Spanish Thistle (Carduus cinereus)
Introduction
Spanish thistle (Carduus cinereus), also called Turkish thistle, is newly introduced to North America. The plant originates from northern Africa and Eurasia. Most recently, Spanish thistle was identified in the Hells Canyon Wilderness in northeastern Oregon and adjacent western Idaho. This species is listed as a noxious weed in Oregon, and no populations are known in Montana. It was first detected in the United States in 2007, when it was misidentified as Italian thistle (Carduus pycnocephalus), but it was correctly identified more recently using morphological traits and genetic analysis. In January 2020, Gaskin and others published a description of Spanish thistle and a revised botanical key to identify Carduus species in North America. Interestingly, populations of Spanish thistle in North America are genetically distant (less than 94% similarity) from other Carduus species on the continent.
Identification and Biology
Spanish thistle, an annual forb in the Asteraceae family, ranges in height from 5 inches to 4 feet and has simple
to openly branched stems. The stems have toothed wings; 0.4 inch spines also occur along stem wings. Basal leaves up to 4 inches long are lobed and taper to winged petioles. Stem leaves are sessile with densely matted hairs on both sides. Flower heads are purple, about 0.5 inch long, and appear individually or in clusters of two to five at the end of wooly-textured branches. Spanish thistle is distinguished from Italian thistle most notably by its loosely clustered flower heads and dry, membranous bract margins.
Habitat and Spread
Reproduction occurs through seeds, which likely spread by animals and wind. In Oregon and Idaho, Spanish thistle was found in areas dominated by native bunchgrasses like Idaho fescue (Festuca idahoensis) and bluebunch wheatgrass (Pseudoroegneria spicata). Sites range from being dry and rocky on south-facing slopes to deeper soils in more moist areas.
Impacts
Spanish thistle is suspected to have similar negative impacts as other exotic Carduus species. The largest known infestations are in backcountry areas where impacts to local plant communities are evident. Agricultural impacts are currently unknown.
Management Options
Be on the lookout for this new thistle species in Montana. Early identification and control of Spanish thistle are key at present time. Because Spanish thistle was thought to be Italian thistle, it was managed as such. However, land managers will need to reevaluate effective management options in the coming years. For instance, the root crown weevil (Trichosirocalus horridus) used on thistle species was released into Spanish thistle infestations in Oregon, but it is believed the weevil did not establish and thus may not be an appropriate biological control for this Carduus species.
Further Information
For more information on this month's Weed Post, contact Extension Invasive Plant Specialist Jane Mangold. Past posts are available in the Monthly Weed Post directory.
Photos provided by the USGS.
This month's Weed Post is also available as a Printable PDF (425KB).
