Perhaps you’re enjoying the views of your backyard, state park, or hiking trail, and you stumble upon a large purple flower with spiny leaves. It’s reminiscent of your grandmother’s shag carpet- only more purple. You may have heard of the perils of invasive thistle, but did you know that there are many native counterparts? Panic not, for this post is your guide to determining if your thistle’s intentions are benign- or something much more sinister.
Centaurea americana (American basketflower)
Gotcha! This isn’t actually a thistle, but it’s capable of fooling the casual observer. American basketflower doesn’t actually have the trademark thorns of a thistle, but the bloom is very similar. This plant is a native. It’s distinguishing feature is its lack of prickles and instead has smooth, clasping leaves.
Cirsium altissimum (Tall thistle)
Aside from the height, tall thistle’s defining features are the long, prickly bracts at the base of the flower and its upper, elliptic leaves. Its lower leaves feature weak prickles with a silver underside. This native thistle is a bountiful nectar source for native bees, and can grow to an astonishing 10 feet tall.
Cirsium carolinianum (Soft thistle)
Soft thistle’s distinguishing feature is its thin, long pinlike pricles on the ovary. Its bloom also includes longer reproductive parts than its close relatives which peer above the petals (which are actually small flowers!). Its leaves are thinner and more lanceolate with prickles that are much shorter than other Cirsium species. For distinguishing features, look at reproductive parts and leaves.
Cirsium discolor (Field thistle)
What distinguishes Field thistle from other native thistles may be a lighter hue and white hairs on stems that lack prickles, which form a mat on the underside of the leaves. Flowers are wider from a more tubular base than its relatives and are supported by stiff sepals at the base. Grows 3-7 feet and blooms June-September. Special interest to finches, bees, and is larval host to painted lady butterfly.
Cirsium engelmannii (Engelmann’s thistle)
Native only to Texas, Oklahoma, Arkansas and Louisiana, Englemann’s thistle can grow 3-6 feet tall and is most likely to be confused with Cirsium texanum. A distinguishing factor can be spiny bracts at the base of the flower, which are lacking in Texas thistle. Its leaves are more pinnately divided, with each pinnate divide more separated with white leaf undersides.
Cirsium horridulum (Horrid thistle, Yellow thistle)
Despite the name, horridulum is native and can be yellow or pink. It can easily be distinguished by its basal leaves, which are a thick radial web of long serrated leaves. The base of the flower is cup-shaped with crosshatching between prickles. The entire plant is covered in thick bristly spine-like prickles and can be 2-5 feet. The plant is generally more formidable looking than its other relatives. The tangle of prickles can lead one to think it’s invasive, but don’t be fooled- it is a native to Texas.
Cirsium muticum (Swamp thistle)
Swam thistle can grow to be 6 feet tall and as the name indicates- lives only in moist soil. Categorized as threatened in Arkansas, swamp thistle has special value to bumble bees This plant features hairy stem, and more narrow flower heads (less than 1 1/4 inch wide). Leaves are deeply lobed, less spiny than most Cirsium species. Stems hairy along base, ovary is covered in cobweb-like hairs.
Cirsium neomexicanum (New Mexico thistle)
Although this plant is not typically found in Texas, it may grow in the western edge of the panhandle. New Mexico thistle has special value to bumblebees. Growing to 6 feet tall, its distinguishing features are silver or gray leaves. New Mexico thistle has radial spiny basal leaves and its blooms may be white or pink. Long, needlelike prickles guard the base of the flower. Until a few inches below the flower, the stems extremely prickly, and then it becomes abruptly smooth closer to the inflorescence. This species is very similar morphologically to C. undulatum, or Wavyleaf thistle.
Cirsium ochrocentrum (Yellowspine thistle)
Easily visible in the third picture, Yellowspine thistle has leaves and bracts armed with numerous stiff yellow spines, can grow 2-4 feet tall. Its stems are blanketed in wooly white hairs. This plant also has a radial formation of basal (bottom) leaves. Long bracts typically occur under bud, guarded with its trademark yellow spines. Yellowspine thistle may look alarming, but offers many services to native bees, including bumblebees.
Yellowspine thistle may be helpful in treating diabetes, and was once taken as contraceptive by the Zuni people.
Cirsium texanum (Texas thistle)
A personal favorite, Texas thistle is virtually indistinguishable from Engelmann’s thistle. Distinctions may be a lack of spiny bracts at the base of the flower. Its leaves are less pinnately divided than engelmannii, with leaves greatly reduced in size near the top of the plant. Prickles are on the tip of each “tooth” on a leaf, but are generally absent elsewhere, and typically a stem will produce only one flower. Branching is not common, and when present the plant will branch towards the base. It can grow to be 6 feet tall.
Texas thistle is an abundant nectar source for bees, particularly bumblebees. It can be found blooming April-August, and is also edible.
Cirsium turneri (Cliff thistle)
Native only in Texas, blooms are primarily red and purple, and prefers moist soils like swamp thistle. A conspicuous difference is that Cliff thistle only grows to become 18 inches tall.
Cirsium undulatum (Wavy leaf thistle)
Wavy leaf thistle has yellow or white prickles at ends of long distinctively wavy leaves. Although it can be invasive outside of its habitat, it is native to Texas. In its native range, Wavyleaf thistle is threatened by a weevil that was originally released to control the invasive Canada thistle. Wavyleaf thistle is similar to Yellowspine thistle, but differs in the undulation of the leaf edges, and the absence of long bracts at the base of the flower. Wavyleaf thistle can grow to be 4 feet tall, and flowers June-July.
Invasive species are an issue because they lack natural predators. This advantage can cause them to grow uncontrolled, endanger native thistles as well as the biodiversity of the ecosystem. One way to fight invasive species is to introduce a natural predator to regulate the invasive species. This also has been done with fire ants at the University of Texas.
Carduus nutans (Musk thistle, Nodding thistle)
With dark green foliage, the base of Musk thistle’s bloom has thick artichoke-like, spine tipped bracts that are extremely distinctive. Flowering heads are completely lacking in cobwebby pubescence that’s so common in Cirsium species. Leaf edges are whitish and pale. Like many thistle species, Musk thistle can grow to be 8 feet tall and can be found flowering May-June.
Cirsium arvense (Canada thistle)
Canada thistle is considered extremely aggressive and invasive in 43 states. Its spines are dispersed, and among its unique features are inflorescences grouped in terminal clusters. Flowers are much smaller than other Cirsium species (less than an inch across, typically), The base of the flower lacks spines and instead has purple-tipped bracts. 2-4 feet tall, Canada thistle is generally shorter than many native thistles. Leaves are green on both sides, rather than a white underside.
Originally fought with weevils, scientists are now hoping a fungus can help fight the good fight against this Canada thistle.
Onopordum acanthium (Scotch thistle)
Also called cotton thistle, this invasive plant is difficult to miss. It’s entirely coated in fine silver hairs and produces extremely broad leaves. Scotch thistle is eye-catchingly bizarre. Its bud is rotund and globe-like, and covered in needle-like spines. The thick stems have curtain-like “wings” on either side, which are spiny. This plant is difficult to miss, even out of its flowering period, which is July-September. Cotton thistle can reach 6 feet in height.
Cirsium vulgare (Spear thistle, Bull thistle)
Spear thistle, or Bull thistle is an invasive species. Introduced from Europe, it’s covered in slender, spear-like spines. Unlike many thistles, its stems are coated in spines as well as bristly white hairs. The leaves on Bull thistle have “spiny wings” extending downwards, greenish undersides, and spines up to 1/2 inch long.
Spear thistle typically forms a large, almost shrub-like plant with high amounts of branching. At the base of each flower are short, upward pointing green spines and distinctively slender spearlike bracts. It can grow to 6 feet tall and blooms June-July. Litter, or dead plant material, can prevent other species from growing around its base.
Silybum marianum (Milk thistle)
This alluringly beautiful thistle was introduced from Europe. Its broad leaves clasp the stem and include white veining that give it a mosaic look. The flowerhead is supported by a large ray of hairless bracts at the base, that look sun-shaped from above. When broken, the leaves produce a milky substance. It flowers June-August.
Although it is invasive, Milk thistle is of medicinal use, and is beneficial to the liver.
Distaff thistle forms yellow flowers with fistlike clusters of spines underneath. Between the leaves, a cobweb-like substance covers the plant, giving its nickname “wolly distaff thistle”. Stems and leaves are tannish-green, and lacking a white underside. This invasive thistle typically grows to 3.5 feet, and flowers May-June. Dead plants may remain standing for up to a year after they die.
If you come across a thistle, and are overwhelmed with the abundance and similarity of thistle species, don’t panic. Begin with leaf patterns and edges, height, and the base of the flower. Most thistles have very similar inflorescences, so the best way to ID a thistle is to look for other clues. The key to identifying is to practice. If you aren’t sure, take pictures of distinctive features (leaves, bud, base, stems) to bring home and identify. If you have experience, you can also try using this dichotomous key, but beware that it is specific to Texas. GoBotany also offers an interactive online key for Cirsium.
If you come across an invasive thistle, I recommend this guide to managing invasive thistles by the US Department of Agriculture.
Want to learn more?
You can learn more information on invasive species control and what you can do at the National Agricultural Library.
For zero dollars, you can become a citizen scientist invasive-species-vigilante.
To view taxon maps of which Cirsium species are present in your area, you can go to The North American Plant Atlas
A special thanks to OK State for this guide to Oklahoma thistles that helped me get started for this guide to Texas species.