Wyoming Thistle
Field Guide
Native and Non-native
Bonnie Heidel
B-1316
February 2022
About the author: Bonnie Heidel is lead botanist at the Wyoming Natural Diversity Database where she collects and maintains information on rare plants in the state. She is a weed control ally in work and pastimes.
Design: Andrea Perez, Tanya Engel
Front cover photo: Wyoming thistle (Cirsium pulcherrimum var. pulcherrimum) has a species scientific name derived from the Latin word “pulchra”, meaning beautiful. Photo: Bonnie Heidel
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Introduction
Thistles are common across Wyoming. Many people associate thistles with invasive weeds, but Wyoming has 19 native thistles! This guide presents all thistles in Wyoming to enable the user to distinguish native from invasive ones. Numerous photos are provided with brief descriptions to assist with identifications. However, the most reliable method for identification is to use a key, which step by step, and character by character, will lead the user to the correct species name. The keys presented in this guide are intended for non‑experts. See pages 6–7 for thistle plant terms. Getting to know native plants is always worthwhile, and learning to distinguish among the many thistle species will allow the user to focus control efforts only on the weed species, and to appreciate the natives as valuable in Wyoming landscapes.
Why is thistle identification important?
Native thistles provide valuable resources to insects, birds and small mammals. They attract an abundance of nectar‑ and pollen-feeding insects, many of which are important pollinators. Bumblebees, longhorned bees, leafcutter bees, sweat bees, hover flies, beetles, moths and butterflies, including monarchs and regal fritillaries, are all visitors to thistle flowers (Eckberg et al. 2017). In addition, many kinds of herbivorous insects, including butterfly and moth caterpillars, feed on the leaves and stems of our native thistles.
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And, of course, a number of bird species rely heavily on thistle seeds for nourishment. Thistle seeds are very high in protein and are available during the breeding season and late into the fall. Even large herbivores, such as elk, feed on thistles.
Non‑native thistles, on the other hand, display a variety of characters that outweigh any benefits they may bring. In particular, non‑native thistles are capable of rapid spread, displacing all kinds of native species.
They impact agriculture, degrade habitat quality, decrease animal and plant diversity, and reduce land value. In fact, non‑native thistles are arguably one of the most economically detrimental groups of invasive plant species across the United States. Not all invasive thistles spread in the same way, so effective control of invasive thistles depends on correct identification.
Unfortunately, native thistles are often mistaken for invasive thistles and treated as weeds, to the detriment of the landscape and inefficient use of treatment resources. This makes identification of native thistles just as important as identification of invasive thistles. Know the difference!
What is a thistle?
The Wyoming Thistle Field Guide addresses a closely related group of plants in the Aster Family (Asteraceae; also called the Sunflower Family). They are in three genera (Cirsium, Carduus,
Onopordum) that all bear a common name of
“thistle", and they belong to the same taxonomic tribe (Cardueae).
You’ll see a common pattern as you use this guide. They have flower heads, each of which is composed of many small flowers (florets), surrounded by series of bracts (the involucre).This is true for all members of the Aster Family. This field guide does not require investigating the very small features of flowers or the resulting seeds (achenes). We know thistles by their prickliness—most have sharp spines on the leaves and flower heads.
“Thistle" is also commonly used in the names of other prickly genera within the Aster Family including Globe thistle and Sow thistle, but they have spherical flower heads (Globe thistle) or yellow flowers (Sow thistle) rather than upturned flower heads with purple to white flowers of the thistles covered in this guide. Do not be deceived by common names—even prickly plants outside of the Aster Family have been called thistles, e.g., Russian thistle.
The line drawings on the next page depict the most common species in each of the thistle genera included in this guide. Learn to recognize them—they are designated as noxious weeds in the state. The non‑technical key will help identify these three and the additional two noxious thistles in the state. It will also help you confirm if a thistle in question is native.
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Canada thistle
(Cirsium arvense)
Musk thistle
(Carduus nutans)
Scotch thistle
(Onopordum acanthium)
Illustrations: John H. Rumley, Vascular Plants of the Intermountain West, USA, Volume 5, reprinted with permission of New York Botanical Garden.
Things to look for
1. Look for bracts on each flower head, in a series from the outer bracts (lower, bottommost) to inner bracts (top, uppermost). The bract series is also called the involucre. The length of the involucre is a useful measurement. Pick the biggest flower head (flowering or past) on the plant to measure and use the ruler on the inside of the front cover. Bracts may have distinctive features such as a ridge, a fringed tip, a particular shape (narrow or broad), or particular surface (smooth, hairy, etc.). A hand lens may be necessary.
2. Here’s an easy clue. What color are the flowers (florets)? Our native thistles have white or creamy‑white flowers; thistles with pink to purple flowers may be native or non‑native.
3. Look at the base of the stem. Dense colonies of separate stems indicates the presence of horizontal underground stems (rhizomes).
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Flower
head
Florets
Involucre
Made up of many bracts
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4. Look for prominent spiny wings on the stem. If spiny wings run up and down the full length, then it is a non‑native thistle. If spiny wings are interrupted or absent along the stem, then it could be native or non‑native.
Plants growing as a cluster of basal leaves radiating out from a central point (rosette) can be difficult to identify. If there are stalks with flower heads from the prior year, they might give you enough clues to determine whether or not the thistle is non-native.
Illustrations: Katherine Benkman
8 | Guide to non-native thistles
How to use this guide
To identify an unknown thistle, turn to the
dichotomous key on page 9. The key consists of a series of two mutually exclusive statements, known as couplets. Starting with the first couplet, choose the statement that best fits the unknown thistle. The corresponding number will lead to another
couplet and ultimately to the plant’s name. Below is an example.
1a. Plant has spiny winged stems the entire length..............................................................2
1b. Plant does not have spiny winged stems the
entire length....................................................5
The two choices, 1a and 1b, form a couplet. If your unknown plant has spiny winged stems the entire length, then proceed to couplet 2.
2a. Involucres are less than ¾" long, and flower
heads are typically densely clustered on short
stalks...Plumeless thistle (Carduus acanthoides)
2b. Involucres are greater than ¾" long, and
flower heads are mostly solitary on longer
stalks...............................................................3
If the plant matches the description in 2a, then choose “Plumeless thistle”. If it matches the description in 2b, then proceed to couplet 3. Always read each couplet carefully before proceeding to the next step. When you arrive at a name, always check the species description to be sure it is consistent with your plant. If it is not, re‑evaluate the choices you made in getting to this place in the key. For native thistles, you might also cross‑check identifications against the map on pages 66–67.
Guide to non-native thistles | 9
Key to Wyoming thistles—Part 1
1a. Plant has spiny winged stems the entire
length..............................................................2
1b. Plant does not have spiny winged stems the
entire length....................................................5
2a. Involucres are less than ¾" long, and flower
heads are typically densely clustered on short
stalks.....................................Plumeless thistle
(Carduus acanthoides)
2b. Involucres are greater than ¾" long, and
flower heads are mostly solitary and on longer
stalks...............................................................3
3a. Foliage is silver gray or bluish gray in appear-
ance....Scotch thistle (Onopordum acanthium)
3b. Foliage is green, not gray...............................4
4a. Bracts are broad and triangular, flower heads
often nod or bend..........................Musk thistle
(Carduus nutans)
4b. Bracts are narrow, needle-like; flower heads
are upright...........Bull thistle (Cirsium vulgare)
5a. Plants arise from rhizomes, often forming
dense colonies; involucre is less than ¾"
long...............Canada thistle (Cirsium arvense)
5b. Plants arise from taproots or branched
rootstock; involucre is greater than ¾" long...6
6a. Upper leaf surface is covered with many
short, sharp spines; outer bracts tend to point
outward rather than upward........Bull thistle
(Cirsium vulgare)
6b. Go to the Part 2 Key, pg. 20
The Part 2 Key covers all native thistles.
Guide to non-native thistles
Bull thistle
Cirsium vulgare
Robert Dorn
Jennifer Ackerfield
Biennial arising from a taproot, with one to many stems 1 ft–5 ft tall. Flowers are dark purple to pinkish-purple. Involucres are 1"–1½" long. Flower heads have urn-shape outlines that tapers on top and are in loose clusters. Bracts have needle-like tips and tend to point outward more than upward. Leaves have narrow lobes tipped with spines, and the upper leaf surface is covered by many short, bristly spines. Stems are usually much-branched, with spiny wings that usually extend for at least ¾" below leaf.
Things to look for:
Bracts with fine needle-like tips that tend to point outward more than upward, leaves with upper leaf surface that is rough and covered by short bristly spines.
10 | Guide to non-native thistles
Noxious Weed
Non-native
Guide to non-native thistles | 11
Ben Legler
Statewide in roadsides, pastures, thickets, lowlands and other disturbed areas, within grasslands and woodlands.
Wendy Haas
Robert Dorn
Canada thistle
Cirsium arvense
Robert Dorn
Perennial arising from underground rhizomes, with stems ½ ft–4 ft tall, often forming colonies. Flowers are pink or purple. Involucres are small, ½"–¾" long. Flower heads are solitary and often forming a flat-topped inflorescence. Bracts are hairless, pressed tight to the flower head, with fine bristles rather than spines, and often have distinct white midribs. Leaves have wavy margins and spine-tipped irregular lobes. Stems and leaves have short hairs, often densely woolly on lower leaf surface.
Things to look for:
Small flower head, bracts that end in fine bristles rather than stiff thorns, flower heads sometimes appear to form a flat-topped inflorescence, wavy leaf margin, often forming dense colonies.
12 | Guide to non-native thistles
Noxious Weed
Non-native
Guide to non-native thistles | 13
Robert Dorn
Bonnie Heidel
Statewide in roadsides, field margins, other disturbed areas and wet meadows, within grasslands, sagebrush and woodlands.
Musk thistle
Carduus nutans
Robert Dorn
Ben Legler
Biennial arising from a stout taproot, with one to several stems 1½ ft–7 ft tall. Flowers are rose purple. Involucres are wider than long, 1¼"–3" wide. Flower heads are solitary and often nod when in full flower. Bracts are broadly triangular, with a spine tip, and lowermost bracts are often bent backward. Leaves have spiny margins and surfaces that range from hairless to densely hairy. Stems are usually much-branched with spiny wings that run for most of the length of the stem without interruption. Basal leaves of vegetative plants have a silvery outline as seen from above.
Things to look for:
Large solitary flower heads that often nod, bracts broadly triangular, lowermost bracts often bend backwards, stems spiny-winged much of length.
14 | Guide to non-native thistles
Noxious Weed
Non-native
Guide to non-native thistles | 15
Ben Legler
Julie Kraft
Robert Dorn
Statewide in roadsides, pastures, lowlands, and other disturbed areas, within grasslands and woodlands.
Plumeless thistle
Carduus acanthoides
Robert Dorn
Biennial arising from a stout taproot, with one to several stems 1½ ft–5 ft tall. Flowers are pale pink to purple. Involucres are small, ½"–¾" long. Flower heads are solitary. Bracts in outer position have a spine tip. Leaf surfaces are sparsely hairy above, more hairy below. Stems are usually much-branched with spiny wings the full length of the stem.
Things to look for:
Small, solitary flower heads, stems spiny-winged the full length.
16 | Guide to non-native thistles
Noxious Weed
Non-native
Guide to non-native thistles | 17
Southeastern Wyoming in roadsides, pastures, fields, other disturbed areas, usually within grasslands.
Robert Dorn
Scotch thistle
Onopordum acanthium
Robert Dorn
Biennial arising from a stout taproot, with one to many stems 1 ft–7 ft tall. Flowers are purple, violet, reddish, or pinkish-white. Involucres are about ¾" long. Flower heads are in loose clusters. Bracts all have spines. Leaves have a bluish gray or silvery gray appearance and margins of leaves ae usually toothed rather than lobed. Stems have spiny wings running the entire length.
Things to look for:
Leaves with cobwebby hairs giving grayish appearance, stems that are spiny-winged the full length.
18 | Guide to non-native thistles
Noxious Weed
Non-native
Guide to non-native thistles | 19
Robert Dorn
Jennifer Ackerfield
Robert Dorn
Statewide in roadsides, pastures, thickets, lowlands, and other disturbed areas, within grasslands and woodlands.
Key to Wyoming thistles—Part 2
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Guide to native thistles
Barneby’s thistle
Cirsium barnebyi
Perennial arising from a taproot, with one to few stems 1 ft–1¾ ft tall. Flowers are lavender‑purple. Involucres are about ¾" long. Flower heads are in a loose cluster. Bracts are hairless to sparsely hairy, with a spine. Leaf surfaces on upper and lower sides are densely white to grayish woolly. Stems are often white-woolly.
Things to look for:
Small flower heads usually in loose clusters and usually as long as wide, leaves hairy on both sides.
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Native
Guide to native thistles | 27
Known from an area of Lincoln County on shale slopes, within desert scrub and sagebrush; also reported from Carbon County.
Beaver Rim thistle
Cirsium pulcherrimum var. aridum (Cirsium aridum)
Bonnie Heidel
Perennial arising from a branched rootstock, with one to few stems ½ ft–1½ ft tall. Flowers are white to rose pink. Involucres are ¾"–1¼" long, wider than long. Flower heads are usually 4 or fewer. Bracts may have long cobwebby hairs or none, with a spine. Leaves are white or gray woolly on top with dense white hairs below, with spiny margins. Stems usually have cobwebby hairs, sometimes spiny-winged for short distances below leaves.
Things to look for:
Flowers usually white, usually four or fewer flower heads, leaf surfaces with dense cob‑webby hairs below and less hairy above.
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Native Sensitive
Guide to native thistles | 29
Bonnie Heidel
Bonnie Heidel
Central Wyoming in barren uplands on chalky, gravelly, or fine-textured sandy-shale, within sagebrush.
Colorado thistle
Cirsium scariosum var. coloradense (Cirsium coloradense, C. tioganum var. coloradense)
Ben Legler
Biennial arising from a taproot, with a stout, single stem ½ ft–4 ft tall. Flowers are usually white, sometimes lavender or pinkish purple. Involucres are 1"–1½" long. Flower heads are in a cluster. Bracts are narrowed to a short spine tip usually with a clear, fringed margin. Leaves and stems have cobwebby hairs, sometimes hairless on upper leaf surface. Bristles on top of the seed are ¾"–1" long.
Things to look for:
Flowers usually white, flower heads in loose cluster on a stout stem, leaves and stems with cobwebby hairs, leaf tips mostly coming to a sharp point.
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Native
Guide to native thistles | 31
Bonnie Heidel
Bonnie Heidel
Southern and central Wyoming in alkaline wet meadows and lowlands, within sagebrush.
Drummond’s thistle
Cirsium drummondii
Robert Dorn
Biennial or short-lived perennial arising from a taproot, with a single stout stem ½ ft–2¼ ft tall, or sometimes stemless. Flowers are rose-purple. Involucres are 1¼"–2¼" long. Flower heads form a cluster. Bracts have a ridge and short hairs on margins. Outer bracts are broader than inner ones. The outer usually have spines at tips and the inner usually have fringed tips and a ridge. Leaves and stems with sparse long hairs, becoming hairless.
Things to look for:
Large flower heads, inner bracts wider than outer bracts and fringed at tip, stout stem or stemless.
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Native
Guide to native thistles | 33
Beth Burkhart
Beth Burkhart
Only known from the Black Hills, in open woodlands, clearings, and meadows.
Elk thistle
Cirsium foliosum
Jennifer Whipple
Biennial or short-lived perennial arising from a taproot, with a single stout stem ¾ ft–2½ ft tall. Flowers are white to pale pink. Involucres are ¾"–1" long. Flower heads are usually in a dense cluster. Bracts are hairless, with a short spine. Leaves have sparse hairs on both sides, and stems are densely hairy. Bristles on top of the seed are as long or longer than florets.
Things to look for:
Flowers usually white, small flower heads usually in a dense cluster, stout stem.
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Native
Guide to native thistles | 35
Jennifer Ackerfield
Jennifer Whipple
Only known from a few locales in Bighorn Mountains and Yellowstone National Park in moist, open woodlands and roadsides.
Flodman’s thistle
Cirsium flodmanii
Robert Dorn
Perennial arising from horizontal roots, with one to few stems 1 ft–3 ft tall. Flowers are deep purple to pink. Involucres are ¾"–1½" long. Flower heads are solitary. Bracts have a ridge and short spine, sometimes with cobwebby hairs. Leaves are white woolly below, becoming greenish above, those on the stem are shallowly to deeply lobed.
Things to look for:
Flower heads solitary, stem leaves shallowly to deeply lobed.
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Native
Guide to native thistles | 37
Robert Dorn
Bonnie Heidel
Statewide in moist meadows, lowlands and roadsides, within grasslands and sagebrush.
Fringed thistle
Cirsium clavatum var. americanum (Cirsium centaureae)
Bonnie Heidel
Biennial or short-lived perennial arising from a taproot, with one to few stems ¾ ft–3 ft tall. Flowers are white to lavender-purple, forming a loose cluster. Involucres are ¾"–1¼" long. Flower heads are in loose clusters. Bracts are fringed, translucent at the tip, hairless or with hairs at the margin. Leaves hairy below. Bristles on top of the seed are ½"–1" long.
Things to look for:
Flowers usually white, inner bracts with fringed margins and translucent tips, leaves hairy below, dark green and hairless above.
38 | Guide to native thistles
Native
Guide to native thistles | 39
Bonnie Heidel
Bonnie Heidel
Southeastern Wyoming in open woodlands and clearings.
Graygreen thistle
Cirsium cymosum var. canovirens (Cirsium canovirens)
Jennifer Whipple
Biennial or short-lived perennial arising from a stout taproot, usually with a single stem, 1 ft–3¼ ft tall. Flowers are creamy white to purplish. Involucres are ¾"–1" long, and wider than long. Flower heads are solitary. Bracts have a ridge, may have cobwebby hairs at the margins, and at least the outer bracts are spine-tipped. Leaves have long, sparse hairs, more on the bottom leaf surface than the upper.
Things to look for:
Flowers usually white, flower heads solitary, bracts have a ridge.
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Native
Guide to native thistles | 41
Jennifer Whipple
Jennifer Whipple
Only known from northwestern corner of Park County in dry meadows and roadsides, within sagebrush and woodlands.
Greene’s thistle
Cirsium inamoenum (Includes var. inamoenum and var. davisii; Cirsium subniveum misappl.)
Bonnie Heidel
Biennial or short-lived perennial arising from a taproot, with one to several stems 1 ft–3¼ ft tall. Flowers are white to pink-purple. Involucres are ¾"–1¼" long. Flower heads are in loose clusters on densely hairy stalks. Bracts have a ridge and cobwebby hairs along the margins. Outer bracts are spine-tipped. Leaves usually have dense hairs on the lower leaf surface, less on the upper.
Things to look for:
Flower heads in loose clusters on densely hairy stalks. Flowers of var. inamoenum are white or pale lavender, those of var. davisii are dark lavender to pink-purple.
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Native
Guide to native thistles | 43
Bonnie Heidel
Bonnie Heidel
Western Wyoming in dry meadows and rocky slopes, within sagebrush and woodlands.
Meadow thistle
Cirsium scariosum var. scariosum
Richard Spellenberg
Bonnie Heidel
Biennial arising from a taproot, with a stout, single stem to 1½ ft tall or occasionally stemless. Flowers are usually pink to purple.
Involucres are 1"–1½" long. Flower heads form a dense cluster. Bracts are narrowed to a spine tip usually with a clear, fringed margin. Leaves and stems have cobwebby hairs, except sometimes hairless on upper leaf surface. Bristles on top of the seed are about ½" long.
Things to look for:
Flowers usually pink to purple, flower heads in dense cluster on stout stem or stemless, leaves and stems with cobwebby hairs, leaf tips mostly rounded.
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Native
Guide to native thistles | 45
Julie Kraft
Northwestern and southern Wyoming in montane meadows and open woods, within sagebrush and woodlands.
Julie Kraft
Murdock’s thistle
Cirsium eatonii var. murdockii (C. murdockii)
Charmaine Delmatier
Perennial arising from a taproot, also having creeping roots, with one to several stems,
¾ ft–2½ ft tall. Flowers are yellowish-white to lavender, pink or purple. Involucres are ¾"–1¼" long. Flower heads form a loose cluster. Bracts are often deep purple, with sparse or dense hairs, and with spines. The lower bracts often have branched spines. Leaves are green, hairless, and finely-divided into narrow spine-tipped lobes. Leaves and stems are variously hairless or hairy.
Things to look for:
Bracts often deep purple, lower bracts often have spines that are branched.
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Native
Guide to native thistles | 47
Charmaine Delmatier
Northern Wyoming in dry meadows and rocky slopes, in woodlands and above tree line.
Charmaine Delmatier
Ownbey’s thistle
Cirsium ownbeyi
Bonnie Heidel
Perennial arising from a taproot, with one to several stems, 1½ ft–2½ ft tall. Flowers are white to rose pink. Involucres are ¾"–1" long. Flower heads are in loose clusters. Bracts have long spines, and are hairless except for slender hairs at the margins. Leaves are green, hairless, and finely-divided into narrow, spine-tipped lobes. Stems are hairless or lightly pubescent and spiny-winged between leaves.
Things to look for:
Flowers usually white, bracts hairless except at margins, leaves hairless and finely-dissected with narrow lobes, stems hairless or lightly pubescent.
48 | Guide to native thistles
Native Sensitive
Guide to native thistles | 49
Bonnie Heidel
Walter Fertig
Bonnie Heidel
Only known from an area of Sweetwater County in barren uplands of shale and sandy‑clay, within sagebrush and pinyon‑juniper woodlands.
Prairie thistle
Cirsium canescens
Bonnie Heidel
Biennial or short-lived perennial arising from a taproot, with one to few stems 1½ ft–3 ft tall, densely hairy. Flowers are creamy white or yellowish-white. Involucres are usually 1¼" or less long, and at least as wide as long. Bracts have a dark ridge and spine tip. Leaves near bottom of stem have elongate lobes 3–6 times longer than wide, leaf surfaces have cobwebby hairs above, dense woolly hairs below.
Things to look for:
Flowers white, bracts with a ridge, lower leaves with elongate lobes.
50 | Guide to native thistles
Native
Guide to native thistles | 51
Bonnie Heidel
Bonnie Heidel
Statewide in uplands and roadsides, within grasslands and sagebrush.
Stemless thistle (Dinnerplate thistle)
Cirsium scariosum var. americanum
(Cirsium tioganum var. tioganum)
Bonnie Heidel
Perennial arising from a taproot, stemless. Flowers are usually white, sometimes pink‑tinged. Involucres are ¾"–1¼" long. Flower heads are in a compact cluster. Bracts are narrowed to a short spine tip usually with a clear fringed margin. Leaves are lobed or unlobed, with leaf tips that are often rounded or obtuse and hairless or nearly so on upper surface, with gray woolly hairs on lower surface. Bristles on top of the seed are about ½"–1" long.
Things to look for:
Flowers usually white, flower heads in a dense cluster, no stem.
52 | Guide to native thistles
Native
Guide to native thistles | 53
Bonnie Heidel
Bonnie Heidel
Southcentral Wyoming in wet meadows, within woodlands.
Tracy’s thistle
Cirsium tracyi
Al Schneider
Perennial arising from a taproot, with one to few stems 1¼ ft–3 ft tall. Flowers are lavender to white or pink purple. Involucres are ¾"–1" long. Flower heads are solitary. Bracts have a ridge and may be hairless or with sparse cobwebby hairs. Leaves are gray woolly on the bottom, less so on top. Stem leaves are shallowly or deeply lobed on the margins.
Things to look for:
Flower heads solitary, stem leaves shallowly to deeply lobed.
54 | Guide to native thistles
Native
Guide to native thistles | 55
Al Schneider
Only known from a couple of records in Uinta County in dry, open, or wooded slopes, within sagebrush.
Wavyleaf thistle
Cirsium undulatum
Robert Dorn
Perennial arising from a taproot, with one to few stems 1¼ ft–3 ft tall. Flowers are deep to pale purple. Involucres are ¾"–1½" long. Flower heads are solitary. Bracts have a ridge and may be hairless or with sparse cobwebby hairs. Leaves are white woolly on both sides, especially below. Stem leaves are toothed or shallowly lobed on the margins, but basal leaves are deeply lobed.
Things to look for:
Flower heads solitary, stem leaves toothed or shallowly lobed.
56 | Guide to native thistles
Native
Guide to native thistles | 57
Ben Legler
Robert Dorn
Statewide in uplands and roadsides, within grasslands and sagebrush.
White thistle
Cirsium hookerianum
Ben Legler
Biennial or short-lived perennial arising from a taproot, usually with a single stem, ¾ ft–2¾ ft tall. Flowers are white to pale purple.
Involucres are ¾"–1¼" long. Flower heads are variously solitary or clustered. Bracts have cobwebby hairs and spines and sometimes have a ridge. Leaves and stems have woolly white hairs, especially on the lower leaf surface.
Things to look for:
Flowers usually white, outer bracts with cobwebby hairs, leaves with densely matted hairs on underside, less hairy above.
58 | Guide to native thistles
Native
Guide to native thistles | 59
Ben Legler
Ben Legler
Northern Wyoming in mountain meadows and woodlands.
Wyoming thistle
Cirsium pulcherrimum var. pulcherrimum
Bonnie Heidel
Perennial arising from a branched rootstock, with one to few stems ½ ft–2½ ft tall. Flowers are white to rose pink. Involucres are ¾"–1¼" long, and wider than long. Flower heads are often in groups of four or more. Bracts may have long cobwebby hairs or be hairless, with a spine. Leaves have dense white hairs below and are hairless or nearly so on top, with spiny margins. Stems usually have cobwebby hairs, sometimes spiny-winged for short distances below leaves.
Things to look for:
Flowers usually white, flower heads four or more, leaf surfaces with dense cobwebby hairs below and little or no hairs above.
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Native
Guide to native thistles | 61
Bonnie Heidel
Jennifer Ackerfield
Most of state in uplands and roadsides, within sagebrush and woodlands.
Yellowspine thistle
Cirsium ochrocentrum (Ours is C. o. var. ochrocentrum)
Robert Dorn
Perennial arising from creeping roots, with one to many stems 1¼ ft–3 ft tall, densely hairy. Flowers are pink to purple, rarely white.
Involucres are 1"–1¾" long, about 2" wide. Flower heads are clustered. Bracts have a ridge, sometimes have cobwebby hairs, and outer bracts have a stout spine that bends outward from the flower head. Leaves have cobwebby hairs, more on the lower leaf surface than the upper.
Things to look for:
Flower heads as wide or wider than long, leaves densely hairy below, less hairy above.
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Native
Guide to native thistles | 63
Robert Dorn
Jennifer Ackerfield
Robert Dorn
Mainly southeastern Wyoming in uplands and roadsides, within grasslands.
References and resources
Dorn, R. D. 2001. Vascular Plants of Wyoming. Mountain West Publishing. Cheyenne, WY. (Available in print through Rocky Mountain Herbarium bookstore).
Eckberg, J., E. Lee-Mäder, J. Hopwood, S. Foltz Jordan, and B. Borders. 2017. Native Thistles: A Conservation Practitioner’s Guide. Plant Ecology, Seed Production Methods, and Habitat Restoration Opportunities. 92 pp. Portland, OR: The Xerces Society for Invertebrate Conservation.
Nelson, B. E. and R. L. Hartman. 2017. Invasive plants of Wyoming. Rocky Mountain Herbarium, University of Wyoming. www.rockymountainherbarium.org.
Nelson, B. E. 2018. Wyoming Flora Checklist. Rocky Mountain Herbarium, University of Wyoming. www.rockymountainherbarium.org.
Parkinson, H. and J. Mangold. 2015. Guide to exotic thistles of Montana and how to differentiate from native thistles. http://msuinvasiveplants.org/documents/publications/extension_publications/Guide%20to%20thistles_EB0221.pdf
Rocky Mountain Herbarium 2021. Online specimen database. www.rockymountainherbarium.org.
University of Wyoming Extension—www.uwyo.edu/uwe
USDA Natural Resources and Conservation Service. The PLANTS Database—plants.usda.gov.
USDI Bureau of Land Management. Sensitive species in Wyoming—www.blm.gov/policy/im-wy-2010-027.
Whitson, T. D., L. C. Burrill, S. A. Dewry, D. W. Cudney, B. E. Nelson, R. D. Lee and R. Parker. 2012, 11th ed. Weeds of the West. Available as an ePub and downloadable PDF, bit.ly/weedswest.
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Wyoming Association of Conservation Districts—conservewy.com/
Wyoming Weed and Pest Council. Wyoming Noxious weed list, https://wyoweed.org
Acknowledgments
This publication was a team project. Dorothy Tuthill was a reviewer and facilitator at all stages. The text on guide useage and key to non-native thistles follows Parkinson and Mangold (2015) as used with permission. The key to native thistles follows Dorn (2001), using less technical terms. Photographs were contributed by Jennifer Ackerfield, Beth Burkhart, Charmaine Delmatier, Robert Dorn, Walter Fertig, Wendy Haas, Julie Kraft, Ben Legler, Al Schneider, Richard Spellenberg, and Jennifer Whipple. Rocky Mountain Herbarium provided invaluable resources including specimen images. Karen Clause, Robert Dorn, Julie Kraft, B. E. Nelson, J. Sheehan, Dan Tekiela, and Kimberly Wahl review of earlier versions improved the manuscript. This publication was supported by Wyoming Weed and Pest Council, Wyoming Association of Conservation Districts, USDI Bureau of Land Management, USDA Forest Service, USDA Natural Resources Conservation Service, UW Extension Service, UW Biodiversity Institute and UW Wyoming Natural Diversity Database.
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Internet addresses change and pages can disappear over time. If you find problems with any of the listed web sites in this publication, please contact the Wyoming Natural Diversity Database, wndd@uwyo.edu, (307) 766-3023.
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NW — Park, Teton
Elk thistle
Flodman’s thistle
Graygreen thistle
Greene’s thistle
Meadow thistle
Murdock’s thistle
Prairie thistle
Wavyleaf thistle
White thistle
Wyoming thistle
Yellowspine thistle
NWC — Big Horn, Hot Springs, Washakie
Colorado thistle
Flodman’s thistle
Greene’s thistle
Meadow thistle
Murdock’s thistle
Wavyleaf thistle
White thistle
Wyoming thistle
NE — Campbell, Crook, Weston
Drummond’s thistle
Flodman’s thistle
Prairie thistle
Wavyleaf thistle
Wyoming thistle
NEC — Johnson, Sheridan
Colorado thistle
Elk thistle
Flodman’s thistle
Meadow thistle
Murdock’s thistle
Prairie thistle
Wavyleaf thistle
White thistle
Wyoming thistle
Yellowspine thistle
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CE — Converse,
Natrona, Niobara
Colorado thistle
Flodman’s thistle
Meadow thistle
Prairie thistle
Wavyleaf thistle
Wyoming thistle
Yellowspine thistle
SE — Goshen, Laramie, Platte
Flodman’s thistle
Meadow thistle
Prairie thistle
Wavyleaf thistle
Wyoming thistle
Yellowspine thistle
SC — Albany, Carbon
Colorado thistle
Flodman’s thistle
Fringed thistle
Meadow thistle
Prairie thistle
Stemless thistle
Wavyleaf thistle
Wyoming thistle
Yellowspine thistle
SW — Sweetwater, Uinta
Beaver Rim thistle
Colorado thistle
Flodman’s thistle
Fringed thistle
Greene’s thistle
Meadow thistle
Ownbey’s thistle
Tracy’s thistle
Wavyleaf thistle
Wyoming thistle
F — Fremont
Beaver Rim thistle
Colorado thistle
Flodman’s thistle
Fringed thistle
Greene’s thistle
Meadow thistle
Murdock’s thistle
Prairie thistle
Wavyleaf thistle
White thistle
Wyoming thistle
Yellowspine thistle
CW — Lincoln, Sublette
Barneby’s thistle
Beaver Rim thistle
Greene’s thistle
Meadow thistle
Murdock’s thistle
Prairie thistle
Wavyleaf thistle
Wyoming thistle
Index
Common name
Barneby’s thistle...............
Beaver Rim thistle............
Bull thistle*.......................
Canada thistle*.................
Colorado thistle................Drummond’s thistle..........
Elk thistle..........................
Flodman’s thistle..............
Fringed thistle...................
Graygreen thistle..............
Greene’s thistle................
Meadow thistle.................
Murdock’s thistle..............
Musk thistle*.....................
Ownbey’s thistle...............Plumeless thistle*.............
Prairie thistle....................
Scotch thistle*..................
Stemless thistle................
Tracy’s thistle...................
Wavyleaf thistle................
White thistle......................
Wyoming thistle................
Yellowspine thistle............
Page
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*Non-native thistle
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Scientific name
Carduus acanthoides*...............................
Carduus nutans*.......................................
Cirsium arvense*.......................................
Cirsium barnebyi.......................................
Cirsium canescens....................................
Cirsium clavatum var. americanum...........
Cirsium cymosum var. canovirens............
Cirsium drummondii..................................
Cirsium eatonii var. murdockii...................
Cirsium flodmanii.......................................
Cirsium foliosum........................................
Cirsium hookerianum................................
Cirsium inamoenum..................................
Cirsium ochrocentrum...............................
Cirsium ownbeyi........................................
Cirsium pulcherrimum var. aridum............
Cirsium pulcherrimum var. pulcherrimum.
Cirsium scariosum var. americanum.........
Cirsium scariosum var. coloradense.........
Cirsium scariosum var. scariosum............
Cirsium tracyi.............................................
Cirsium undulatum....................................
Cirsium vulgare*........................................
Onopordum acanthium*............................
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Musk thistle (Carduus nutans) is a common weed of roadsides and disturbed places in Wyoming. Photo: Robert Dorn
UW Extension, Biodiversity Institute, Wyoming Natural Diversity Database
UW Extension B-1316
February 2022
Issued in furtherance of extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kelly Crane, director, University of Wyoming Extension, University of Wyoming, Laramie, Wyoming 82071. • The University’s policy has been, and will continue to be, one of nondiscrimination, offering equal opportunity to all employees and applicants for employment on the basis of their demonstrated ability and competence without regard to such matters as race, sex, gender, color, religion, national origin, disability, age, veteran status, sexual orientation, genetic information, political belief, or other status protected by state and federal statutes or University Regulations.