Think twice before killing those thistles: Thistle Identification

Fewleaf thistle (Cirsium remotifoilum), a native thistle

Thistles are one of the most common weed problems that property owners deal with in Clackamas County.  Many of the thistles we encounter are invasive and can grow to dominate a property.  But did you know there are at least four thistles that are not only non-invasive, but native to western Oregon?  These native thistles are not only well-behaved, but are beneficial to the health of our working lands and natural areas.

Native Thistles are Important!

Native thistle provide important habitat and food sources for native fauna. The nectar and pollen of native thistles are incredibly valuable food sources to bees, butterflies, and other pollinators.  Many insects feed on the leaves, stems, flowers and seeds, while some songbirds also feed on thistle seeds. These nectar sources help support pollinators year-round, and can help to increase yields for many valuable crops.

The presence of native thistles also makes it harder for the aggressive non-native thistles to invade an area.  Native thistles help to support healthy populations of beneficial insects that will also consume non-native thistles. Our native thistles also remain in balance with other native plants and do not aggressively displace other plants.  Despite their benefits, native thistles are either knowingly or unknowingly killed because it is assumed they will become a big problem, or simply because they have spines, or are considered “weeds”.  Because of this, many native thistles are in danger of being completely eradicated from our area.

 

The Problem with Non-Native Thistles

Milk thistle (Silybum marianum) infestation in Clackamas County

On the other hand, non-native thistles can overrun an area and displace native plants, reduce agricultural yield, and create problems for grazing animals when they infest a field or pasture. They also cost a lot of money and time to control.  One of the struggles with controlling thistles is that it can be difficult to distinguish between them.  We are here to help!

 

General Thistle Characteristics

Thistles are members of the sunflower family (Asteraceae) and can be annual, biennal, or perennial.  True thistles have spines along the leaf margins and their flower heads are generally pink-purple and surrounded by bracts that are typically spiny. Below are some questions to help you determine whether your thistle is native or not:

  • Is the thistle spiny along the entire length of the stem? (a YES answer indicates Italian thistle, slenderflower thistle, Scot’s thistle, or bull thistle)
  • Are the bracts triangular, firm, and spine-tipped? (a YES answer indicates Italian thistle or slenderflower thistle)
  • Are the bracts thick, and leathery, and jagged? (a YES answer indicates milk thistle)
  • Are the roots rhizomatous? (a YES answer indicates Canada thistle)

If the answer to all of these questions is “NO”, it is likely you have a native thistle!

Native Thistles

  • Clustered thistle/Indian thistle/Short-styled thistle (Cirsium Brevistylum):
    • Annual, biennial or short-lived perennial
    • Stems are fuzzy and mostly unbranched (branched/clustered and more fuzzy near the top)
    • Flower heads are “cobwebby”
    • Flower heads are red- purple and have small white stalks (the “styles”) extending out individual flower tubes less than 1 mm
    • Bracts around the flower heads are long and tapered
    • Leaves point upward, with the top leaves extending above the flower head
    • Leaves are shallowly lobed, have somewhat weak spines, and are woolly on the underside (sometimes slightly woolly on the top)
    • Plant grows up to 7 feet tall
  • Edible thistle (Cirsium edule):
    • Biennial or short-lived perennial
    • Looks similar to the clustered thistle, but stems are more branched, and leaves are more strongly spiny
    • Flower head is very cobwebby and hangs on young plants
    • Flower tubes have styles that extend out 3-8 mm
    • Leaves are somewhat woolly on the underside, especially along the midrib
    • Leaves at the base can be 15 inches long
    • Plant grows up to 9 feet tall
  • Few-leaved thistle (Cirsium remotifolium)
    • Biennial or short-lived perennial with a lot of variation between plants
    • Stems can be either woolly or smooth
    • The plant has weak spines, and is sparingly branched on the upper half
    • Flower heads can be creamy-white to purple and are only moderately cobwebby
    • Bracts are spiny with papery margins that might by slightly fringed
    • Leaves are woolly on the underside, deeply lobed, and spiny
    • Leaves may have short stalks, or they may be clasping the stem with no stalk
    • Plant grows up to 6 feet tall
  • Wavy-leaved thistle (Cirsium undulatum)
    • Perennial
    • Stems are branched on the upper half and are matted with white hairs
    • Flower heads are typically solitary, pale lavender to white, and up to 2 inches wide
    • Bracts have a sticky midrib and are typically tipped with spines that stick out
    • Leaves are hairy on top and bottom
    • Leaf margins have shallow wavy lobes with a yellow spine at the tip
    • Plant can grow up to 4 feet tall

Non-native Invasive Thistles

  • Bull thistle (Cirsium vulgare):
    • Common in Clackamas County, moderately invasive, see our weed profile
    • Biennial
    • Stems are leafy, stout, branched, and somewhat woolly
    • Flower heads are large, rose to purple with many sharp spine-tipped bracts
    • Leaves are irregularly lobed, with the end lobe being longer than the side lobes, and all tipped with yellow prickles
    • Leaves are woolly on the underside
    • Leaves have noticeable “wings,” (flat projections extending down the stem)
    • Plant can grow up to 6.5 feet tall
  • Canada thistle (Cirsium arvense)
    • Common in Clackamas County, highly invasive, see our weed profile
    • Perennial plant that spreads by underground stems
    • Stems are slender, ridged, and hairy
    • Flower heads are purple, small (less than 2 cm), and clustered
    • Bracts are broadly triangular, and typically not very spiny
    • Leaves are small, lobed, and the margins have very sharp yellowish prickles
    • Plant can grow up to 6.5 feet tall
  • Blessed milk thistle (Silybum marianum)
    • Uncommon but present in Clackamas County, highly invasive, see our weed profile or Best Management Practices
    • Biennial (sometimes annual)
    • Stems are ridged and branched in the upper half, with sparse hairs
    • Flower heads are large (up to 2 inches) and purple
    • Bracts are stout, jagged, very sharp, 1-2 inches long
    • Leaves are dark green with distinct white marbling
    • Leaves at the base can be up to 20 inches long
    • Plant can grow up to 8 feet tall
  • Italian plumeless thistle (Carduus pycnocephalus) and slenderflower thistle (Carduus tenuiflorus)
    • Not known to exist in Clackamas County
    • Two annual, similar-looking species
    • Stems are woolly and very spiny-winged, with branches that point upward
    • Flower heads are pink – purple, small (0.5 inches wide), and grow in spiny clusters of 2 to 5 (on Italian) and 5-20 (on slenderflower)
    • Bracts are hairy and spiny, the hairs on Italian thistle bracts are stiff and point forward
    • Leaves are deeply lobed and woolly on the underside, lobes are tipped with spines
    • Plant can grow up to 6 feet tall
  • Scot’s thistle (Onopordum acanthium)
    • Not known to exist in Clackamas County
    • Biennial (sometimes annual or short-lived perennial)
    • The whole plant is very branched, strongly spiny, with a bluish gray appearance
    • The stem is woody and very spiny-winged (flat leaf projections extend down stem)
    • Flower heads are purple and 1-3 inches wide
    • Bracts are tipped with flat, pale, orange spines
    • Leaves have triangular lobes, are spiny, covered in silvery hairs, and can be up to 1 foot long
    • Plant can grow up to 8 feet tall

Want to dig deeper?

There are several other species that we did not discuss here, either because they aren’t known to be in western Oregon, or because they aren’t true thistles… and this post is already long enough! Follow the links below for more information about these “thistle-like” counterparts .

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Do NOT follow this link or you will be banned from the site!