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Tuesday - April 19, 2011

From: San Marcos, TX
Region: Southwest
Topic: Invasive Plants
Title: How to tell the difference between native and European thistles
Answered by: Nan Hampton

QUESTION:

How can I tell the difference between invasive (European) thistles and thistles that are native to Texas? And what is the best way to eradicate the invasive varieties?

ANSWER:

According to the USDA Plants Database there are six different genera of plants that occur in Texas that have species with 'thistle' as part of their common name. Some of the species in those genera are native and others aren't. All of them, even those that are native, have been listed by at least one state as noxious. They are:

Centaurea melitensis (Maltese star-thistle)—non-native and noxious, appears on the Texas Invasives Database. You can see a description of two of the species below from eFloras. Occurs in Hays County.

Silybum marianum (blessed milkthistle) non-native and noxious. You can see a description of this species at eFloras. Occurs in Hays County.

Carthamus lanatus (woolly distaff thistle) non-native and noxious. You can see a description of this species at eFloras. No record in or near Hays County.

Onopordum acanthium (Scotch cottonthistle) non-native and noxious, appears in the Texas Invasives Database. You can see a description of the species in eFloras. No record in or near Hays County.

Cirsium sp. You can see the descriptions of the species below from eFloras. (Note: Arkansas and Iowa have put all Cirsium spp., native and non-native, on their Noxious Weeds lists):

Cirsium altissimum (tall thistle)—native and noxious. No record in or near Hays County.
Cirsium carolinianum (soft thistle)—native and noxious. No record in or near Hays County.
Cirsium engelmannii (Engelmann's thistle)—native and noxious. No record in or near Hays County.
Cirsium horridulum (yellow thistle)—native and noxious. Occurs in Hays County.
Cirsium x iowense—native and noxious [cross between C. altissimum and C. discolor]
Cirsium muticum (swamp thistle)—native and noxious. No record in or near Hays County.
Cirsium ochrocentrum (yellowspine thistle)—native and noxious. Occurs in Hays County.
Cirsium texanum (Texas thistle)—native and noxious. Occurs in Hays County.
Cirsium turneri (cliff thistle)—native and noxious. No record in or near Hays County.
Cirsium undulatum (plumed thistle)—native and noxious. Recorded in Comal County, but not Hays County.
Cirsium vulgare (bull thistle)—non-native and noxious, appears on the Texas Invasives Database. Occurs in Hays County.

Carduus sp. You can see descriptions of the species below on eFloras. (All species appearing in Texas are non-native and considered noxious.):

Carduus acanthoides (spiny plumeless thistle) non-native and noxious. No record in or near Hays County.
Carduus nutans (nodding plumeless thistle) non-native and noxious, appears on the Texas Invasives Database. Reported by USDA as occurring in Blanco County, but not Hays County.
Carduus pycnocephalus (Italian plumeless thistle) non-native and noxious. No record in our near Hays County.
Carduus tenuiflorus (winged plumeless thistle) non-native and noxious, appears on the Texas Invasives Database. Reported by USDA as occurring in Travis County, but not Hays County.

Salsola sp. You can see descriptions of the species below on eFloras:

Salsola collina (slender Russian thistle) non-native and noxious. No record in or near Hays County.
Salsola kali (Russian thistle) non-native and noxious. No record in or near Hays County.
Salsola tragus (prickly Russian thistle)—non-native and noxious, appears in the Texas Invasives Database. No record in or near Hays County.

To summarize, assuming you are asking advice for the thistles that occur in Hays County, Cirsium horridulum (Yellow thistle), Cirsium ochrocentrum (Yellowspine thistle) and Cirsium texanum (Texas thistle) are native thistles that occur in Hays County and Cirsium undulatum (Plumed thistle) is a native that occurs in adjacent Comal County.

The following non-native thistles occur in Hays or adjacent counties: Centaurea melitensis (Maltese star-thistle), Silybum marianum (blessed milkthistle), Cirsium vulgare (bull thistle), Carduus nutans (nodding plumeless thistle) occurs in adjacent Blanco County and Carduus tenuiflorus (winged plumeless thistle) occurs in adjacent Travis County. The first two, Maltese star-thistle and blessed milkthistle, are relatively easy to distinguish from any of the native species in Hays County. The other three may be a bit more difficult to distinguish from the natives, but the eFloras descriptions for each should help in determining which is which. All of the invasives except Silybum marianum (blessed milkthistle) appear in the Texas Invasives Database with Management guidelines. You can find information for managing Silybum marianum from King County (Washington) Noxious Weed Control Program.

Cirsium horridulum

Cirsium ochrocentrum

Cirsium texanum

Cirsium undulatum