Carduus nutans

Size and shape of the imbricate, spine-tipped involucral bracts is used to distinguish members of the group from closely related species and from each other.

Members of the genus Carduus are distinguished by their simple pappus hairs from members of the genus Cirsium, which have feathery, plumose pappus hairs. Within the genus Carduus, members of the nutans group are distinguished by their large nodding heads from closely related, small- flowered plumeless and Italian thistles (C. acanthoides, C. crispus, C. pycnocephalus, and C. tenuiflorus) (McCarty 1984, Mulligan and Frankton 1954, Trumble and Kok 1982).

Carduus thoermeri is distinguished from other members of the nutans group by the broad (4-8 mm) fairly short blade of the involucral bract, which converges to a short awn tip. Carduus macrocephalus is distinguished from other North American members of the nutans group by the raised mid-vein of the long, broad, uniformly tapering involucral bract. Carduus nutans conforms to the illustration in the 3rd edition of Britton and Brown's illustrated flora (Gleason 1957). The involucral appendage is much narrower than in other members of the group. The involucral blade is narrow (1.5-3 mm), more or less hairy, and tapers gradually to an awn. Carduus sp. from British Columbia is characterized by a broad, fairly short involucral bract, converging slowly but not uniformly to the tip (McCarty 1985, Tutin et al. 1976).

In addition to these morphologically distinct species, hybrids of intermediate appearance have been reported between Carduus sp X C. thoermeri, C. thoermeri X C. macrocephalus and Carduus nutans (sensu latu) X C. acanthoides (McCarty 1985, Moore and Mulligan 1956, 1964, Mulligan and Moore 1961).

Musk thistle is most prevalent in disturbed areas such as roadsides, grazed pastures and old fields, but can invade deferred pastures and native grasslands (Feldman et al. 1968, Nagel pers. comm., McCarty pers. comm.). This species forms impenetrable stands in rangelands and pastures, and is one of the most serious weeds in North America due to its unpalatability to livestock (FNA 2006a).

Musk thistle is a monocarpic species requiring a cool period of vernalization in order to bloom (Medd and Lovett 1978, Hadding and McCarty 1980). Under natural conditions, musk thistle most often functions as a spring biennial, fall biennial, or winter annual (Lee and Hamrick 1983, McCarty et al. 1969). The species is very plastic (McCarty pers. comm.). Ten percent of plants in a Kentucky nursery study functioned as true annuals (Lacefield and Gray 1970).

There appears to be a cline in flowering strategy from south to north. Although plants are reported to behave as biennials and winter annuals from Oklahoma (O'Bryan and Peeper 1986) as far north as Minnesota (Durgan pers. comm.), Canadian plants are treated as biennials (Mulligan and Frankton 1954).

Plants of all ages overwinter as rosettes. Both flowering and seed production are positively correlated with rosette size. In one Kansas study, plants greater than 14 cm in rosette size in late April flowered the following summer regardless of their age (Lee and Hamrick 1983).

Bolting begins as early as March in Kentucky (Lacefield and Gray 1970) until as late as May in Minnesota (Durgan pers. comm.). Flowering begins from early June in the south to as late as mid-July in the north and may continue for up to seven weeks (McCarty 1982). Within a single flowering head, florets develop centripetally over a period of 36 to 48 hours. Pollinators include bees (Apis mellifera), bumblebees (Bombus spp.), and sphinx moths (Hyles spp.). Florets on the same head are self-compatible (Lee and Hamrick 1983).

Seed maturity and dispersal occur within 7 to 10 days of flowering (McCarty and Scifres 1969) and begin as early as the first week in June in Kentucky (Lacefield and Gray 1970). Seed production can be as great as 11,000 seeds per plant (McCarty and Scifres 1969). Terminal heads average 1000 seeds per head, whereas the last blooming side branches average only 125 seeds. Early-maturing, terminal seeds are heavier and exhibit a higher rate of viability than later-maturing seeds from secondary branches (McCarty 1982). The bulk of the seeds fall near the parent plant with less than 1% being carried further. Experimental studies in Virginia suggest that seeds do not travel far from the parent plant, with over 80% of seeds deposited within 40 m of the parent plant (Smith and Kok 1984). However, McCarty (pers. comm.) reports that a pilot in Nebraska flew through a cloud of musk thistle seeds at an altitude of 500 feet.

Seeds have been reported to remain viable in the soil for periods as long as ten years (Burnside et. al. 1981).

In one Kansas study, less than 2% of the seeds falling within the boundaries of the population germinated the following year and about 30% of the new seedling cohort came from seed carried over from previous years (Lee and Hamrick 1983).

Studies of germination requirements in Nebraska indicate that a period of dormancy is not necessary before germination (McCarty et al. 1969). However, in Kentucky only 2% of fresh seed germinated, whereas 50% of seeds germinated after 8 weeks and 90% of year-old seeds germinated (Lacefield and Gray 1970). McCarty et al. found in laboratory studies that cold, moist treatment resulted in low rates of germination. Even when moisture is adequate, soil cover is required before a high percentage of seeds will germinate (McCarty et al. 1969).

In Kansas, both musk thistle rosette survival and earlier germination were enhanced in study plots dominated by Bromus japonicus, a winter annual that formed dense protective litter retaining moisture during the dry summer months. Summer mortality and later germination were observed in plots dominated by perennial weeds that lost their lower leaves but continued evapotranspiration during the summer months, decreasing protection of Carduus nutans and increasing competition (Lee and Hamrick 1983). In Kansas greenhouse experiments optimum levels of germination, survival and growth occurred in habitats with a light covering of litter that reduced evapotranspiration. Thick litter layers reduced germination and establishment by preventing seeds from reaching the soil surface (Hamrick and Lee 1987).