silybum marianum, Blessed Milkthistle

Gardens & Landscapes, Invasive Species, Extension Master Gardener October 11, 2012|Print

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

Plant profiles - www.CABI.org

Noxious Weed Control - Oregon DOA Plant Division


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