nodding thistle | skymeadows

nodding thistle (Carduus nutans)

COMMON NAMES:
chardon penche
musk thistle
nodding plumeless thistle
nodding thistle
plumeless thistle

SCIENTIFIC SYNONYMS:
Carduus macrocephalus Desf.
Carduus macrolepis Peterm.
Carduus nutans L. ssp. leiophyllus (Petrovic) Stojanov & Stef.
Carduus nutans L. ssp. macrocephalus (Desf.) Nyman
Carduus nutans L. ssp. macrolepis (Peterm.) Kazmi
Carduus nutans L. ssp. nutans
Carduus nutans L. var. leiophyllus (Petrovic) Arènes
Carduus nutans L. var. macrocephalus (Desf.) B. Boivin
Carduus nutans L. var. vestitus (H.M. Hallier) B. Boivin

CONFIRMATION STATUS: Pending confirmation.

TAXONOMY: The currently accepted scientific name of nodding thistle
is Carduus nutans L. Carduus nutans is part of a variable complex that has
been treated as one to several species or as a single species with several
subspecies or varieties. The New World plants apparently represent mul-

tiple introductions, probably representing more than one of these taxa.
Various intermediates are evident, and many specimens cannot be reliably
assigned. Insufficient evidence exists to reliably apply the names of the
various segregate entities to North American material.

Hybrids between Carduus acanthoides and Carduus nutans (Carduus × orthocephalus Wallroth) have been documented from Ontario and Wiscon-

sin and probably occur at other sites where the parental taxa co-occur.

This is a very spiny thistle that can become quite tall. It is easily distin-

guished from many of the native thistles by the prickly bracts at the base

of the flowerheads. These large bracts curl outward and narrow into sharp

points, while the bracts of native thistles are appressed together and resem-

ble green fish scales (fine spines are usually present on the outer bracts for

some native species). The leaf undersides of the bull thistle are light green

or whitish green, while the leaf undersides for some native thistles, such as

Cirsium discolor (pasture thistle) and Cirsium altissimum (tall thistle), are

powdery white in appearance. Other native thistles, however, don't have

this latter characteristic. The bull thistle also has spines on its stems (from

the decurrent extensions of the leaves), while the stems of native thistles

are spineless.

NATIVE STATUS: Introduced, United States and Canada.

GENERAL BOTANICAL CHARACTERISTICS:

Habit: Nodding thistle in an invasive biennial, and sometimes summer or

winter annual forb. Musk thistle may have 1 to 7 branched stems that grow

2 to 6 feet (0.6-1.8 m) tall. Stem leaves are 3 to 6 inches (7.6-15.2 cm) long

and spiny. Stems have spiny wings their full lengths except for a few inch-

es below flowerheads.

Leaves: As a biennial, nodding thistle initially forms a prostrate rosette.

Rosette leaves can grow up to 10 inches (25 cm) long and 4 inches (10

cm) wide, and rosettes can be 2 feet (0.6 m) or more in diameter.

Flowers: Flowerheads are large, 1.5 to 3 inches (3.8-7.6 cm) in diameter,

solitary, and terminal on shoots. Flowers are subtended by numerous large,

lance-shaped, spine-tipped bracts that resemble a pinecone.

Fruit/Seeds: The fruit is an achene bearing 0.3-0.5 cm seeds with a hair-

like pappus.

Nodding thistle is capable of forming dense stands, especially on highly

disturbed sites where competition is low, or in overgrazed or disturbed

pastureland. Population size may fluctuate in response to climatic con-

ditions. Nodding thistle patches are usually denser than patches of other

biennial thistles, but less dense than those of perennial thistles.

Roots: Nodding thistle rosettes have numerous small roots in the fall, and

develop a large, fleshy taproot in the spring that is hollow near the soil sur-

face. The root crown and upper root tissues contain buds, normally sup-

pressed by apical dominance, which may sprout following damage to

plants.

REGENERATION PROCESS: Nodding thistle is usually a biennial,
requiring 2 years to complete a reproductive cycle, but may germinate and
flower in a single year in warmer climates. Seedlings emerge in mid to late
July and develop into a rosette where plants can reach 4 feet in diameter.
Plants overwinter in the rosette stage until they begin to bolt in mid-March.
During the bolting stage plants form multi-branched stems to a height of 6
feet. The number of seedheads per plant is site-dependent and ranges from
about 24 to 56 on favorable sites and 1 to 18 on less favorable sites. A sin-

gle flower head may produce 1,200 seeds and a single plant up to 120,000
seeds, which may be wind blown for miles. Seed may remain viable in the
soil for over ten years, making it a difficult plant to control.

Most seed is dispersed within the immediate vicinity of the parent plant.
This leads to a clumped pattern of seedling development and results in
intraspecific competition and mortality. Wind and water are good dissem-

ination methods and seeds are also spread by animals, farm machinery

and other vehicles. Less than 5 percent of seed remains attached to the

pappus when it breaks off the flowering head and floats away on wind

currents.

Researchers have found some evidence of allelopathy in nodding thistle.
Aqueous extracts, leachates, and ground plant material from musk thistle
all showed some inhibition of germination and radicle growth rate of sev-

eral pasture species. Additionally, growth of musk thistle seedlings appears

to be stimulated by adding musk thistle tissue to the soil. Nodding thistle

may thus stimulate recruitment of its own kind as it invades.

HABITAT TYPES: Nodding thistle is adventive from Eurasia, and has
existed in the United States since the 19th century, if not earlier. Habitats
include meadows, prairies, grassy balds, pastures, abandoned fields, fence
rows, areas along roadsides and railroads, cut-over woods, building sites,
and miscellaneous waste areas. This species prefers disturbed areas and is
not common in high quality natural habitats. It spreads rapidly in areas
subjected to frequent natural disturbance events such as landslides and
flooding but does not grow well in excessively wet, dry or shady conditions.
Vigorously growing grass competes with musk thistle, and fewer thistles
occur in pastures where grazing is deferred. However, nodding thistle also
can become a problem in pasture or rangeland that is in good condition. It
grows from sea level to about 8,000 ft elevation.

SITE CHARACTERISTICS: Nodding thistle grows in full sun (it does
not tolerate shade) and prefers light (sandy), medium (loamy) and heavy
(clay) soils. It also tends to do well in soil situations that are slightly stony.
It also prefers acid, neutral and basic (alkaline) soils that are moist.

Occasionally the foliage is affected by mildew during the summer or fall.

Individual plants can produce a great abundance of seed, which have a

high germination and survival rate. Thus, this plant can be quite aggres-

sive. Because the seeds remain viable for only 1-2 years, one control strat-

egy consists of destroying individual plants before they reach the flower-

ing stage. It is possible for a plant to reestablish itself if a portion of the

taproot remains in the ground.

Germination and seedling establishment are correlated with moisture and
light. Thus, more seeds germinate and establish plants in open pastures
and other degraded areas.

SEASONAL DEVELOPMENT: Flowers emerge in early May to August
and seed dissemination occurs approximately one month after the flowers
form. It is common to observe musk thistle with heads in several stages of
floral development and senescence. Thus, musk thistle sets seed over an
extended time period.

GENERAL DISTRIBUTION: Nodding thistle is found throughout the

U.S., except for Maine, Vermont, Florida, Alaska and Hawaii, and is
found in all the southern Canadian provinces.

SKY MEADOWS DISTRIBUTION: To be determined.

IMPORTANCE AND USES: The plant is very attractive to bees, hover-
flies and butterflies, it is a food plant for the caterpillars of many Lepido-

ptera species. A number of species of birds also graze on mature musk

thistle seed in Australia.

Livestock rarely eat musk thistle foliage; however, cattle and domestic
sheep and goats have been observed consuming flowers and seedheads.
Horses seem to be particularly fond of musk thistle flowers. It is unclear
whether this results in musk thistle seed dispersal. Dense infestations of
the plant discourage animals from occupying that portion of the field in
which it grows.

The pith of the stem can be boiled and is said to have a pleasant taste.

The flowers are febrifuge and are used to purify the blood. The seeds
contain a fixed oil that is rich in linoleic acid; this has proved of benefit
in the prevention of atherosclerosis.

The down of the plant is used to make paper. The seed of all species of
thistles yields a good oil; this species contains 41 - 44% oil.

While nodding thistle offers some benefits to wildlife and humans, its
has clear disadvantages to those who raise livestock. Nodding thistle
is generally conceded to be one of the most serious weeds in North
America. It is unpalatable to larger mammalian wildlife and livestock
and often forms dense, impenetrable stands in pastures and rangelands.
It readily colonizes disturbed sites in many different habitats. A single
large terminal head can produce as many as 1200 cypselae. Efforts to
control nodding thistle infestations with Rhinocyllus conicus, a European
seed head weevil, have met with some success, but concerns have been
raised because this parasite also attacks native Cirsium species.