(Plant Identification for everyone)

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.

Silybum marianum_Milk Thistle_Marin Headlands_2012-03-20__BHS

Milky leaf color is diagnostic.  Spines on leaf edges.

Silybum marianum_Milk Thistle_Bull Point at Point Reyes_2012-07-09__SGM

Spines below flower are long
and point out horizontally.

Silybum marianum_Milk Thistle_Tennessee Valley_1976-06-27__WF

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.

Centaurea solstitialis_Yellow Star Thistle_Black Point_1977-11-26__WF

Yellow flower.

Yellow Star Thistle-2

Fluffy seed head.

Centaurea solstitialis_Yellow Star Thistle_Old Mine Trail on Mt. Tam_1977-08-30__WF

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.

Bull Thistle 2_Silybum marianum__SGM-2012-07-09 Bull Point at Point Reyes

Pear-shaped flower base, leaves spear-shaped and spiny.

Bull Thistle-9-2012-08-28 Pirates Cove

Fluffy seed structures, like dandelions.

Bull Thistle-2013-01-21 Pirates Cove

Dead flowers stay on plant a long time.

Cobweb Thistle

Cirsium occidentale var. occidentale


Dunes and beaches at Point Reyes


Aster Family



· 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.

Cobweb Thistle

Base leaves large and flat on the ground.

Cobweb Thistle 2_Cirsium occidentale var occidentale__SGM

Flower base has many long

Cirsium occidentale_Cobweb Thistle_Steep Ravine_2004-03-23__WF

Flower base spherical, covered with cobwebby hairs.

Venus Thistle

Cirsium occidentale var. venustum


Dry sunny slopes


Aster Family



· 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.

Cirsium occidentale var. venustum_Venus Thistle_Pantoll Road_1984-05-12__WF-2

Base leaves not on the ground, and narrower than Cobweb.

Cirsium occidentale var. venustum 4_Venus Thistle_Pantoll Road_1984-05-12__WF

Flower base without cobweb

Cirsium occidentale var. venustum_Venus Thistle_Pantoll Road_1984-05-12__WF-3

Plant not covered in cobwebby hairs.

Brownie Thistle

Cirsium quercetorum


Grasslands and brushy slopes near the coast


Aster Family


· Blooms May - July

· Spiny leaves

· Grows low to the ground.

· Flowers tan to purple

· Found in coastal grasslands and open woodlands.

Cirsium quercetorum_Brownie Thistle_Dillon Beach_1976-04-25__WF-2

Leaves puckered, with long spines.

Brownie Thistle 2_Cirsium quercetorum__SGM

Brownie flower color.

Cirsium quercetorum_Brownie Thistle_Dillon Beach_1976-04-25__WF

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.

Italian Thistle

Relatively flat leaves with
spines on the tips of each lobe.

Italian Thistle_Carduus pycnocephalus__SGM

Stems winged with spines.

Carduus pycnocephalus_Italian Thistle_Fort Barry_1989-04-20__WF

2-5 flower heads per cluster.

Clustered Thistle

Cirsium brevistylum


Moist areas, including coastal marshes.


Aster Family



· 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.

Cirsium brevistylum 2_Clustered Thistle_Tomales Bay State Park_1990-05-26__WF

Thorny leaves with strong central vein.

Cirsium brevistylum 4_Clustered Thistle_Tomales Bay State Park_1990-05-26__WF

White or pink flowers, about 1” wide.

Cirsium brevistylum_Clustered Thistle_Tomales Bay State Park_1990-05-26__WF

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

Cirsium hydrophilum var. vaseyi 2_Mt. Tamalpais Thistle_Below Bootjack_1984-05-12__WF

Only found on serpentine soil.

Cirsium hydrophilum var. vaseyi_Mt. Tamalpais Thistle_Below Bootjack_1984-05-12__WF

Flower base spines are sticky.

Cirsium hydrophilum var. vaseyi 3_Mt. Tamalpais Thistle_Below Bootjack_1984-05-12__WF

Very long leaves with many spiny lobes.


Cirsium andrewsii




Aster Family

Native – found only in California.


· Blooms March to July

· Many branches, often thick with cobwebby fibres.

· Leaves spiny on the edge.

Cirsium andrewsii 3_Fransciscan Thistle_East of Lighthouse at Point Reyes_1987-07-05__WF--__WF

Flower pink to purpleish red.

Cirsium andrewsii_Franciscan Thistle__DLS--__DLS

Spines at base of flower point upwards

Cirsium andrewsii 2_Franciscan Thistle__DLS--__DLS

Many branches, often thick with cobwebby fibres.

© Creative Commons BY NC 3.0.  Contributors (identified by initials) are acknowledged at http://PlantID.Net/Contributors.htm

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.