Thistles belong to a large plant family and are mostly native to Asia and Europe. Thistles may be part of the Cirsium, Carduus, Carthamus or Onopordum genus. Some thistles are considered noxious weeds or invasive plant species because of their vigorous reproduction and sometimes toxic qualities. Some thistles are not considered invasive, mostly because their spread is naturally controlled by native plants or certain feeding insects. Proper thistle identification is important in knowing whether the plant’s presence warrants any control measures.
Identify Invasive Thistles
Spot the bull thistle (Cirsium vulgare) by looking for its deeply lobed leaves that are prickly and leathery on the top. The tall, erect stems each sprout a single pinkish flower head that rests atop a greenish, spiny thistle.
Identify the musk thistle (Carduus nutans), which is commonly found invading pastures, fields and meadows, by its hairless, spiny and alternately arranged leaves that have spiky and silvery edges. Musk thistle can grow up to 7 feet tall and flowers with 50 to 100 reddish-purple blooms that are each 2 to 3 inches wide.
Look for slender, green and freely branching 1- to 4-foot stems that bear purple flowers to identify the Canada thistle (Cir. arvense). This thistle has many small alternately arranged, deeply lobed leaves with stiff yellowish spines around their edges. The Canada thistle bears 1/8-inch long “fruits” that are flattened and brownish with a circle of long hairs.
Look for a spiny thistle plant that grows up to 4 feet tall and has yellow flowers with red veins on spiny heads to identify the distaff thistle (Carthamus lanatus). The distaff thistle has straw-colored, 2- to 4-foot stems and flowers that have a whorl of spiny leaves right below the head. The leaves and stems are covered with cobweb-like or woolly hairs.
Identify Scotch thistle (Onopordum acanthium) by its broad, large grayish-green leaves with thick, cottony hairs. The Scotch thistle has globe-shaped reddish-purple thistle flowers and its leaves aren’t deeply lobed like other thistle leaves. The stems have spiny wings and are 3 to 9 feet tall.
Spot the purple starthistle (Centaurea calcitrapa) invading fields and other grasslands, with leaves and stems that are covered with cobweb-like hairs and rosette leaves that have circular spines in their centers. The purple starthistle’s lavender to deep-purple flowers grow atop spine-tipped bracts. The lower leaves are deeply lobed or divided, while the upper leaves are undivided and narrow.
Look for spiny-winged stems that are covered with cobweb-like hairs on the undersides to identify the Italian thistle (Carduus pycnocephalus), which invades pastures and other grazed lands. Growing up to 8 feet tall, the Italian thistle blooms clusters of one to five pinkish, cylinder-shaped flower heads and has spiny-toothed, lobed leaves that have rigid, distinct spines between the leaf segments.
Study the thistle weed for sparse branches and alternately arranged, deeply lobed leaves with pointed tips to spot the milk thistle (Silybum marianum). The milk thistle has distinct, shiny dark-green leaves that are marbled with white, which can be up to 20 inches long and 10 inches wide. Each stout, rigid stem bears a purple, 2-inch-wide flower with a broad, spine-tipped bract.
Spot the yellow starthistle (Centaurea solstitialis) by its dull-green leaves and stems that are covered in woolly hairs. The leaves are alternate and are “wing-like.” The yellow starthistle has thistle-like yellow flowers with 3/8- to ¾-inch yellow spines that are arranged in a star-like pattern at the base of each flower head.
Spot Noninvasive Thistles
Identify the tall thistle (Cirsium altissimum) by its extreme height, with stems that grow up to 10 feet tall, making it one of the tallest thistle species. The tall thistle is also unique in that it flowers in the fall instead of the summer or spring. The tall thistle’s leaves are green and mostly hairless on the upper surfaces, with white fuzz on the lower surfaces. The flower heads are light purple and shaped like shaving brushes.
Spot the wavyleaf thistle (Cir. undulatum) by its distinct wavy leaves that have dense fuzz on both upper and bottom surfaces. The leaves are greenish-gray on top and whitish on the bottom, with 1/8- to ¼-inch spines growing from each leaf lobe tip. The wavyleaf thistle has 1- to 4-foot stems that are whitish, thick and fuzzy, as well as pink to deep-red, globe-shaped flowers.
Look for mostly hairless, green and oblong leaves that have edges with spine-tipped lobes to spot the yellow thistle (Cir. horridulum). The yellow thistle has thick, hairy, 2- to 5-foot stems and buff-yellow to reddish-purple flower heads that sit atop a whorl of spiny leaves.
Identify the yellowspine thistle (Cir. ochrocentrum) by its leaves and flower bracts that are edged or covered with many stiff, ¼- to 3/8-inch yellow spines. The purple flower heads are globe-shaped, and the 2- to 5-foot-tall stems are matted with whitish fuzz or hairs. The leaves are also covered in whitish hairs, with greenish-gray upper leaf surfaces and whitish undersides.