Canada Thistle or Creeping Thistle (Cirsium arvense) Aster Family / Thistle Subfamily By Thomas J. Elpel with additions by Pamela G. Sherman
About Canada Thistle: Thistles belong to the Thistle Subfamily of the Aster Family, along with artichokes. Compare an artichoke and a thistle, and notice that they are very similar, except in size. Like other members of the Aster Family, these are composite flowers, with many very small flowers clustered together on a disk, which is typically protected by overlapping layers of bracts. In other words, those are not sepals surrounding the flowerhead, but modified leaves called bracts. With a cooked artichoke, you can peel the bracts off one at a time, dip them in butter and eat the tender base of each one. You could do the same thing with thistles, except that the bracts are too small to bother with.
Canada thistle or Canadian thistle, featured here, is native to Eurasia, where it is known as creeping thistle, due to its ability to spread via roots into dense patches. It is also known as cursed thistle, probably due to the fact that chopping the roots with a plow effectively spreads the thistle across a field. Thus, it is considered an invasive species across much of the globe, including in its native lands.
Canada thistle is misnamed, since it was brought to this continent from Europe in the early 1600's. Due to competition with crop plants, weed control legislation for Canada thistle was passed as early as 1795 in Vermont and 1831 in New York. The name Canada thistle is an American misnomer. I would rather call it creeping thistle, but it is so well known by the erroneous name that it would just confuse the issue to change it now.
Canada thistle is widespread and locally abundant in plowed fields and pastures, as well as riparian areas and woodlands near places of human habitation. Wherever we go, it goes. Learn to recognize the plant in its adult form, and you may soon realize that smaller versions of it are everywhere, possibly including the spiny greens underfoot in your lawn.
Unlike other thistles, male and female flowers appear on separate plants in the Canada thistle. (Male and female parts may appear together, but only one sex is fertile.) Honeybees are the main pollinators of Canada thistle. Individual plants produce an average of 1500 seeds, but there must be both male and female plants in the vicinity for successful pollination. About 90% of the seeds will germinate within one year, but other seeds can remain viable for about 20 years. Seeds can be blown a half mile in the wind. T h i s t l e s e e d s a r e a f a v o r i t e w i t h s e v e r a l s e e d - e a t i n g b i r d s , l i k e g o l d f i n c h e s .
Seedlings require full sun for normal development. Growth is reduced if full sunlight is not available, and the seedlings die when shade reduces light intensity to 20%.
Also unlike other thistles, the Canada thistle has a deep and wide-spreading root system. Dense Canada thistle patches are formed where a single male or female plant has spread by its roots. Individual roots only live for about two years, and older patches will occasionally die out and disappear. Small fragments of the roots can sprout new thistle plants, so the plant is especially troublesome in farm fields where it is spread by cultivation.
Edibility: Young Canada thistle leaves are edible and remarkably tasty. Individual leaves can be rolled up to smash the spines and eaten like that, or placed whole on sandwiches, as noted in Foraging the Mountain West. Young thistles or thistle tops can also be boiled as a potherb. The stalks, while still flexible, can be peeled and eaten raw. The plants seem highly nutritious and might make a healthy green drink with the aid of a blender.
U s i n g t h e g r e e n s , s t e m , r o o t , a n d f l o w e r s, K a t r i n a B l a i r o f D u r a n g o , Colorado s e r v e s r a w t h i s t l e - b a s e d f o o d a n d d r i n k a t h e r w i l d f o o d c a f e , f a r m e r s m a r k e t s t a n d , a n d w i l d f o o d c s a : g r e e n d r i n k l e m o n a d e , M e x i c a n s t e m s n a c k s , f l o w e r h e a d s , r o o t k r a u t , r o o t c h a i , t e a , a n d l i q u e u r . [ R e t i r e d U S D A E t h n o b o t a n i s t J i m D u k e l i k e s t h e c o o k e d y o u n g l e a v e s a n d s t a l k s . O t h e r s l i k e j u s t - e m e r g e d , f e w - d a y - o l d b a b y t h i s t l e s i n s t i r f r i e s , q u i c h e s , a n d a n y w h e r e e l s e s p i n a c h m i g h t b e u s e d . W h e n t h e v e r y y o u n g t h i s t l e s a r e w e l l c o o k e d ( o r s t r a i n e d o u t , a s i n a g r e e n d r i n k ) , t h e s p i n e s a r e n o t a n i s s u e f o r m o s t p e o p l e a n d d o n o t n e e d t o b e r e m o v e d . S e n s i t i v e i n d i v i d u a l s , a s e v e r y o n e , s h o u l d t a k e a l l n e c e s s a r y p r e c a u t i o n s w h e n t r y i n g a n e w f o o d .
M e d i c i n e : D e l a w a r e I n d i a n s u s e d Canada thistle f o r c a n c e r a n d h e m o r r h a g i c h e m o r r h o i d s . N a v a h o u s e d i t t o i n d u c e v o m i t i n g . O j i b w a u s e d i t f o r s t o m a c h c r a m p s . C a n a d a t h i s t l e h a s b e e n u s e d t o c o a g u l a t e m i l k . M o h e g a n I n d i a n s u s e d i t i n a m o u t h w a s h f o r i n f a n t s a n d i n a t u b e r c u l o s i s r e m e d y f o r a d u l t s . M o n t a g n a i s a l s o u s e i t f o r t u b e r c u l o s i s . O j i b w a u s e d i t a s a b o w e l t o n i c .
A n u m b e r o f d i f f e r e n t t h i s t l e s p e c i e s a r e l i s t e d i n H a r t w e l l ' s P l a n t s U s e d A g a i n s t C a n c e r , f o r c a n c e r o f t h e b r e a s t a n d n o s e , e d e m a t o u s t u m o r s , a n d s c i r r h u s .
O t h e r U s e s : T h i s t l e d o w n c a n b e u s e d a s t i n d e r t o s t a r t c a m p o r w o o d s t o v e f i r e s i n w o o d s t o v e . L e a v e s a n d s t a l k , b u t n o t h e a d s a n d r o o t s , c a n b e p u t i n t h e c o m p o s t , a s i t i s a p e r e n n i a l a n d s p r e a d s b y s e e d a n d r o o t .
Weed Control Strategies
Canada thistle sometimes integrates well into local ecology, forming small, stable, non-threatening patches, or scattered individual stems, without becoming overly invasive. However, Canada thistle can also be a persistent annoyance underfoot in a lawn or flowerbeds. It can quickly take over sufficiently moist pastures, especially if livestock graze away grass and other competition. Using mulch to smother other plants can encourage the spread of Canada thistle. The plants can form a tall, dense, and nearly impenetrable prickly forest as an understory among willows and other shrubbery along waterways. And it can quickly spread across farm fields when the roots are chopped and redistributed by plow.
Fire: Late spring burns have been an effective tool to control Canada thistle (May-June), while early spring burns stimulate the plants to increase sprouting and reproduction. Late spring burns should be conducted three years in a row to be most effective. These studies were done in non-brittle prairies of Illinois. Repetitive burning in more brittle habitats could have serious negative consequences.
Mechanical Controls: H a n d - p u l l i n g o r c u t t i n g C a n a d a t h i s t l e c a n s t i m u l a t e t h e p l a n t s t o s e n d u p m o r e s p r o u t s f r o m t h e r o o t s , h o w e v e r r e p e t i t i v e t r e a t m e n t s w i l l r e p o r t e d l y s t a r v e u n d e r g r o u n d s t e m s , p r o b a b l y o n l y w h e r e t h e r e i s d e n s e v e g e t a t i o n , l i k e a l f a l f a , t o c o m p e t e w i t h i t . C l i p t h e a b o v e - g r o u n d p o r t i o n s b e f o r e s e e d s e t . T h e i d e a i s t o f o r c e i t t o u s e u p i t s r o o t e n e r g y r e s e r v e s w h i l e n o t r e p r o d u c i n g b y s e e d e i t h e r .
Mowing in pastures can be an effective control, if the thistles are cut at least once during the early bud stage. There must be dense vegetation present to compete with and shade the thistles. An alfalfa field in Montana mowed two times per year eliminated virtually all thistles after four years. Note that cultivating chops the roots into little pieces that sprout new thistles. Cultivating is only successful if it is repeated every 10 to15 days through the growing season for up to two years.
According to Heather Roads from the website www.gardeningknowhow.com, "Controlling Canada thistle organically is done with a sharp eye and an even sharper pair of scissors. Find the base of the Canada thistle plant and simply snip it off at the base. Do not pull Canada thistle out, as this can split the root, which causes two Canada thistles to grow back. Check the location weekly and snip off any new growth that you may see. The idea is to force the weed to use up its energy reserves by regrowing but removing the new leaves before the Canada thistle has a chance to build its energy reserves back up."
Biological Controls: World-wide there are at least 84 species of insects that feed on Canada thistle, and about half of them feed inside various parts of the plants. Several insects have been introduced into this continent to help control thistles, including the thistle defoliating beetle, Cassida rubiginosa, the thistle-stem gall fly, Urophora cardui, a stem-mining weevil, Ceutorhynchus litura, the thistle-head weevil, Rhinocyllus conicus and the thistle rosette weevil, Trichosirocalus horridus.
A type of fungus (Puccinia spp.) weakens the thistles enough to make them more susceptible to 2,4-D and more susceptible to other insects. Thistles treated with this fungus and a weevil (Ceutorhynchus litura) had a 50% increase in damage over untreated plants.
T h e N a t i o n a l S u s t a i n a b l e A g r i c u l t u r e I n f o r m a t i o n S e r v i c e ( A T T R A ) , f u n d e d m o s t l y b y a c o o p e r a t i v e a g r e e m e n t w i t h t h e U S D A , n o t e s t h a t t h e b i o c o n t r o l s l i s t e d a r e o f t e n s l o w t o g e t e s t a b l i s h e d a n d w o u l d n o t b e o f m u c h h e l p i n f r e e i n g f i e l d s o f t h i s t l e i n t h e f i r s t y e a r . O n t h e o t h e r h a n d , o n c e t h e y a r e e s t a b l i s h e d , a n d i f t h e y a r e n o t k i l l e d o f f b y p e s t i c i d e s , t h e y c a n p r o v i d e l o n g - t e r m b i o l o g i c a l c o n t r o l o f t h i s t l e . . . I n g e n e r a l i t s e e m s a p p a r e n t t h a t s o m e t h i s t l e p l a n t s w i l l h a v e t o r e m a i n i n o r d e r f o r t h e s e b e n e f i c i a l i n s e c t s t o r e t a i n t h e i r p o p u l a t i o n s .
Grazing: Research has shown that Canada thistle is equal or greater than alfalfa in nutrition. Livestock usually avoid the plant because of the spines, but they often eat the flowers. Sheep and goats eat the tender young thistles in the spring. Later in the year the plants can be treated with salt or molasses to encourage grazing and trampling. K a t h y V o t h h a s a s m a l l b u s i n e s s t e a c h i n g c o w s a l l o v e r t h e W e s t t o e a t w e e d s , i n c l u d i n g C a n a d a t h i s t l e. Even horses eat the thistles if they are cut first, apparently so they can graze without being poked in the nose. Grazing or trampling does not kill the plants, but it does reduce energy reserves in the roots and well-timed grazing can prevent seed production.
Animal Impact: Focused animal impact may effectively trample concentrations of the thistle while stimulating more desirable plant species to compete. Controlled tests with livestock need to be conducted with this weed.
Seeding: C a n a d a t h i s t l e c a n b e p a r t l y c o n t r o l l e d w i t h t h e a i d o f " s m o t h e r c r o p s " w h i c h d e v e l o p e a r l i e r i n t h e s e a s o n , f o r m i n g a d e n s e c o v e r t o s h a d e t h i s t l e s e e d l i n g s . S m o t h e r c r o p s a r e m o s t e f f e c t i v e i f c o m b i n e d w i t h r e g u l a r c u t t i n g s , s u c h a s a l f a l f a , o r f o r a g e g r a s s e s . C u t t i n g C a n a d a t h i s t l e o n a r e g u l a r b a s i s d e p l e t e s c a r b o h y d r a t e f o o d r e s e r v e s i n t h e r o o t s . A d e n s e s t a n d o f p r e f e r r e d p l a n t s e e d e d o r t r a n s p l a n t e d a n d s u p p o r t e d w i t h g o o d f e r t i l i t y s o o n a f t e r t h i s t l e i s c o n t r o l l e d . . . g o e s a l o n g w a y t o w a r d g o o d t h i s t l e c o n t r o l . S e e d i n g i n t e r m e d i a t e w h e a t g r a s s a n d t a l l f e s c u e r e d u c e d C a n a d a t h i s t l e d e n s i t y b y 6 0 p e r c e n t w h e n c o m b i n e d w i t h m o w i n g . S u g a r b e e t s , b u c k w h e a t a n d s m a l l g r a i n s h a v e a l s o b e e n p l a n t e d a s s m o t h e r c r o p s .
Vinegar: U.S. Department of Agriculture Researchers in Maryland tested various strengths of vinegar on Canada thistle. They found that either a 5% or 10% solution of vinegar burned off the top growth of Canada thistle. The plants, however regrew from the roots. Acetic acid in vinegar kills plant tissue by dissolving the cell membrane, which causes the plant to dry out. Vinegar works best when used in the sun. A word of caution, however: vinegar in concentrations greater than 5% acetic acid may be hazardous-burning the skin or damaging the eyes-and should be handled with care.
Chemical Controls: For small infestations, spot applications of glufonsinate-ammonium (Finale®) or glyphosate (Roundup®) are effective means of control, especially early in the season. These are non-selective herbicides, so care should be taken to avoid hitting non-target plants. The herbicides can be dribbled on the plants for wiped on with a sponge, but be sure to wear rubber gloves and goggles.
Large infestations of Canada thistle require selective herbicides to avoid killing desirable plant species. The herbicide 2,4-D amine is effective during the bud to flower stage when root carbohydrate reserves are low. Triclopyr and dicamba herbicides may be more effective, but also more persistent in the soil. These herbicides can migrate into the groundwater, especially in sandy soils.
Spraying should ideally be done when there is plenty of moisture for growth, otherwise the herbicides may not translocate well through the plants. Also note that there are many varieties of Canada thistle, some with resistance to certain herbicides. If one herbicide fails to get results then consider trying another. Nitrogen fertilizer may be added to stimulate better translocation of the chemicals.
According to ATTRA, The National Sustainable Agriculture Information Service, spraying thistles with herbicides kills the adult plants but doesn't stop seeds in the soil from sprouting and growing new plants. Biological controls (mentioned above) are slow to become established, but can provide long-term control to keep thistle populations in check, provided some plants remain as hosts for the beneficial insects.
Bottom Line: In a yard or small, manageable pasture, it is sensible to strive for total elimination. Try cutting the thistles off at ground level to remove all leaves, or spot spray the plants with vinegar, or if necessary, herbicide. Canada thistle has resilient roots and seems to bounce back after most treatments, including herbicides, so expect to revisit the issue a few times during the growing season, possibly over a few years, to completely eliminate the thistles.
For bigger thistle problems in larger pastures, consider spraying the weeds with a solution of molasses and water to make them more appealing to wildlife and livestock. Otherwise, consider buying or borrowing sheep or goats to help control the problem. As a last resort, utilize herbicides and be prepared to revisit the problem multiple times over several years.
Important: Most "weed problems" are really "people problems" from poor land management and a lack of ecological insight. It is easy to reach for a tool like fire, mowing, or herbicides to attack an out-of-control weed, but often those tools do little to get to the root cause of the weed infestation, and sometimes make the problems worse. Please read more about range ecology, desertification, and invasive weeds on this website before applying any tool of weed control Go to: Desertification and Invasive Weeds.
Notes
______. "Cirsium arvense." Plants for a Future:
https://www.pfaf.org/database/plants.php?Cirsium+arvense. June 8, 2009.
______. "Cirsium arvense." Species Abstracts of Highly Disruptive Exotic Plants at Effigy Mounds National Monument. Northern Prairie Wildlife Research Center:
https://www.npwrc.usgs.gov/resource/othrdata/explant/cirsarve.htm
B l a i r , K a t r i n a . T h e W i l d W i s d o m o f W e e d s C h e l s e a G r e e n P u b l i s h i n g: W h i t e R i v e r J u n c t i o n, V T. 2 0 1 4 .
Wildflowers-and-Weeds.com 
U s i n g t h e g r e e n s , s t e m , r o o t , a n d f l o w e r s, K a t r i n a B l a i r o f D u r a n g o , Colorado s e r v e s r a w t h i s t l e - b a s e d f o o d a n d d r i n k a t h e r w i l d f o o d c a f e , f a r m e r s m a r k e t s t a n d , a n d w i l d f o o d c s a : g r e e n d r i n k l e m o n a d e , M e x i c a n s t e m s n a c k s , f l o w e r h e a d s , r o o t k r a u t , r o o t c h a i , t e a , a n d l i q u e u r . [ R e t i r e d U S D A E t h n o b o t a n i s t J i m D u k e l i k e s t h e c o o k e d y o u n g l e a v e s a n d s t a l k s . O t h e r s l i k e j u s t - e m e r g e d , f e w - d a y - o l d b a b y t h i s t l e s i n s t i r f r i e s , q u i c h e s , a n d a n y w h e r e e l s e s p i n a c h m i g h t b e u s e d . W h e n t h e v e r y y o u n g t h i s t l e s a r e w e l l c o o k e d ( o r s t r a i n e d o u t , a s i n a g r e e n d r i n k ) , t h e s p i n e s a r e n o t a n i s s u e f o r m o s t p e o p l e a n d d o n o t n e e d t o b e r e m o v e d . S e n s i t i v e i n d i v i d u a l s , a s e v e r y o n e , s h o u l d t a k e a l l n e c e s s a r y p r e c a u t i o n s w h e n t r y i n g a n e w f o o d . 
