Ask Mr. Smarty Plants

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 ochrocentrum (Yellowspine thistle), a native species, has been reported as occurring in Hays County.  You can see a photo of the basal rosette of a young plant at New Mexico Weed Information database and from the County of Santa Barbara, CA Weed Management page.  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). 

• Cirsium undulatum (Wavyleaf thistle) has been reported from neighboring Comal County.  You can see a photo of a basal rosette from Wild Utah. 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. 

• Centaurea americana (American star-thistle or basket-flower) is a native with a flower that resembles the Cirsium species but without all the prickles.  Here are more photos from the University of Texas Biological Sciences' Archive of Central Texas Plant Species.

• The other species that occurs in Hays County is the non-native Centaurea melitensis (Maltese star-thistle).  It is one of the "bad guys" and appears in the Texas Invasives Database.  You can see photos of its basal rosette on the Santa Monica National Recreation Area website.

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.  

• Carduus nutans (nodding plumeless thistle) is also a possibility (but is reportedly rare) occurring in Blanco and Kendall Counties and west.  You can see a photo of a young plant at the New Mexico State University Weed Information database.  Here is more information from Plant Conservation Alliance's Alien Plant Working Group's "Least Wanted" list

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:

• Sonchus asper (Spiny sowthistle) is native to Europe and occurs in Hays County.  Here are photos of young plants from the Virginia Tech Weed Identification Guide and more from the Weedalogue.

• Sonchus oleraceus (Common sowthistle) is native to Europe and Asia and occurs in Hays County.  You can see a photo of a young plant at the New Mexico State University Weed Information database and more from the Virginia Tech Weed Identification Guide.

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.