Family: Asteraceae (Aster Family)
Duration and Habit: Biennial Herb
Branched, robust biennial (or sometimes annual) that often grows 2.5 m or more in height and 2 m in width. Main stems may be up to 10 cm wide at the base. Stems have vertical rows of prominent, spiny, ribbon-like leaf material or "wings" that extend to the base of the flower heads. Leaves, which are armed with sharp, yellow spines, are up to 60 cm long and 30 cm wide. Upper and lower leaf surfaces are covered with a thick mat of cotton-like or woolly hairs, which give the foliage a gray-green appearance. The globe-shaped flower heads are borne in groups of 2 or 3 on branch tips. Flower heads are up to 5 cm in diameter, with long, stiff, needle-like bracts at the base. Flowers range from dark pink to lavender. Seeds are smooth, slender, and plumed.
Native Lookalikes: Currently no information available here yet, or there are no native Texas species that could be confused with Scotch thistle.
Ecological Threat: Listed as a noxious weed or otherwise problem plant in 14 US states.
Biology & Spread: Onopordum acanthium is a herb of the sunflower family (Asteraceae) that is native to Europe and Asia. It has been introduced to temperate climates elsewhere, including much of North America and Australia. In North America, O. acanthium is a weed problem on western rangeland and produces significant economic losses for ranchers.
U.S. Habitat: In its native Europe, O. acanthium is well established in continental areas with summer-dry climates. In the western U.S., O. acanthium infests wet meadows and pastures, as well as more arid big sagebrush (Artemesia tridentata Nutt.) sites. Onopordum acanthium is often associated with waste places, as well as rivers, streams, canals, or other waterways. It can also be abundant in dry pastures, fields, and rangeland. In particular, the plant thrives in light, well drained, and sandy or stony soils. Temperature and moisture, rather than soil nutrient concentrations determine the ecological performance of Onopordum species.
U.S. Nativity: Introduced to U.S.
Native Origin: Europe and Asia
U.S. Present: AZ, CA, CO, CT, DC, FL, ID, IL, IN, KS, KY, MA, MI, MN, MO, MT, NE, NJ, NM, NV, NY, OH, OR, PA, RI, TX, UT, VA, VT, WA, WI, WV, WY
Distribution: May be found in most of North America with highest population concentration and impact in arid and semi-arid, western lands.
List All Observations of Onopordum acanthium reported by Citizen Scientists
Canada thistle [Cirsium arvense (L.) Scop.][CIRAR], bull thistle [Cirsium vulgare (Savi) Ten][CIRVU], and Nodding thistle [Carduus nutans (L.)][CANU4] may be confused with Scotch thistle.
Canada thistle is perennial, with creeping roots and small unisexual flower heads unlike Carduus thistles. Plants are either male or female (dioecious). In addition, Canada thistle has smooth stems and plumose pappus bristles.
Bull thistle is a coarse biennial with plumose pappus bristles and upper leaf surfaces covered with stiff bristly hairs that are rough to touch.
Nodding thistle has leaves that are dark green, coarsely lobed, with a smooth waxy surface and a yellowish to white spine at the tip. Flower heads will droop to a 90-degree angle from the stem when mature.
Physical: Small areas can be eradicated by digging. Plants must be cut off below the soil, leaving no leaves attached. Mowing has limited effectiveness for controlling O. acanthium. It usually only prevents seed production if done either immediately prior to flowering or when plants are just starting to flower. When mowing is conducted too early, it may only delay flowering. However, when plants are cut too late in the flowering process, viable seed may still develop in the capitula following cutting. Because there can be a wide variety in the maturity of plants, a single mowing is unlikely to provide satisfactory control. Onopordum acanthium invasions may be prevented by manipulating the cropping environment (cultural control methods). For example, establishing and maintaining dense, vigorous, competitive pasture can effectively prevent O. acanthium establishment. Healthy pasture is particularly important in the autumn, when most O. acanthium seeds germinate.
Chemical: For herbicide control, Picloram, dicamba, 2,4-D, dicamba + 2,4,-D, and metsulfuron are effective for controlling O. acanthium. Application rates vary depending on stand density and environmental conditions. Herbicides should be applied in the spring before O. acanthium bolts, or in the fall to rosettes.
Biological: Thistle invasion in unlikely to occur in ungrazed pasture. Goats will graze O. acanthium, reducing plant numbers and preventing seed production. No biological controls are currently available in the United States. Australia has released several biocontrol insects. Four control agents have been used in the biocontrol of O. acanthium a seed-feeding weevil Larinus latus was first released on 200 sites during 1993. It was found that the agents had eaten through 83% of the seed on released sites. A second control agent Lixus cardui, slower to spread than the first one was released a year later, this affected the growth of the plant. There are plans to release four more agents; thistle rosette destroying weevil Trichosirocalus sp. and a moth, Eublemma amoena, and two flies, Botanophila spinosa and Urophora terebrans which attack rosettes and seed respectivly. These control agents however, have failed host specificity tests in the U.S. Additional insects are being evaluated for release in the U.S.
USDA, NRCS. 2006. The PLANTS Database (http://plants.usda.gov, 7 November 2006). National Plant Data Center, Baton Rouge, LA 70874-4490 USA. (http://plants.usda.gov/java/profile?symbol=ONAC)
Global Invasive Species Database, 2005. Onopordum acanthium. Available from: http://www.issg.org/database/species/ecology.asp?si=295&fr=1&sts=(Accessed November 7, 2006)
USDA, NRCS. 2006. The PLANTS Database (http://plants.usda.gov, 7 November 2006). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
Global Invasive Species Database (http://www.issg.org/database)
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