Thistles are spiky plants in the Cardueae tribe of the aster family. Woodland has one native thistle species and twelve invasive weed thistle species.

Native Species

Peregrine Thistle (Cirsium cymosum) is a three-foot-tall perennial thistle that is native to a portion of west-central Woodland and to the Yolo County foothills (but not to the Capay Hills). It grows only in dry areas. It has white flowers. It is ranked 9 out of 10 on the Ogren Plant Allergy Scale, indicating that it tends to cause severe hay fever.

Invasive Weeds

Perennials

Milk Thistle (Silybum marianum) is an invasive weed that can grow over 6 feet tall. Photo by queerbychoice.

Purple Starthistle (Centaurea calcitrapa) is a biennial thistle and invasive weed from Europe that is rare in its native range but aggressive in many other parts of the world. It can grow a little over 4 feet tall. It has purple flowers.

Diffuse Starthistle (Centaurea diffusa), also called Diffuse Knapweed, is a biennial thistle and invasive weed from eastern Europe and Western Asia. It can grow 2 feet tall. It has white flowers.

Creeping Thistle (Cirsium arvense), also called Canada Thistle, is a perennial thistle and invasive weed from the Mediterranean region. Root fragments as small as one centimeters can sprout to form a new plant. Creeping Thistle can grow 5 feet tall. It has pink flowers. It is ranked 9 out of 10 on the Ogren Plant Allergy Scale, indicating that it tends to cause severe hay fever.

Bull Thistle (Cirsium vulgare) is a perennial thistle and invasive weed from the Mediterranean region. It can grow nearly 6 feet tall. It has purple flowers. It is ranked 9 out of 10 on the Ogren Plant Allergy Scale, indicating that it tends to cause severe hay fever.

Scotch Cottonthistle (Onopordum acanthium), also called Scottish Cottonthistle, is a biennial thistle and invasive weed from Europe and Asia. It can grow nearly 10 feet tall and 4 feet wide. It has purple flowers.

Milk Thistle (Silybum marianum) is a biennial thistle and invasive weed from Europe and Asia. It can grow 6 and a half feet tall. It has purple flowers.

Annuals

Yellow Starthistle (Centaurea solstitialis) has invaded 12 million acres in California, making it one of California's worst invasive weeds. Photo by queerbychoice.

Italian Thistle (Carduus pycnocephalus) is an annual thistle and invasive weed from the Mediterranean region. It can grow 6 and a half feet tall. It has purple flowers.

Slender-Flowered Thistle (Carduus tenuiflorus), also called Winged Thistle, is an annual thistle and invasive weed from Europe and northern Africa. It can grow 7 feet tall. It has purple flowers.

Woolly Distaff Thistle (Carthamus lanatus), also called Saffron Thistle, is an annual thistle and invasive weed from the Mediterranean region. It has yellow flowers.

Maltese Starthistle (Centaurea melitensis), also called Tocalote, is an annual thistle and invasive weed from Europe and northern Africa. It can grow 3 and a half feet tall. It has yellow flowers.

Yellow Starthistle (Centaurea solstitialis) is an annual thistle and invasive weed from the Mediterranean region. A single Yellow Starthistle plant can produce nearly 75,000 seeds. Yellow Starthistle has invaded 12 million acres in California, making it one of California's worst invasive weeds. It can grow 3 and a half feet tall. It has yellow flowers.