Carduus tenuiflorus, Slenderflower Thistle

Gardens & Landscapes, Invasive Species, Extension Master Gardener January 28, 2013 Print Friendly and PDF

Invasive Species: Carduus tenuiflorus, Slenderflower Thistle

Slenderflower thistle is an herbaceous winter annual or biennial plant that invades disturbed areas. The most serious infestations occur in the Pacific coastal states, but slenderflower thistle also occurs sporadically in the eastern United States. Plants grow up to 6 ft. (1.8 m) tall with single or multiple spiny stems with wings arising from the base of the spines. Stem wings can be up to 0.4 in. (10 mm) wide. Leaves are deeply lobed and also very spiny. The small flower heads (less than 1 in. [2.5 cm] long) are slender, purple, and lack stalks. Flowering occurs from May to July. Slenderflower thistle invades pastures, old fields, vacant lots, and other disturbed areas. Its presence reduces the forage productivity of pastures. Slenderflower thistle is native to Europe and was recognized as a serious invader in the western United States as early as 1956.

What are invasive species and why should we be concerned about them? 


Taxonomy: Scientific and Common Names for This Species

Asterales > Asteraceae > Carduus tenuiflorus W. Curtis

Synonym(s): Italian thistle, winged plumeless thistle

Carduus tenuiflorus - USDA PLANTS Profile

Distribution Maps

Slenderflower thistle - 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

Slenderflower thistle is an herbaceous winter annual or biennial invasive plant that grows up to 6 ft. (1.8 m) tall with single or multiple spiny stems with wings arising from the base of the spines. Stem wings can be up to 0.4 in. (10 mm) wide.

 
Evelyn Healy, University of California - Davis, bugwood.org bugwood.org

Foliage

Leaves are deeply lobed and also very spiny.

Eric Coombs, Oregon Department of Agriculture, bugwood.org Eric Coombs, Oregon Department of Agriculture, bugwood.org

Flower

The small flower heads (less than 1 in. [2.5 cm] long) are slender, purple, and lack stalks. Flowering occurs from May to July.

Joseph M. DiTomaso, University of California - Davis, bugwood.org Joseph M. DiTomaso, University of California - Davis, bugwood.org

Fruit

 

   
bugwood.org bugwood.org

Native Thistle Species That Can Resemble Slenderflower Thistle

 

Centaurea americana, American star-thistle - Images at invasive.org

Karan A. Rawlins, University of Georgia,  bugwood.org Karan A. Rawlins, University of Georgia,  bugwood.org

 

- Images at invasive.org

   
bugwood.org bugwood.org

 

Additional Images for Slenderflower Thistle

Slenderflower thistle - Images at Invasive.org

 

Learning Resources for Slenderflower Thistle

 

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 Service, or other related partner on this map provided by USDA.

Jepson Herbarium - University of California

Plant Profiles - Cal-IPC

Noxious weeds - King County

Slenderflower Thistle (Winged Slender or Seaside Thistle) - BUGWOOD Wiki

Slenderflower thistle - Invasive Plant Atlas


 

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.