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Mr. Smarty Plants - How to tell the difference between native and non-native thistles

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Wednesday - March 13, 2013

From: San Marcos, TX
Region: Southwest
Topic: Non-Natives, Plant Identification, Herbs/Forbs
Title: How to tell the difference between native and non-native thistles
Answered by: Nan Hampton

QUESTION:

It's thistle time already. There are many plants in the aster family with thistle in their common name. Are "real" thistles only those in the genus Cirsium, or are there others as well? We are trying to learn how to identify non-native thistles before they bloom without disturbing our natives. Any hints on differences (other than flowers) would be welcome. Thanks.

ANSWER:

There are several native plants that have "thistle" as part of their names (e.g., Argemone polyanthemos (Thistle poppy or Annual pricklepoppy) in the Family Papaveraceae (Poppy Family) and  Eryngium heterophyllum (Mexican thistle) in the Family Apiaceae (Carrot Family); but, as you mentioned Cirsium in the Family Asteraceae (Aster Family) is the major genus of what are generally thought of as native thistles in North America. The basal rosettes and general leaf morphology are the features to use for identifying which are native and which are non-native before they flower.  The ones that occur in Hays County or in adjacent counties are:

  • Cirsium horridulum (Yellow thistle), despite its unfortunate species name, is a native of Central Texas.  The flower color can be pink, yellow or white.  You can see more photos and information as well as a photo of the basal rosette of a young plant at Natives for Your Neighborhood from the Institute for Regional Conservation in South Florida. You can read a detailed botanical description from eFloras.org (the online version of the Flora of North America). 
  • Cirsium texanum (Texas thistle) occurs over nearly all of Texas with the exception of the Panhandle, far West Texas and East Texas bordering Louisiana. You can see a photo of a basal rosette of a young plant on Dave's Garden page and here are photos and more information from the University of Texas Biological Sciences' Archive of Central Texas Plant Species. You can read a detailed botanical description from eFloras.org (the online version of the Flora of North America). 

The only non-native Cirsium you are likely to find in Hays County is Cirsium vulgare (bull thistle).  You can see a photo of the basal rosette of a young plant at New Mexico Weed Information database and more photos from the Virginia Tech Weed Identification Guide.  You can read a detailed botanical description from eFloras.org (the online version of the Flora of North America).

There are a couple of species occurring in or near Hays County that are in the Genus Centaurea

Carduus is genus of thistles that are all introduced from Europe, Asia or North Africa. 

  • The species of Carduus that you are most likely to see in or around San Marcos and Hays County, Texas is Carduus tenuiflorus.  You can see more photos from CalPhotos Berkeley. If you click on Texas on the distribution map on the species page in the USDA Plants Database, you will get a county distribution map of Texas to see that it has been reported in Travis County.  

Silybum marianum (Blessed milkthistle) is another species of thistle from Europe and Asia that has been reported in Hays County. Its seeds have been used as herbal medicines since ancient times to treat liver diseases and other ailments.  Its leaves and basal rosette are easily identified since they have white net-like lines on them.

There are a couple of other non-native members of the Family Asteraceae that are called thistles that occur in Hays County:

Finally, there are a couple of plants in the Family Apiaceae (Carrot Family) that don't have the common name of thistle, but do look very much like thistles.  They both are native and occur in or adjacent to Hays County:

Eryngium leavenworthii (Leavenworth's eryngo) occurs in Hays County and Eryngium hookeri (Hooker's eryngo) occurs in Travis County.

Below are photos from our Image Gallery of the thistle or thistle-like species native to Central Texas.

 

From the Image Gallery


Yellow thistle
Cirsium horridulum

Yellow thistle
Cirsium horridulum

Yellowspine thistle
Cirsium ochrocentrum

Texas thistle
Cirsium texanum

Wavyleaf thistle
Cirsium undulatum

American basket-flower
Centaurea americana

Leavenworth's eryngo
Eryngium leavenworthii

Hooker's eryngo
Eryngium hookeri

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