He that has a good harvest must be content with some thistles—Spanish Proverb

Bull thistle (Cirsium vulgare) photo by Nina Munteanu

On my daily walks to the local marshy stream by a large meadow, a cluster of Bull thistles has recently burst into brilliant purple-pink flower. The Bull thistle goes by the latin name Cirsium vulgare (vulgare meaning ‘common’) and was previously known as Cirsium lanceolatum—likely for its highly lanceolate leaves. Also known as the Spear thistle (for obvious reasons), the genus Cirsium derives from the Greek word for “swollen vein” because this plant was used in its treatment, along with other medicinal uses.

Attracted to its beautiful flowers, I decided to inspect it more closely. But this stunning deep purple-pink flowered plant is as sharp as it is beautiful. Brian Johnston of Microscopy-UK describes the thistle this way:

“The ubiquitous thistle flamboyantly discourages close investigation. Almost the entire plant is covered in very sharp spiny bristles that easily penetrate all but the most sturdy materials.”

And yet, close investigation is exactly what I wished to do. So, I did some research and then, after lunch, I returned to the Bull thistles armed with my friend’s gardening gloves, a set of small garden shears, a pair of tweezers and my trusty SLR camera.

Bull thistle flower head (photo by Nina Munteanu)

Description of Bull Thistle

To begin, the Bull thistle is a biennial plant (e.g., forms a leafy rosette during its first year of growth and a flowering stem in the second year). It’s also a member of the Aster family.

Bull thistle flower head (Capitula) and involucre (photo by Nina Munteanu)

Bull thistle flower head showing hundreds of tubular florets and style coated with white pollen (photo by Nina Munteanu)

Like the aster, the Bull thistle is a composite plant with a dome-like flower head or capitulum of up to 200 hundred tubular florets (together called an inflorescence); flower heads may occur solitarily or in terminal clusters of two or three on a single plant. Unlike asters, the Bull thistle flower head contains only disk (not ray) florets.

Each floret is bisexual (containing female stigma and style and male anthers). The pink corolla (flower petals), longer than the floret tube itself, have throats with five lobes from which emerges the long style. The style is tightly surrounded by five stamens, shorter than the style.

Close-up of a Thistle floret, showing male (fused anthers) and female parts (stigma/style and ovary/achene) and pappus (modified sepal and achene transportation mechanism) (photo by Nina Munteanu

Several of the thistles I observed in the woodland-field ecotone where I walked had white flowers amid the deep purple ones. CABI tells us that white-flowers are rare worldwide but have been spotted in Canada, especially in British Columbia and in Ontario, where I currently reside.

The flower head sits on a narrow vase-shaped involucre (collection of bracts) which surrounds and protects the enclosed achene-pappus units (the dry seeded fruit and protective calyx).

Emerging flower head from the spiny involucre of the Bull thistle (photo by Nina Munteanu)

The involucre consists of hundreds of grayish-green bracts (modified leaves) or phyllaries, armed with very sharp, stiff yellow spines. The spines help guard the thistle against insects wishing to climb the stem in search of the nutritious seeds (achenes). The upper bracts normally curve up towards the flower head (reflexed) and the lower ones bend down (decurrent).

Cross-section of Bull thistle, showing receptacle (thistle heart) inside the protective involucre and on which the seeds are embedded (photo by Nina Munteanu)

Inside the protective shell of bracts, the ovaries of each floret attach to the receptacle, the widest part of the involucre and a fleshy part of the flower-base. It’s the heart of the inflorescence (think ‘artichoke heart’ or the fleshy part of the strawberry on which seeds are embedded). When fertilized, the ovaries develop into dry fruits called achenes (a dry fruit that doesn’t dehisce or open to disperse its seed). The achene of the thistle is called a cypsela—a fruit that looks like a seed.

Lancelate leaf of Bull thistle showing sharp spines (photo by Nina Munteanu)

The stem and leaves of the Bull thistle are also protected by sharp prickles. In fact, the bull thistle can be distinguished from other thistles by the covering of the short, sharp prickles on the upper, dark green surface of each highly lobed lanceolate leave blade.

Bull Thistle Sex

Bumble bee with legs full of pollen feeding on Bull thistle nectar (photo by Nina Munteanu)

Because the pollen won’t germinate on the thistle flower that produces it, the Bull thistle needs to get its pollen to another flower head through cross-pollination by pollinators such as an insect or bird. The Bull Thistle presents its pollen to pollinators through a stigma/style piston mechanism. The style pushes the stigmas out from between the pollen-carrying anthers that surround it. The anthers release their pollen on the inside and as the style and stigma push past, they collect pollen from the dehisced anthers. The globs of sticky pollen are then ready to be picked up by pollinators searching for the nectar inside the tiny flowers. As the insects move from one flower to the next, they release some pollen on the sticky stigma; the pollen then travels down the tube to fertilize the egg at the base. The fertilized egg develops into an achene or seed among many others embedded in the receptacle inside the protective shell of the involucre (collection of bracts below the flower head) awaiting dispersal.

I’m told that thistles of the genus Cirsium are described as Plume Thistles because the fruit has a pappus (modified sepals) of feathery hairs; each hair holds many side-branches. The branched pappus acts as a parachute to reduce the terminal velocity of the falling achene so that wind can disperse it farther. The branching hairs also help to loosen the achenes from the capitulum when captured by the wind.

Dried involucre of Bull thistle revealing achenes and branching pappus (photo by Nina Munteanu)

Why Some People Don’t Like the Bull Thistle

Close-up of achene and pappus (photo by Nina Munteanu)

Originally from Eurasia, the Bull thistle was introduced to North America and has thrived here. CABInternational informs us that this thistle “has the potential to compete with many crops and natural species and displace them from their natural habitats. Since this species can tolerate adverse environmental conditions and adapt to different habitats, it continues to spread and occupies new areas despite control measures. High seed production, variation in seed dormancy, and vigorous growth make this species a serious invader. It competes with other species in pastures, rangelands and agricultural fields and causes both wool fault and physical injury to animals.

Bull Thistle Ecology

Although it is considered an invasive species and weed, the Bull thistle still contributes beneficial ecosystem services such as serving as nectar sources for pollinators such as honey bees (Apis spp.), bumblebees (Bombus spp.), sweat bees (Agapostemon spp.) and butterflies such as the monarch, tiger swallowtail, skippers, among others).

a honey bee looking for nectar on the Bull thistle flowerhead (photo by Nina Munteanu)

Cirsium vulgare also ranked in the top 10 for nectar production in a recent UK survey. Bull thistle was also a top producer of nectar sugar in another study in Britain; it ranked third with a production per floral unit of (2323 ± 418μg).

Bumble bee feeding on a Bull thistle (photo by Nina Munteanu)

The Bull thistle also provides seeds to granivores (consumers of grains and seeds) such as goldfinches. The goldfinch is particularly attracted to the thistle. Not only does the goldfinch pull the seeds out to eat; it also uses the thistle down (the pappus) to line its nest.

The Bull thistle was used by early humans as a warm medicinal tea. The roots were also good against poor digestion and helped treat stomach cramps. The leaves were used to treat neuralgia.

Eflora describes several uses of thistle from tinder (seed fluff) and papermaking (using the inner bark) to medicinal uses in teas and poultices (e.g., for sore jaws, treating rheumatic joints, or bleeding piles.

How to Identify the Bull Thistle

You can identify the Bull thistle by several features:

• Flower head or capitula (larger than Canada thistle) often gumdrop-shaped, containing hundreds of tiny purple flowers, all being disk florets (no ray florets).
• Fluted shape of the involucre below the flower head (not rounded like the Scotch thistle) more slender and usually more numerous than in Nodding thistle or Plumeless thistle. The phyllaries (involucral bracts) are each tipped with a very sharp, stiff yellow spine. The upper bracts normally curve up towards the flower head and the lower ones bend down. Hairs that look like cobwebs lie scattered among the series of phyllaries.
• Its stem is light green with dense white hair on its angular circumference, punctuated by a series of winged spines.
• Sharply lobed leaves, each with a stiff spine at its tip, that are coarse on the top with spines and soft on the bottom.

Eating the Bull Thistle 

Given that the Bull thistle belongs to the Artichoke sub-family (Carduoideae) and tribe (Cynareae), I wondered if the Thistle was edible. What we normally eat of the artichoke are the soft pads inside the bracts that surround the flower cluster, and the receptacle (the heart) to which the florets attach. 

After watching two videos about parts of the Bull thistle being edible (e.g. leaf, flower receptacle, and roots), I had to try it for myself. The trick was convincing my naturalist friend Merridy to join in the experiment… 

Collecting thistle heads (photo by Merridy Cox)

Cutting away spiny involucre to expose the heart (photo by Merridy Cox)

EatTheWeeds demonstrated that a sharp knife could cut away the spines of a leaf, leaving the edible leaf rib in tact. They also showed how you can harvest the root (which means destroying an entire plant), but it looked like too much work and I decided that I wasn’t prepared to sacrifice an entire plant to satisfy my curiosity. What would the goldfinches do then? However, The Northwest Forager demonstrated how one can easily harvest a few flower heads for a taste of thistle hearts.

Merridy was curious enough to join me and we left for the thicket by the marsh meadow. First we collected late-blooming flowerheads that had fairly sizable involucres—and therefore large receptacles inside. We collected about ten then returned to our outside lab to process them. 

Removing the achenes and pappus, revealing the thistle heart (photo by Merridy Cox)

Thistle heart (photo by Nina Munteanu)

The next step was to cut away the involucre near the stem and remove the florets with the pappas and achenes. I had to pull them away firmly, revealing a rounded whitish receptacle that looked like a pocked mini-dome—the heart of the thistle inflorescence—with visible ‘spiral’ arrangement.

As instructed by The Northwest Forager, we then boiled the hearts for 10 minutes then drained and followed by frying the hearts in butter with some sea salt for a few minutes.

Frying up the thistle hearts (photo by Nina Munteanu)

Merridy and I then simply took each tiny thistle heart and gnawed on it with our teeth. I was overjoyed by a lovely subtle nutty kohlrabi-like flavour. When I asked Merridy what it tasted like to her, she responded in her direct way: “like a Jerusalem artichoke.” I have eaten many an artichoke heart and found these mini-versions an excellent similarity. 

When we were done with the process, Merridy thought someone might like to eat the seeds we’d removed to eat the thistle hearts. They were thrown on the patio and very soon a curious chipmunk came to test them. 

Chipmunk checks out thistle seeds (photo by Merridy Cox)

Cutting off the involucre and pulling apart the florets to reveal the receptacle was a bit of work; but it was worth it for me to discover and appreciate more about this complex plant.

References:

CABI Invasive Species Compendium. 2020. “Cirsium vulgare (spear thistle)”.

Cronodon.com. 2020. “Asteraceae—Thistles”

Eflora.neocities.org. “Bull Thistle—Cirsium vulgare”

Friends of the Wild Flower Garden, Inc. 2020. “Bull Thistle (Spear Thistle, Plume Thistle, Lance-leaved Thistle)”

Forcella and Randall. 1994. “Biology of bull thistle, Cirsium vulgare (Sai) Tenore. Reviews of Weed Science, 6: 29-50.

De Jong, T.J. and Klinkhamer, P.G.L. 1998. population ecology of the biennials Cirsium vulgareand Cynoglossum officinale in a coastal sand-dune area. J. Ecol. 76: 366-382.

EatTheWeeds. 2009. “Episode 109: Bull Thistle”

Govan, Carol. 2019. “Common/Bull Thistle” SeeingNatureCCG, March 10, 2019.

Grime et al. 1988. “Comparative plant ecology. A functional approach to common British species.” Unwin Hyman Ltd., London, UK. 679pp.

Holm et al., 1997. “World weeds: natural histories and distribution.” Wiley-Blackwell, 1129pp.

Johnston, Brian. 2004. “A Close-up View of Two Thistles: “Bull” and “Nodding””

Michaux, B. 1989. Reproductive and vegetative biology of Cirsium vulgare (Savi) Ten. (Compositae: Cynareae). New Zealand Journal of Botany 27: 401-414.

Moore and Frankton. 1974. “The Thistles of Canada. Monograph, Research Branch, Canada Department of Agriculture, No. 10: 112pp.

Ontario Ministry of Agriculture, Food and Rural Affairs. 2016. “Ontario weeds: Bull thistle”

Sheldon, J.C. and Burrows, F.M. 1973. The dispersal effectiveness of the achene-pappus units of selected compositae in steady winds with convection. New Phytol. 72: 665-675.

The Northwest Forager. 2017. “Mini artichokes for FREE! Foraging the Bull Thistle”

Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press(Vancouver) was selected by Margaret Atwood in the New York Times‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” was released by Inanna Publications (Toronto) in June 2020.