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Federal Noxious Weed TDA Noxious Weed TPWD Prohibited Exotic Species 
Invasive Plant Atlas of the USNOTE:
means species is on that list.
Silybum marianum
Blessed Milk Thistle
Synonym(s): Milk thistle, spotted thistle, variegated thistle
Family: Asteraceae
Duration and Habit: Annual, Biennial Herb
Photographer: Charles T. Bryson
Source: USDA Agricultural Research Service, Bugwood.org
Description
A robust, branching annual/biennial growing from 2 to 6 feet in height. Flowerheads are presented in many small florets in heads from 2-6 inches wide. Spine-tipped, bracts are present around heads in several rows up to 2 inches in length. Leaves are somewhat hairy, presenting lobed basal leaves from 8-20 inches in length. Leaves are variegated green and white and very spiny.
Native Lookalikes: Currently no information available here yet, or there are no native Texas species that could be confused with Blessed Milk Thistle.
Ecological Threat: Milk thistle is poisonous and can cause nitrate poisoning in cattle and sheep. Well established infestations reduce forage in pastures and rangeland settings. Also, their large rosettes (up to 3 feet in diameter) block light to nearby vegetation and suppress germination and growth. Plants can reach 2-3 m in height in dense stands, and plant skeletons continue to stand for several months, keeping an area bare of other vegetation.
Biology & Spread: S. marianum reproduces by seed, producing over 6,000 seeds per plant annually. Seeds are spread by erosion, human activity, rain or livestock. Seeds can remain viable for over 9 years in the soil.
History: Likely introduced as a folk medicine.
U.S. Habitat: Likely to be found in disturbed areas, pastures or right-of-ways.
Distribution
U.S. Nativity: Introduced to U.S.
Native Origin: Mediterranean region.
U.S. Present: AL, AR, AZ, CA, CO, CT, DC, GA, IN, LA, MA, MD, MI, MS, NH, NM, NV, NY, OH, OK, OR, PA, TN, TX, VA, WA, WV
Distribution in Texas: Distribution in Texas is limited and only known to be present in northeast and central Texas.
Mapping
Invaders of Texas Map: Silybum marianum
EDDMapS: Silybum marianum
USDA Plants Texas County Map: Silybum marianum
Invaders of Texas Observations
List All Observations of Silybum marianum reported by Citizen Scientists
Native Alternatives
- Cirsium texanum (Texas thistle)
- Cirsium horridulum (Yellow thistle)
- Centaurea americana (American basket-flower)
Management
For small infestations, milk thistle can be controlled by pulling or digging up rosettes or the bolted plant before seed heads develop. Use a shovel to cut the taproot under the soil so the plant will not resprout.
For larger infestations, foliar applied, broadleaf herbicides are most effective when applied during active plant growth. 2,4-D, metsulfuron methyl or aminopyralid are known to be effective and will not harm grasses if properly applied. Non-selective herbicides such as glyphosate are effective, but will harm grasses.
Text References
Noxious Weeds of Australia. W. Parsons and E. Cuthbertson, 1992, pages 229–233. World Weeds: Natural Histories and Distribution. Holm et al., 1997, pages 775–786.
Sindel, B.M. 1997. "The persistance and management of thistles in Australian pastures." New Zealand Plant Protection Society.
Whitson, T.D. (ed.) et al. Weeds of the West. Western Society of Weed Science in cooperation with Cooperative Extension Services, University of Wyoming, Laramie, Wyoming.
Online Resources
Search Online
Google Search: Silybum marianum
Google Images: Silybum marianum
NatureServe Explorer: Silybum marianum
USDA Plants: Silybum marianum
Invasive Plant Atlas of the United States: Silybum marianum
Bugwood Network Images: Silybum marianum
