Montana Field Guide

PLANTS: Taprooted, biennial forbs. Stems are stout, erect, often branched, and 30–120 cm tall. Stems are spiny, and winged with green, spreading long, stiff (hirsute) to sparsely entangled, cobweb-like (arachnoid) hairs. Source: Lesica et al. 2022

LEAVES: Basal leaves absent or present at flowering. Basal leaves taper to their base with a winged petiole. Stem leaves are alternately arranged with short petioles that become sessile upwards; primary petioles are winged. Leaf blades are oblong-lanceolate to obovate, 5-30 cm long, deeply pinnately lobed with rigidly divergent lobes, and spiny margins that are plane to revolute. Upper leaf surface has bristle-like spines. Lower leaf surface is green with distinct whitish veins (not tomentose). Sources: Keil in Flora of North America (FNA) 2006; Giblin et al. [eds.] 2018; Lesica et al. 2022

INFLORESCENCE: Purple (white) flower heads are mostly solitary at stem tips and arranged in open corymbiform arrays. Flower stalks (peduncles) are short, but at least some are more than 2 cm long. Involucres are 2.5–4 cm high. Involucral bracts are imbricate in 10 to 12 rows; each bract is green, and linear-lanceolate in shape, and sparsely arachnoid. The outer bracts are keeled, not resinous, and tipped with an erect or spreading spine of 2-10mm long. Sources: Giblin et al. [eds.] 2018; Lesica et al. 2022

TAXONOMY & NOMENCLATURE
Cirsium is derived from the Greek word kirsos which means "a swollen vein" and refers to the effect when pricked by the spines (Mitich 1998). The specific epithet of vulgare comes from the Latin word for "common" (Mitich 1998).

The common name of Bull Thistle was used in the USA and Ireland, while Spear Thistle was used in England (Mitich 1998). Bull Thistle first appeared in the English language in 1878 in the A Dictionary of English Plant-Names by Britten and Holland (Mitich 1998). In the 11th century, the Scots referred to Bull Thistle as their "guardian thistle". The legend is told that during the nighttime an invasion of Danish soldiers trodden on thistles, and created much bellowing which alerted the Scottish forces (Mitich 1998). It is possible that the Bull Thistle is the original Scottish Thistle – or not (Mitich 1998; Plant Lore).

Typically flowering from June to September, or year-round in mild climates (Keil in FNA 2006).

On first-glance thistles can look similar, but upon a closer inspection differences become apparent. Thistles belong to the genera of Cirsium, Carduus, and Onopordum, which all have spiny-margined leaves and often have flower heads with spiny bracts. Ecologically, native and non-native thistles are very different.

NATIVE versus NON-NATIVE THISTLES [Parkinson and Mangold 2015]
Native Thistles
* Plants grow relatively sparsely and possess few or gentler spines, intermix with many plant species, and are slow to colonize disturbed ground.
* Flowers provide nectar and pollen for numerous native birds and insects, and forage for some wildlife. For example, elk eat the flowers of Elk Thistle.
* Involucral bracts tend to adhere to the flower head for most of their length (except for the spine).
* Plants are not rhizomatous except for Flodman’s Thistle which can produce horizontal runner roots.

Non-native Thistles
* Plants colonize disturbed ground quickly, often form dense patches, and produce nastier spines - limiting recreational activities, injuring people/animals, and reducing native plant species diversity.
* Flowers provide nectar and pollen for some birds and insects, but not forage for wildlife or livestock.
* Some species are aggressively rhizomatous and outcompete native plants that provide nutritional forage.
* Require management to control, reduce, or remove. Refer to the MANAGEMENT subsection.

DIFFERENTIATING THISTLE GENERA
Carduus
* Stems: Winged.
* Pappus: Barbellate - minutely barbed, narrow bristles. Bristles usually fall separately.
* Flower Head - Receptacle: Not obviously fleshy or honeycombed. Densely bristly. In the flower head, look for bristles between the florets.

Cirsium
* Stems: Winged or not winged.
* Pappus: Feathery (plumose) - fine, long hairs on each side of the central axis (rib).
* Flower Head - Receptacle: Densely bristly. In the flower head, look for bristles between the florets.

Onopordum
* Stems: Spiny and winged along their entire length.
* Pappus: Barbellate - minutely barbed, narrow bristles. Bristles connected at base.
* Flower Head - Receptacle: Definitively fleshy and honeycombed. No or very sparse and short bristles. In the flower head, look between the florets to find nothing.

Montana has 15 thistle species, and only 5 are described below:

Bull Thistle – Cirsium vulgare, exotic and undesirable
* Flower heads are mostly single at stem tips and arranged in an open inflorescence.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
* On the flower head the outer bracts tend to point outwards and upwards, are needle-like and long.
* Leaves are deeply lobed, green beneath with cobwebby hairs and obvious white veins.
* Leaves have many sharp, short spines. Entire plant has spines, some very long, making it difficult to touch without injury.
* Plants are taprooted.

Canada Thistle - Cirsium arvense, exotic and Noxious
* Flower heads have involucres less than 2 cm tall [examine larger heads].
* Each flower head consists of either male florets or female florets.
* Leaves are arachnoid-villous, but the green leaf remains visible.
* Stems lack an obvious winged stem.
* Plants are strongly rhizomatous.

Scotch Thistle – Onopordum acanthium, exotic and undesirable
* Receptacle of flower head has no bristles.
* Entire lengths of stems have spiny wings, becoming broad and spiny.
* Foliage is silvery gray and can grow taller than 6 feet.

Musk Thistle - Carduus nutans, exotic and undesirable
* Flower heads have involucral bracts that are broadly triangular, have smooth margins, and a short spine-tip.
* Heads nod as flowers mature.

Wavyleaf Thistle - Cirsium undulatum, native and desirable
* Upper leaf surface lacks spines AND white-tomentose hairs making it appear gray.
* Involucral bracts tend to point upwards with inner bracts acuminate.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
* Most flower heads not clustered and peduncles more than 2 cm long.

Flodman’s Thistle - Cirsium flodmanii, native and desirable
* Upper leaf surface lacks spines AND has sparse white-tomentose hairs making it appear green.
* Involucral bracts tend to point upwards with inner bracts acuminate.
* Flower heads have involucres more than 2 cm tall [examine larger heads].
* Most flower heads are not clustered and some peduncles are more than 2 cm long.

Long-styled Thistle - Cirsium longistylum, native, Montana endemic, and SOC
* Upper leaf surface lacks spines.
* Inner and outer bracts are wide, scarious, and with erose tips AND outer bracts have a raised, darkened, and resinous keel.
* Flower heads have involucres more than 2 cm tall [examine larger heads].

Bull Thistle is an early successional species that establishes in disturbed meadows, thickets, and roadsides in the plains, valleys, and montane zones of Montana (Lesica et al. 2022). Bull thistle establishes after disturbance, such as from ground-disturbing activities, fire, and forest logging (Zouhar 2002). In the Intermountain region, plants can occupy disturbed habitats that range greatly in moisture and elevation, such as forests dominated by Grand Fir (Abies grandis), Subalpine Fir (Abies lasiocarpa), Ponderosa Pine (Pinus ponderosa), and Western Redcedar (Thuja plicata), and grasslands in the Great Plains region (Zouhar 2002). In addition, plants have colonized alpine sites in Utah and a subalpine riparian site in Montana (Zouhar 2002). Although plants establish in wet or dry sites, intermediate moisture provides optimal conditions (Zouhar 2002).

ESTABLISHMENT
Bull Thistle plants are promoted where soil is disturbed, or vegetation is removed. Disturbance regimes include excessively grazed rangelands or pastures, harvested forests, burned land, gopher digging, and disturbance corridors along fences, roads, and ditches. In clearcuts and plantations that are at least eight years old and lack further disturbance, few Bull Thistle plants remain.

POLLINATORS
Bees and butterflies feed on the nectar produced from flowerheads (Zouhar 2002). The following bee species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus appositus, Bombus auricomus, Bombus bifarius, Bombus borealis, Bombus centralis, Bombus fervidus, Bombus flavifrons, Bombus frigidus, Bombus huntii, Bombus mixtus, Bombus nevadensis, Bombus rufocinctus, Bombus sylvicola, Bombus ternarius, Bombus terricola, Bombus sitkensis, Bombus occidentalis, Bombus pensylvanicus, Bombus bimaculatus, Bombus griseocollis, Bombus impatiens, Bombus insularis, Bombus suckleyi, Bombus bohemicus, and Bombus flavidus (Thorp et al. 1983, Mayer et al. 2000, Wilson et al. 2010, Colla and Dumesh 2010, Colla et al. 2011, Koch et al. 2012, Williams et al. 2014, Tripoldi and Szalanski 2015).

Plants reproduce by seed.

FRUITS & SEEDS [Adapted from Zouchar 2002]
Fruit is an achene (or cypselae), light brown with dark streaks, 3-5 mm long, and with an undifferentiated apical collar (Lesica et al. 2022). Chromosome number: 2n=68. Attached to the achene is a pappus, 20-30 mm long. While the pappus facilitates travel by wind, it readily disassociates as the seed matures; therefore, wind is not the primary means for dispersal (Mitich 1998).

Seed production and viability are affected by the flowering period, maturity and size of plants, environmental conditions, herbivory, and plant competition. In the Netherlands, seed production was positively correlated with July rainfall. In Australia Bull Thistle plants produced an average of seeds nearly three times higher in grazed pasture than in ungrazed pasture. Seed production is highest in flowerheads that bloom during the peak of flowering for a given location.

Seeds are dispersed by wind and water, and are carried by vehicles and farm machinery, on the fur or feathers of animals, through animal manure, and as a contaminant in baled and loose hay and in commercial legume and cereal seed supplies (Mitich 1989). Wind dispersal is not the primary vector for dispersal. The most common vector for long-distance dispersal comes from infested hay (Mitich 1989). A great majority of seeds fall within a distance that is 1.5 times the height of the parent plant; this has been attributed to a pappus that easily detaches from the seed.

Bull thistle seeds are generally short-lived, and develop a transient or very short persistent seed bank. In natural areas, seeds that fall on the ground either germinate, are eaten by animals, or are destroyed by microbes. Therefore, the species is not capable of producing a persistent seedbank that would maintain a population.
However, buried seeds exhibit an induced dormancy, decay more slowly, and have the potential to form a more persistent seedbank. In one study, seeds buried to at least six inches had a 50% viability rate after a 3-year period of burial. Studies have shown the potential for seeds buried in forest litter and soil to germinate after a disturbance in places without the parent plants. While populations often establish from seeds that colonizeg early seral conditions, establishment could also come from disturbance events that uncover buried, dormant seeds.

Seed germination rates are typically high, ranging from 60 to 90% or greater. Up to 90% of seeds may germinate within a year following dispersal. In the Netherlands, seed germination appears to be regulated by the interactions between moisture, soil nitrate levels, and temperature. Germination rates decrease for seeds exposed to two or more cycles of wetting and drying. Germination rates also decrease as light levels decline. For seeds that do not germinate in the fall, the darkness of winter can induce a dormancy (preventing germination). Studies have also found that seeds landing in small canopy gaps germinate at lower rates than those falling in large canopy gaps. Thus, seeds remaining in darkness or buried in soil and litter can help form a persistent seed bank.

Temperature also has a role in seed germination. Germination is optimal when temperatures range from 50 to 86 degrees Fahrenheit. Fresh seeds require higher temperatures to germinate than stored seeds.

LIFE CYCLE [Adapted from Zouhar 2002]
Populations are short-lived. They establish best where the site is disturbed, dominate for a few years, and then decline as other vegetation recovers. Seeds germinate in the spring when soil temperatures rise or in response to autumn rains. The stage from seedlings to rosettes is when plants have the highest death rate. A seedling's survival increases where soil nutrient levels are higher. Annual death rates at the rosette stage range from 10 to 69%. Survival rates of rosettes improve with their size.

In the first growing season, an individual Bull Thistle produces a small rosette of spiny leaves and a fleshy taproot, and then suspends growth to overwinter. In the second growing season, the rosette grows rapidly, stems bolt, and produces a flowering stalk. Rosettes need to attain a certain size and before exposed to low temperatures (vernalization period) before bolting and flowering. However, the vernalization period varies with geography with some plants flowering without vernalization. As air temperature increases, the timer interval between bolting and seed maturation increases. Plants flower from mid- to late-summer; although, flowering can continue until the first frost or autumn snowfall. A single Bull Thistle plant may remain in a state of flowering from 1 to 6 weeks. Each plant produces from 1 to more than 400 flowerheads. Each flowerhead can produce about 100 to 300 or more seeds. Within the flowerhead, florets on the outside mature before those on the inside.

Flowering Bull Thistle plants release seed in late summer to early autumn. Within a given flowerhead, seeds immediately disperse upon maturity. The lifespan of the flowerhead lasts for only a few days, upon which tissues wilt and the involucre bracts turn brown and open up. As the flowerhead dries, the receptacle arches upwards and the innermost seeds disperse first. The entire plant dies after they set seeds. The dead plant can remain standing for up to two years.

PLANT BIOLOGY
Bull Thistle is the only Cirsium to have bristle-like spines on the upper leaf surface (Keil in FNA 2006). In the literature, the bristle-like spines go by many names, including trichome (spreading hirsute, coarsely hispid, setae, etc.), prickles, and spines. In examining the structures, Keil found that the bristle-like spines emerge from fine veinlets within the leaf's tissues, which means that "spine" is the correct term (FNA 2006). The structures are not trichomes or prickles which arise from epidermal outgrowths (Keil in FNA 2006).

CULTURAL USES
Carduus and Cirsium thistles have been used for foods and medicines by humans for centuries. Young stems and roots of Bull Thistle are edible (Zouhar 2002). In fact, all thistles are edible in small quantities though palatability differs by species and individual plants (Tilford 1997). However, some thistle species contain alkaloids which are potentially carcinogenic (Tilford 1997).

Native Americans used different species of Cirsium thistles to treat respiratory congestion, dermatitis, parasitic infections, and venereal disease, and to help stop bleeding, stimulate milk production in nursing mothers, and as a contraceptive (Tilford 1997).

Bull Thistle is an invasive plant. Its establishment, spread, and increase in abundance are associated with soil or vegetative disturbances from natural and human-caused origins. Many land owners, land managers, and researchers consider Bull Thistle to be the most common and widespread non-native thistle in pastures and rangelands (Zouhar 2002).

COUNTY & STATE DESIGNATIONS
As of 2024, Bull Thistle is listed as County Noxious in Glacier and Meagher Counties, Montana.


INTEGRATED VEGETATION MANAGEMENT
Successful management of non-native thistles requires that land-use objectives and a desired plant community be identified (Shelly et al. in Sheley and Petroff 1999). Once identified, an integrated weed management strategy that promotes a weed-resistant plant community and serves other land-use objectives, such as for livestock forage, wildlife habitat, or recreation, can be developed. Often a combination of compatible control efforts will better achieve the desired objectives.

PREVENTION [Adapted from Zouhar 2002]
Preventing the establishment of non-native thistles can be accomplished by many practices:
* Learn how to differentiate “native” from “non-native” thistles and their species. Refer also to Diagnostic Characteristics.
* Learn how to differentiate Bull Thistle from other non-native thistles.
* The key to successfully management of Bull Thistle is to prevent seed production. However, zero seed production may not be realistic. Target seedlings and rosettes to effectively and efficiently control Bull Thistle.
* Prevent vehicles from driving through, and animals from grazing within, infested areas.
* Thoroughly wash the undercarriage of vehicles and wheels in a designated area before moving to an un-infested area.
* Frequently monitor for new plants, and when found implement effective control methods (Early-Detection and Rapid-Response).
* Maintain proper grazing management that creates resilience to noxious and non-native weed invasion.

RE-VEGETATION CONTROL [Adapted from Zouhar 2002]
Establish desirable and competitive vegetation to deter Bull thistle germination and establishment. Land rehabilitation isn't complete until a diversity of desirable plant species is established, and Bull Thistle is absent or reduced to a minor component of the plant community. Revegetation using aggressive desirable plant species has been demonstrated to inhibit reinvasion of Bull Thistle, especially when used with appropriate biocontrol and grazing management practices. Readers wanting to know more should consult with their local County Extension Agent, Weed District, Natural Resource Conservation Service off, and/or Montana State University Extension Service.

BIOLOGICAL CONTROL [Adapted from Winston et al. 2012 and 2016]
A variety of biocontrol agents (insects, fungi, and others) have been brought into North American to control non-native thistle plants. A fair amount of preliminary work and an array of factors must be considered when developing a biocontrol plan for controlling thistles. Readers are encouraged to consult the "Useful Links" and cited literature in this profile.

Biocontrol agents approved in the USA include:
Musk Thistle Flower Fly (Cheilosia corydon) feeds on Bull Thistle.

Bull Thistle Gall Fly (Urophora stylata) feeds on Bull Thistle.

Biological control insects no longer approved or not recommended include:
Thistle Seedhead Weevil (Rhinocyllus conicus) was intentionally released in 1968, in Canada, and 1969, in USA, to control exotic thistles – specifically Musk, Scotch (failed), and other species (Keil in FNA 2006; Winston et al. 2016). It is unknown i>Rhinocyllus conicus is effective on Bull Thistle (Zouhar 2002). After establishment, it was discovered that Thistle Seedhead Weevil fed on numerous native Cirsium thistles. Therefore, interstate shipment permits were revoked in 2000, and Thistle Seedhead Weevil is no longer approved for re-distribution for any use. Thistle Seedhead Weevil has been documented to attack 22 of 90 native Cirsium species in the US – even in places with non-native thistle species.

PHYSICAL and CULTURAL CONTROLS [Adapted from Zouhar 2002]
Physical and mechanical methods that sever the roots below ground will kill Bull Thistle plants. However, the site must be re-vegetated with the desirable plant community to prevent Bull Thistle seeds from germinating.

Tilling & Hoeing can effectively control Bull Thistle if done prior to flowering to prevent seed production. Bull Thistle does not persist where land is cultivated. However, most Bull Thistle populations grow in natural areas, wildlands, and rangelands where tillage is not appropriate because it can cause other ecological damage.

Slicing stems at the ground level can be effective to kill plants. In Yosemite National Park, 5% of Bull Thistle plants that were slice resprouted while in control plots 80% of plants survived and flowered. For the plants that re-sprouted the number of flowers and seeds produced was also lower.

Hand-pulling can effectively control Bull Thistle if done prior to flowering to prevent seed production.

Mowing alone is not effective, as mowed plants can still flower and produce seed. In addition, populations are often of mixed ages, so a single mowing will not effectively control the population. Mowing can trigger re-sprouting if done too early in the growing season.

GRAZING CONTROLS [Adapted from Zouhar 2002]
Proper grazing management will stimulate grass growth and keep rangelands and pastures in a healthy condition. Healthy rangelands and pastures are resilient to Bull Thistle invasion. Resting land at prescribed intervals will allow grasses to recover from grazing. Timing rest with precipitation cycles will allow available soil moisture to stimulate grass growth.

Bull Thistle is considered not palatable, and is usually avoided by native and domesticated ungulates because of its spines. In addition, Bull Thistle has no nutritive value for livestock animals. In pastures, Bull Thistle responds well to nitrogen fertilization, but has no relationship to potassium or phosphorus content. A study in New Zealand found that damaged rosettes were stimulated to re-grow. Therefore, in excessively grazed pastures and rangelands, Bull Thistle populations persist and/or increase, which reduces desirable forage. In Australia, a study found that in grazed pastures about 15% of seeds developed into seedlings, and of those about 1% survived. In ungrazed pastures the researchers found that 10% of seeds grew into seedlings, and 0.2% of them survived. However, in both pasture types 50% of rosettes grew into adult plants.

Domestic goats and sheep eat Bull Thistle seedlings. Where sheep grazing reduces plant competition, Bull Thistle plants can exhibit increased growth, flower and seed production, and plant survival. However, timing might be critical in that spring grazing may encourage growth while late summer and autumn grazing may eat rosettes and seedlings.

FORESTRY [Adapted from Zouhar 2002]
In the Oregon Coast Range, tree growth of planted Douglas-fir seedlings improved after weeds, including Bull Thistle were controlled. In California, the density of thistles was negatively correlated to the growth of Ponderosa Pine saplings growing within a 7-foot distance.

FIRE [Adapted from Zouhar 2002]
The effect of fire on Bull Thistle depends upon many pre-and post-burn factors – including fire conditions (severity, timing, and weather conditions before and after the burning), soil moisture, and vegetative composition and response. Fire may or may not kill Bull Thistle plants. In south-central Idaho, bull thistle was growing on a Douglas-fir site where a prescribed fire was implemented. The frequency of Bull Thistle declined following the fire but increased 3 years after the burn.

Fire may enhance or depress colonization by Bull Thistle plants. Fire removes vegetation providing a surface that is potentially conducive to its establishment. However, in South Dakota a late spring prescribed burn conducted on a 4- to 5-year cycle encouraged the re-growth of native plants and discouraged the growth of Canada, Musk, and Bull Thistle plants. Researchers in South Dakota suggested that a late spring prescribed burn would encourage greater grass and less forb production.

CHEMICAL CONTROLS [Adapted from Zouhar 2002]
Herbicides are effective in gaining initial control of new invasions or very large infestations but
are a temporary control tactic because the chemicals don't change the environmental conditions. Chemicals are most effective when carefully planned, timed, and incorporated with long-term control tactics that include replacing weeds with desirable plant species, modifying land use management, and preventing new infestations.

As a biennial plant, autumn is a good time to control Bull Thistle because all live plants are seedlings or rosettes and easy to spray, but could be more difficult to locate. Autumn can bring colder temperatures, which could hamper the effectiveness or application of herbicides.

The herbicide type and concentration, application time and method, environmental constraints, land use practices, local regulations, and other factors will determine its effectiveness and impact to non-target species. Strict adherence to application requirements defined on the herbicide label will reduce risks to human and environmental health. Consult your County Extension Agent and/or Weed District for information on herbicidal control. Chemical information is also available at Greenbook.Clopyralid, dicamba, MCPA, picloram, 2,3-D, metsulfuron, and chlorsulfuron chemicals will all kill Bull Thistle plants. It is also critical that herbicides be applied to prevent damage to broadleaf and grass forage species and other non-target plants. Readers can also consult the references in Zouhar (2002).

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Field Guide for Biological Control of Weeds in Montana
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association - Weed District Contacts
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides

Bull Thistle is an early successional species the readily colonizes disturbed lands that represent a spectrum of habitats, such as forests, rangelands, prairies, grassland, riparian, roadsides, and ditches, from valleys to the subalpine (Zouhar 2002). Its widespread distribution and broad ecological tolerances, allows it to colonize almost anywhere where there is soil or vegetative disturbance. Bull Thistle is problematic in pastures, rangelands, and forest management lands, because it competes strong with and decreased desirable forage and has no significant nutritive value for domesticated livestock or native ungulates. The extremely spiny plant makes it not palatable and deters herbivory which could control populations. Plants have no natural competitors which has aided its ability to colonize and spread. Bull Thistle seeds are also easily transported by vehicles and farm equipment, as a contaminant in hay, legume, and cereal production, and on the fur or feather of animals. Plants are highly competitive, producing a large number of viable seeds that germinate upon contact with the soil surface and is followed by rapid growth. As a result, plants grow ungrazed, increase in numbers, and decrease the amount of forage or other desirable plants in a given area.

In forests, studies have found correlations between adjacent dense populations of Bull Thistle and reduced growth rates of Douglas-fir and Ponderosa Pine trees. Bull Thistle populations can be problematic in clearcuts and conifer plantations due to their strong competitive growth.

In protected areas, such as Yellowstone and Glacier National Parks, Bull Thistle is a serious pest because it changes the natural plant community. The spiny plants interfere with recreational activities, especially for people who hike through patches. Populations that establish on trails which are naturally prone to disturbance require additional maintenance. At the Jerry Johnson hot springs in Idaho, Bull Thistle populations in the 1990s were problematic and dangerous for visitors hiking and soaking (Pipp personal communication).