Synonym(s): Milk thistle, spotted thistle, variegated thistle
Duration and Habit: Annual, Biennial Herb
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.
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.
U.S. Nativity: Introduced to U.S.
Native Origin: S. marianum is an annual/biennial herb native to the 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.
List All Observations of Silybum marianum reported by Citizen Scientists
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.
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.
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.
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