Montana Field Guide

PLANTS: Stout, tall, biennials, occasionally annuals, with erect spiny-winged stems, 30-150 cm tall. Sources: Lesica et al. 2022

LEAVES: Basal and cauline (stem) – all spiny margined. Basal leaves have winged petioles. Stem leaves alternately arranged; reduced upwards in size. Blades oblanceolate, deeply pinnately lobed, and 10-20 cm long. Sources: Lesica et al. 2022

INFLORESCENCE: An array of racemes with solitary. purple flower heads on long, wingless, and hairy (tomentose) peduncles, 2-30 cm long. Flower heads consist of involucral bracts, bristles, and disc florets (discoid) on the receptacle, 4-8 cm wide. Flower heads are erect, but nod with maturity. Involucral Bracts lanceolate in shape, often wider than appressed base, unequal, 2-4 cm tall, and 2-7 mm wide. Forming 7-10 rows, the outer bracts spine-tipped and inner unarmed. Disc Florets have stamens and pistil (perfect), 15-25 mm long; petals purple, occasionally white, narrow, and linear-lobed. Pappus minutely barbed. Sources: Keil in Flora of North America (FNA) 2006; Giblin et al. [eds.] 2018; Lesica et al. 2022

Across its range, seeds typically germinate in spring, and also from autumn rains (Winston et al. 2012). Flowering occurs from May through September (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
* 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.
* Tend to have involucral bracts that adhere to the flower head for most of their length (except for the spine).
* Are not rhizomatous except for Flodman’s Thistle which can produce horizontal runner roots.

Non-native Thistles
* 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 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 (capillary) - of 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) - of 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 (capillary) - of 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.

IDENTIFYING CARDUUS SPECIES

Musk Thistle - Carduus nutans, non-native and undesirable
* Plants: Usually biennial; occasionally annual.
* Flower Heads: Mostly solitary. Involucres of 2-4 cm tall and 4-8 cm wide. Nodding with maturity.
* Involucral Bracts: Broadly triangular – at least 2mm wide - with smooth margins and wide at mid-length. Outer (lower) bracts spine-tipped.
* Disk flowers: purple.

Spiny Plumeless-thistle - Carduus acanthoides, non-native and undesirable
* Plants: Annual or biennial.
* Flower Heads: Several per stem and sessile or short-pedunculate. Involucres of less than 2 cm tall and less than 4 cm wide. Generally, erect with maturity.
* Involucral Bracts: Narrowly triangular - less than 2mm wide- with smooth margins and widest at base. Outer (lower) bracts spine-tipped and a short spine-tip.
* Disk flowers: white or purple.

TAXONOMY & NOMENCLATURE
Carduus nutans is part of a complex that exhibits great morphological variation (Keil in FNA 2006). Depending upon the taxonomist, this plant has been treated as several species or as a single species with many subspecies or varieties (Keil in FNA 2006). Plants in North America arose through multiple introductions from various taxonomic sources which cannot reliably be separated based on morphology (Winston et al. 2016). Sufficient evidence to consistently apply names to the various segregated entities is lacking (Keil in FNA 2006).

Carduus is an old name for ‘thistle’.

Musk Thistle grows in fields, roadsides, disturbed grasslands, and riparian meadows in valley and montane zones in Montana (Lesica et al. 2022). Plants grow best in disturbed, neutral to acidic soils with moist conditions (Winston et al. 2016).

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 auricomus, Bombus borealis, Bombus fervidus, Bombus pensylvanicus, Bombus bimaculatus, Bombus griseocollis, and Bombus impatiens (Thorp et al. 1983, Colla and Dumesh 2010, Williams et al. 2014, Tripoldi and Szalanski 2015).

Plants reproduce by seed.

FRUIT
Fruit is a cypsela or an achene. A single flower head can produce about 1200 cypselae (Keil in FNA 2006). It is a dry, single-seeded fruit that formed from a double ovary. The pappus (calyx) of Musk Thistle separates from the achene. The pericarp (fruit wall) and seed coat are separate (free from each other). Pericarp is thick-walled, hard, nutlike, and with receptacular attachments (Keil in FNA 2006). The achene is ovoid, glabrous, and 4-5 mm long. Chromosome number of 2n=16 (Keil in FNA 2006).

HYBRIDS
Carduus nutans and Carduus acanthoides have been documented to hybridize (C. x orthocephalus) in Ontario, Canada and Wisconsin, USA where parent species co-occur (Keil in FNA 2006).

LIFE CYCLE [Adapted from Winston et al. 2012 and 2016]
Musk Thistle is typically a biennial, requiring two growing seasons to complete its life cycle. Seeds usually germinate in the spring of the first year to form a basal rosette; although autumn rains can also stimulate germination. Stems bolt in late spring to early summer. Flowering occurs from May through August or into autumn if appropriate conditions persist. Flower occurs throughout the remainder of the summer into late fall. Seeds remain viable for many years. Seeds are transported by water, native animals, domesticated livestock, and human activity.

The musk thistle complex has been found in at least 3,068 counties in 40 state in the continental USA, with 12% of those countries rating their infestation as "economic" (Dunn 1976)

COUNTY & STATE DESIGNATIONS
In 2024, Musk Thistle is listed as County Noxious in 16 counties of 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.

PREVENTION [Adapted from Piper in Sheley and Petroff 1999]
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.
* 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.

BIOLOGICAL CONTROL [Adapted from Winston et al. 2012 and 2016; NRCS 2019)
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.

Musk Thistle was the first plant species targeted for biological control in the USA.

Biological control agents approved for Musk Thistle in the USA include:
Musk Thistle Flower Fly (Cheilosia corydon)

Nodding Thistle Gall Fly (Urophora solstitialis) was released in Montana in 1990. Populations have not yet established on Musk Thistle.

Puccinia carduorum [fungus]

Cheilosia grossa [European hoverfly species]

Psylliodes chalcomera [flea beetle]

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). 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 is documented to attack 22 of 90 native Cirsium species – even in places where non-native thistle plants.

Thistle Crown Weevil (Trichosirocalus horridus) was released in 1974, in USA, and 1975, in Canada, to control exotic thistles – specifically Musk Thistle and Spiny Plumeless-thistle (Winston et al. 2016). However, through natural spread Thistle Crown Weevil attacked both non-native and native Cirsium thistles. As a result, interstate transport is not permitted in the USA, and some states have prohibited redistribution of the insect within their borders. It is critical that re-distribution of this species does not occur into places where it is either banned or not present and native thistles are known or suspected to occur.

In Montana Thistle Crown Weevil prefers Musk Thistle over Spiny Plumeless-thistle. The weevil is effective in controlling thistle populations when in combination with either other biocontrol agents and/or competition from other plant species. At many sites the insect has been ineffective with limited damage. Thistle Crown Weevil attacks the rosettes, but there has been enough survival to allow these thistles to produce some seed.

CHEMICAL CONTROLS [Adapted from Harvey and Mangold 2018]
Herbicides can be effective, especially when properly integrated with a weed management plan. 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.

The most widely used products include, but are not limited to:
Aminopyralid can be applied in the spring or early summer on rosettes and bolting plants or in the autumn on rosettes. Aminopyralid is less detrimental to desirable broadleaf plants – though legume species are an exception.

Clopyralid can be used on young, actively growing thistles prior to the bud stage or on rosettes in the autumn. Clopyralid does not injure established grasses.

Picloram can be used on thistles at all life stages. Picloram is a restrict-use herbicide because it is persistent, yet mobile in the soil, and can contaminate water.

Dicamba can be used on Carduus species that are emerging or actively growing. It is best to apply on small thistles, prior to flowering, or in autumn on rosettes.

2,4-D can be used on young and actively growing annual and biennial species of non-native thistle.

Metsulfuron should be applied after emergence when non-native thistles are actively growing. Results are better when used with a nonionic or organosilicone surfactant. Restrictions on grazing or with some grasses apply with its use.

PHYSICAL CONTROLS [Adapted from Winston et al. 2012]
Tilling in agricultural settings can effectively control Carduus species of thistle if done on a timely basis and if roots are cut below the soil surface (root crown). Tilling can be more effective when combined with an herbicide application. Tilling is not practical or desirable in wildlands or nature preserves. Tilling and disking are not compatible where biological control is used because it disrupts the organism’s life cycle.

Mowing as close to the ground, as practical, and before flower heads bloom can reduce seed production and spread of Carduus species of thistle. Once the terminal flower heads start blooming, un-cut stems can recover, branch, flower, and seed. A single mowing treatment will not injure the root system but is effective for annual and biennial species. Mowing in the summer prior to an autumn herbicide application will improve coverage of the chemical that falls on the rosette leaves. Mowing may be incompatible with some biological control agents. Mowing should be timed appropriate to work with the life cycle of the biological control agent to prevent unwanted removal of insects, galls, etc.

CULTURAL CONTROLS [Adapted from Winston et al. 2012]
Seeding can help establish competitive native and desirable grasses and forbs where there is an infestation of Carduus species of thistle. Within two years, typically, of establishing competitive grasses then biological control agents can be introduced. Establishing native and desirable competitive vegetation is one of the more successful long-term weed control methods.

Prescribed or cultural fire can control non-native thistles, but success is dependent upon the season of burn, burn severity, site conditions, and plant community composition and phenology. If used in conjunction with native plant seeding, especially those that flourish with fire, then plants will create natural competition against non-native thistles. If used in conjunction with herbicides than non-native thistle control can improve though impacts to soils should be considered. Biological control agents must be able to survive controlled burns that aid in returning native vegetation to previous thistle-infested areas – as determined by the agent, fire severity, and timing of the burn.

Grazing by livestock is generally not an effective control for non-native thistle infestations because the spiny leaves, stems, and flower heads are not very palatable. Cattle have been observed in many places to nip the flower heads of Musk Thistle and later spit them out onto the road (Pipp personal communication).

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

Musk Thistle is an aggressive non-native plant that is one of the most serious weeds in North America (Keil in FNA 2006). Musk Thistle are unpalatable to wildlife and livestock. Plants can form dense, impenetrable stands in pastures and rangelands. Plants easily colonize disturbed sites that represent a wide range of vegetation types.