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Marin County | Plant Groups

Common Marin Thistles
Thistles are easy to find in Marin because of their abundance and because several species tend to do well next to roads. They’re also easy to distinguish. Use: · Flower color, which varies from brown to yellow to purple to red · Leaf shape, color, location on the plant, and thorniness · The bulge just below the flower. This varies in shape, the arrangement of spines on it, and hairiness.
Milk Thistle Silybum marianum Fields, pastures and beside roads. Aster Family Not Native	· Blooms April – July · Large leaves at base. · Leaves shiny green with obvious milk-white veins. · Leaves spiny. · Stem is grooved. · Flowers red-purple. · Flower base (involucre) has distinctive long spines pointing out horizontally in a couple of layers.	Milky leaf color is diagnostic. Spines on leaf edges.	Spines below flower are long and point out horizontally.	Can grow 6 feet tall.
Yellow Star Thistle Centaurea solstitialis Roads, fields, and disturbed areas Aster Family Not Native – highly invasive	· Blooms Apr - Sept. · Often found beside roads. · Base leaves not spiny. · Leaves uncommon above the base leaves. · Yellow flower looks a bit like a dandelion flower. · Flower base has distinctive long spines pointing out horizontally in one layer.	Yellow flower.	Fluffy seed head.	Horizontal spines are distinctive.
Bull Thistle Cirsium vulgare Hills and valleys, along roads and trails Aster Family Not Native – moderately invasive	· Blooms June - Sept · Spear-shaped spiny leaves at base and along the length of the stem. · Pear-shaped spiny flower base. · Flower 1”-2” across, pink purple, soft looking. · Seeds have fine hairs, making thistledown. · Dead flowers stay on plant a long time.	Pear-shaped flower base, leaves spear-shaped and spiny.	Fluffy seed structures, like dandelions.	Dead flowers stay on plant a long time.
Cobweb Thistle Cirsium occidentale var. occidentale Dunes and beaches at Point Reyes Aster Family Native	· Blooms May – July · Leaves grey-green with many hairs. · Leaf lobes are triangular. · Flower base is spherical, with long spines, covered in cobwebby hairs. · Flowers, white to red, can be up to 3” wide. · If not in Point Reyes, check Venus Thistle, below.	Base leaves large and flat on the ground.	Flower base has many long spines.	Flower base spherical, covered with cobwebby hairs.
Venus Thistle Cirsium occidentale var. venustum Dry sunny slopes Aster Family Native	· Blooms May – July · Very similar to Cobweb Thistle except: · Base leaves are a different shape and don’t lie on the ground. · Body and flower base are not hairy.	Base leaves not on the ground, and narrower than Cobweb.	Flower base without cobweb hairs.	Plant not covered in cobwebby hairs.
Brownie Thistle Cirsium quercetorum Grasslands and brushy slopes near the coast Aster Family Native	· Blooms May - July · Spiny leaves · Grows low to the ground. · Flowers tan to purple · Found in coastal grasslands and open woodlands.	Leaves puckered, with long spines.	Brownie flower color.	Grows low to the ground.
Italian Thistle Carduus pycnocephalus Grassy areas and the edge of woods Aster Family Not Native – moderately invasive	· Blooms Feb - July · Base leaves all around the stem quite long (10” to 14”). · Leaves flatter than other thistles. · Flowers pink to purple. · Flower base is a long oval with big spines.	Relatively flat leaves with spines on the tips of each lobe.	Stems winged with spines.	2-5 flower heads per cluster.
Clustered Thistle Cirsium brevistylum Moist areas, including coastal marshes. Aster Family Native	· Blooms Apr – July · Generally a single stem, growing to about 6 feet tall · Leaves long, with many spines. · Flower generally smaller than the flower base.	Thorny leaves with strong central vein.	White or pink flowers, about 1” wide.	Notice the flower base is quite a bit wider than the flower.
Mt. Tamalpais Thistle Cirsium hydrophilum var. vaseyi Serpentine seeps Aster Family Native – only in California.	· Blooms May – Aug · Flowers pink to purple · Branching stem to about 6 feet tall. · Very long leaves with many pointed spiny lobes. · Flower base roundish with sticky twisted spines. · Found on Mt. Tam and Pine Mountain	Only found on serpentine soil.	Flower base spines are sticky.	Very long leaves with many spiny lobes.
FranciscanThistle Cirsium andrewsii Coastal Aster Family Native – found only in California.	· Blooms March to July · Many branches, often thick with cobwebby fibres. · Leaves spiny on the edge.	Flower pink to purpleish red.	Spines at base of flower point upwards	Many branches, often thick with cobwebby fibres.

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Reviewed by KJ and DLS. Last Updated 6/2/2014 by BHS.

Note to botanists – this page uses common vocabulary, sacrificing more precise scientific terms in the interest of general communication. We hope the loss of precision in wording is, to some extent, made up for by photographs showing key identification points.

Note to non-botanists – here is a botanical term you might like to know:

· Involucre – The base, or cup that supports a number of flowers. In thistles, the involucre is the greenish, roundish, often spiky base of the flower. Its shape, and spike pattern are often the key to identifying which species you’re looking at. The colored strands growing out of the involucre are each separate flowers.