Montana Field Guide

PLANTS: Strongly rhizomatous, perennial forbs. Stems are erect, often branched above, and 30–100 cm. Stems have sparse hairs (glabrate). Individual stems are unisexual. Source: Lesica et al. 2012

LEAVES: Basal leaves have regularly spaced, coarse, marginal hairs, and shallowly lobed. Stem leaves are alternately arranged with short petioles. Leaf blades are oblanceolate to elliptic, usually with crinkled edges and spiny-toothed margins, very irregularly lobed, 3-15 cm long, and terminate in a spine. Leaves are sometimes tomentose beneath. Upper leaves become small and decurrent. The lowest leaves usually become deciduous. Sources: Morishita in Sheley and Petroff 1999; Lesica et al. 2012.

INFLORESCENCE: Purple flower heads are arranged as several per stem in a corymbiform array with peduncles (stalks) of 0–4 cm long. Involucres are 1–2 cm high. Involucral bracts are imbricate in 6 to 8 rows. The outer bracts are ovate with a darkened resinous keel-tip. The inner bracts are linear, Bracts either have short or absent spines. Source: Lesica et al. 2012

On first-glance thistles can look similar, but upon closer inspection differences become apparent.
Thistles belong to the genera of Cirsium, Carduus, and Onopordum. They are separated by:

Cirsium
* Feathery (plumose) pappus, which have fine, long hairs on each side of the main bristle.
* Receptacle of flower head has bristles. Look between florets within a flower head to find them.

Carduus
* Capillary pappus, which are minutely barbed, narrow bristles.
* Receptacle of flower head has bristles. Look between florets within a flower head to find them.

Onopordum
* Receptacle of flower head has no bristles. Look between florets within a flower head to find nothing.
* Entire lengths of stems have spiny wings.
* Foliage is silvery gray.

Native versus Exotic (Source: Parkinson and Mangold 2015)
* Native thistles tend to have involucral bracts adhere to the flower head for most of their length (except for the spine).
* Native thistles tend to grow scattered across a habitat, spreading slowly with disturbance, and contribute to plant diversity.
* Exotic thistles grow quickly with disturbance, form dense patches that interfere with access, and through competition often reduces plant diversity.

Montana has 12 species of Cirsium, and only 5 are described below.

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.

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.

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].

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.

In Montana it occurs in moist, usually disturbed soil of fields, meadows, thickets, roadsides, woodlands, open forests, often along streams, wetlands in the plains, valleys, and montane zones (Lesica et al. 2012).

It frequently occurs along roadsides, railroad rights-of-way, rangeland, forest land, lawns, gardens, cropland, abandoned fields, stream banks, lake shores, and other riparian habitats (Morishita in Sheley and Petroff 1999). It infrequently occurs in sand dunes and open sandy areas (Morishita in Sheley and Petroff 1999). It has been observed that the diversity of annual broadleaf species increases near patches of Canada thistle (Morishita in Sheley and Petroff 1999).

Canada Thistle grows best where temperatures range from 32 to 90 degrees Fahrenheit and precipitation levels range from 16 to 30 inches per year (Moore 1975). It colonizes clay soils very well, is adaptable to many other soil types, and is most productive in well-aerated soils (Morishita in Sheley and Petroff 1999). It tends to survive dry conditions better than wet conditions (Morishita in Sheley and Petroff 1999).

VULNERABLE HABITAT
Canada Thistle can occupy a wide range of habitats and is fairly adaptable (Morishita in Sheley and Petroff 1999). it usually occurs in open areas with moderate moisture conditions.

ANIMAL INTERACTIONS
Honeybees are the primary pollinator. They use flowers to collect nectar. Female flowers can have a vanilla-like odor while male flowers are unscented.

Potential and Known Pollinators:
The following animal 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).

CULTURAL
Canada Thistle roots are used medicinally to induce vomiting (emetic).

The Mohegan people used it as a mouthwash for infants by making an infusion of leaves (Moerman 1986). They also used the plant to treat lung problems.
Both the Mohegan and Montagnais people made an extract by decocting the plant that they consumed to treat tuberculosis (Moerman 1986).

The Ojibwa people used Canada Thistle as a “bowel tonic” (Moerman 1986).

Plants reproduce by seed and rhizomes.

FLOWERS / FRUITS
Flower heads consist of only disk florets. Florets have pinkish, purple, or occasionally white petals (corollas). Male disk florets are 10–16 mm long and longer than the pappus. Female disk forets are 14–18 mm long and shorter than mature pappus.
Fruits are achenes, 2–4 mm long. Collars are absent.

Most of the time male (produce pollen) and female (produce seeds) flower heads are found on separate plants (imperfectly dioecious). If the male and female plants are not close enough for pollination to occur, then sterile flowers can be formed. A single female flower head can contain 84 to 132 florets, averaging 100 florets. The number of seeds produced will reflect the level of pollination.

Seeds will germinate at the soil surface but germinate best at depths from 0.5 to 1.5 cm. Seeds can germinate upon release during the summer to form rosettes before winter. Other seeds remain dormant until the next spring or later.

RHIZOMES
Underground stems (rhizomes) are extensive, growing from 2 to 22 feet deep, and horizontally to more than 19 feet long in a single season (Morishita in Sheley and Petroff 1999)! Most roots develop in the upper two feet of soil. As the patch matures, the root system breaks up creating independent groups of plansts. Mature roots live for about two years before being replaced by new roots that developed off the old roots (Morishita in Sheley and Petroff 1999).

LIFE CYCLE Adapted from Morishita in Sheley and Petroff 1999.
Seeds germinate in spring, producing two oblong and fleshy cotyledons. The first true leaves are oblong, have no petiole (stalk), and have margins with regularly-spaced, coarse hairs spines. As the plant develops the leaves become serrate, then lobed, and spines become more prominent. When the plant has produced 2 true leaves, the root system has branched, grown to 6 inches long, and begins to thicken. Root development proceeds by growing deeper and longer.

Seedlings survive only if competition is limited and the daytime light intensity remains above 20% of full sunlight. Shading reduces seedling survival and growth. Plants require 16-hour photoperiods to induce shoot elongation and flowering. Less than 16-hour photoperiods causes newly emerged shoots to remain short (low-growing rosettes).

Established plants re-sprout in the spring from adventitious buds on the rhizomes. Shoots may emerge when average weekly temperatures reach 41 degrees Fahrenheit, but will definitely grow once temperatures reach at last 46 degrees Fahrenheit. A plant can produce up to 100 flower heads on a single season, but typically will range from 32 to 69 heads.

Canada Thistle seeds disperse poorly by wind because the tuft of hairs (pappus) on the seed does not remain firmly attached. Dispersal occurs more often by human activities. Canada Thistle seeds contaminate crop seed, feed, and manure. They float on water making them easily transported.

Canada Thistle seeds have been shown to survive in soil for up to 22 years. Seed viability increases with the depth buried. Less than 1% of seeds buried 1 to 3 inches were viable after 2.5 to 5 years. Seeds buried to 8 inches had higher viability.

Persistent control over many years is required to eliminate Canada Thistle because of its extensive root system. Combining cultural, mechanical, biological, and chemical techniques will best exhaust the nutrients in its root system (but must be designed to the site’s specific conditions). An integrated vegetative management approach provides the best long-term control and 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 can be developed that can promote a weed-resistant plant community and serves other land-use objectives such as livestock forage, wildlife habitat, or recreation.

PREVENTION [Adapted from Jacobs et al. 2007]
Preventing the establishment of Canada Thistle can be accomplished by many practices:
* Learn how to accurately identify Canada Thistle in order to detect occurrences and know where to implement control methods.
* 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 uninfested areas.
* Frequently monitor for new plants, and when found implement effective control methods.
* Maintain proper grazing management that creates resilience to noxious weed invasion.
* Do not pick the flowers or transport plants. Where possible, contribute to or develop educational campaigns to help eradicate or reduce Canada Thistle populations.

PHYSICAL and CULTURAL CONTROLS [Adapted from Jacobs et al. 2007]
Hand-pulling is effective for young populations if done several times each season to starve the root-system. Hand-pulling plants in combination with other control methods will likely be more successful at removing the population.

Tilling fragments the rhizomes allowing Canada thistle to increase in abundance. Suppression might occur where tilled at 21-day intervals throughout the growing season followed by establishing perennial forage plants or winter annual cereals; these plants emerge in early spring and can inhibit the emergence of late Canada Thistle shoots.

Prescribed Burning that is done in early spring can encourage growth (sprouting and reproduction). Prescribed burning in late spring (May to June) may help control Canada Thistleganic matter and deletes nutrients, creating conditions that favor its re-establishment.

Mowing can be effective when done 3 or more times during the growing season for several consecutive years. Mowing in combination with using herbicides such as piclorum, piclorum + 2,4-D, or Dicamba can be more effective. In Canada mowing Canada Thistle 3 or 4 times each year almost eliminated it after 3 years, but at other sites only worked to prevent flowering.

CHEMICAL CONTROLS [Adapted from Jacobs et al. 2007]
Herbicides can be effective, especially when properly integrated with intensive pasture management. 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.

Effective control of Canada Thistle requires that an appropriate toxic level of the active ingredient be put into the root system. This means applying the appropriate herbicide on enough leaf area at a time when the plant translocates it to the root system is necessary. Avoid mixing a fast-acting herbicide with a systemic herbicide because the foliage will die before the plant can move the active ingredient into the root system.

Herbicides can be applied in the bud stage or in fall re-growth. In the bud stage leaf area is maximized and root reserves are depleted. In the fall, translocation to the root system will be at its greatest. Canada Thistle plants convert starch into sugar in their roots. A study has found that a fall herbicide treatment can prevent the conversion of starch into sugars in the roots. The sugars act as an anti-freeze by preventing ice-crystals from forming within root cells. Without sugars, roots are susceptible to winter kill. Applying the herbicide onto the correct plant is important, because roots of crops (especially legumes) can also be damaged.

Aminopyralid, clopyralid, and Picloram provide similar suppression in pastures and rangelands when applied at label rates and at times when the plant can get it into the root system. Picloram is a restrict-use herbicide because it is persistent, yet mobile in the soil, and can contaminate water.

BIOLOGICAL CONTROLS [Adapted from Jacobs et al. 2007]
Canada Thistle Stem Weevil (Hadroplontus litura) is a stem-boring weevil that attacks Canada Thistle rosettes. Adults lay a few eggs into a cavity (1-2.5 mm wide) on the underside of leaves that are at least 5 cm long over a 4 to 5 week period. Larvae emerge and mine down the leaf’s mid-veins into the root crown and sometimes the upper root to feed on callus tissues. Older larvae mine the stem and then emerge to pupate, impacting root reserves for overwintering (https://integratedweedcontrol.com). Canada Thistle Stem Weevil will cause the most damage if it attacks the stem before it grows. Larvae pupate in cocoons of soil particles and the adults emerge in late summer to early fall to feed on the upper leaves and stems. They over-winter in soil litter, and in the spring will emerge to eat rosette leaves by puncturing them. While this will not usually kill the plant, the holes left in the root crown makes the plant susceptible to a fatal rust fungus.

Thistle Seed Head Weevil (Rhinocyllus conicus) is the most widely distributed insect for thistle control in the U.S.; however, the U.S. Department of Agriculture prohibits moving these weevils between states because it can feed on native thistles. It has been shown to reduce 90-95% of thistles over an 8 to 9 year period.

Leaf Beetle (Altica carduorum) adults feed on all Cirsium species; although, it is predicted that it will favor Canada Thistle.

Canada Thistle Stem Gall Fly (Urophora cardui) adults lay eggs into stem tissue (https://integratedweedcontrol.com). The developing larvae then cause the plant’s formation of a hard woody gall, which takes energy from the plant. Stems, buds, foliage, and flowers above the galls are often malformed or stunted and prone to dry up ahead of unattacked stems, thereby, not contributing energy to root reserves.

GRAZING CONTROLS [Adapted from Jacobs et al. 2007]
Canada Thistle leaves are spiny and unpalatable to most livestock, and thus grazing is not usually used to control populations. However, in Australia intensive sheep grazing reduced the spread of Canada Thistle when compared to ungrazed pastures. In Canada goats have been observed to eat plants, preventing it from flowering.

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

Canada Thistle forms dense infestations if left unmanaged (Morishita in Sheley and Petroff 1999). Rapid growth occurs vegetatively by rhizomes, creating cloned individuals. Patches can also grow quickly because some seeds germinate immediately to develop rosettes before spring while other seeds will remain dormant until spring or later (Morishita in Sheley and Petroff 1999).

Canada Thistle decreases forage for livestock and native ungulates on rangeland (Morishita in Sheley and Petroff 1999). Plants are unpalatable because of their spines.

The dense patches of spiny leaved plants reduces access and space for recreational activities (Morishita in Sheley and Petroff 1999).

Canada Thistle infestations reduce crop production, property values, and increase land management costs (Morishita in Sheley and Petroff 1999).