Hello everyone,

Last week we talked about how to kill Canada thistle.  This week I would like to point out two more thistle species, and some thistle look-alikes.  Bull thistle (Cirsium vulgare) and blessed milk thistle (Silybum marianum) are prevalent in the Willamette Valley.  Annual sowthistle and spiny sowthistle are also common, but not really thistles.  Prickly lettuce is another plant that looks thistle-ish, but is not.  

All plants discussed today are in the family Asteraceae.  These plants are threatening in appearance, some are painful to the touch, and a menace to anyone responsible for their control.   All are invasive weeds native to Europe and/or Asia.


Bull thistle (Cirsium vulgare)

Bull thistle is one of the most common thistles in our area.  In fact, the specific name vulgare means 'common'.  

Bull thistle is a biennial that forms a rosette the first year.  Foliage is hairy, lobed with prickles, and has a dark bluish-green color..  Erect foliage and flower stems are formed the second year.  Flowers are larger than Canada thistle, developing to 1 inch tall and wide.

Bull thistle is the ugliest of the thistles, but it's much easier to control than Canada thistle.  It is a biennial that reproduces by seed.  Use of preemergence herbicides in combination with good sanitation will prevent seed germination.  If preemergence control fails, spray with Roundup or 2,4-D on actively growing plants prior to flower bud emergence.  Beyond that, mow or prune flower heads to prevent seed formation.  Bull thistle will not survive cultivation, which is probably why I rarely see it in well-maintained nursery fields.  

bull thistle
bull thistle
bull thislte
Bull thistle flower heads are pink to purple, and approximately 1 inch tall and wide.
Bull thistle foliage is dark green and often with a slight grey tint.  Foliage is very pubescent, deeply lobed, and armed with stiff prickles at the tips.
At maturity, plants grow 2 to 5 feet tall and wide.

Blessed milk thistle (Silybum marianum)

Blessed milk thistle is not a true thistle (if you consider thistles to be only those plants in the genus Cirsium).  It is a monstrous weed, and is as harmful to the touch as bull thistle.  It is not as common as bull thistle; I've only seen this weed along roadsides and in fencerows.  I've never seen it in a nursery field, but I've heard reports that it is becoming increasingly invasive in the Willamette Valley. 

Blessed milk thistle is easy to identify by the creamy white veins that cover its foliage.  Flower heads are large (2 to 3 inches in diameter) with thistle-like purple bristles.

The specific name marianum is applied to several plants with white mottled leaves.  According to some, the spots formed when milk from Mary, the mother of Jesus, dropped on the foliage (Smith, 1997).

Blessed milk thistle is a biennial or winter annual.  Control of this weed is covered thoroughly in the Pacific Northwest Weed Control Handbook, however, I will summarize here.  Herbicides containing clopyralid (Lontrel, Stinger, or Curtail) should be applied to actively growing plants in the spring.  Roundup can be applied to rosettes in the spring or fall (if in the fall, before the first hard freeze).  Herbicides containing 2,4-D should be applied on fall rosettes, or in the spring before flower stalks emerge.

blessed milk thistle
blessed milk thistle
blessed milk thistle
Flower heads are large and spiny.
White-mottled foliage is unique.
Habit is large, growing up to 6 feet tall and wide.

Sowthistles (Sonchus spp.) and prickly lettuce (Lactuca serriola) are plants that also resemble thistles in that they have similarly lobed and prickly foliage.  However, these plants have yellow flowers that do not resemble thistles in any manner, and they are generally not painful when touched (foliage and prickles are soft).  We'll cover these plants in more detail in a later email.


More photos from the overflow bin.  I have posted extra pictures on a different page, so viewers can see more details of each species without slowing the download time of this page.

For more photos of bull thistle, click here.

For more photos of blessed milk thistle, click here.



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Literature

Smith, A.W.  1997.  A Gardener's Handbook of Plant Names, Their Meanings and Origins.  Dover Publications, Mineola, NY.