BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Carduus nutans | Musk Thistle 



GENERAL BOTANICAL CHARACTERISTICS : 
Musk thistle is a biennial or winter annual forb that generally grows
1.6 to 6.6 feet (0.5-2.0 m) tall.  Large flowerheads (up to 3 inches [7
cm] in diameter) on terminal branches are usually nodding.  Leaves and
stems are spiny [5].  Musk thistle has multiple branches, and a single
large plant may have 100 or more flowerheads [21,24].  The seeds are
0.14 to 0.18 inch (0.35-0.45 cm) long, and the attached pappus is not
feathery.  Musk thistle has a long, fleshy taproot [5].


RAUNKIAER LIFE FORM : 
   Hemicryptophyte
   Therophyte


REGENERATION PROCESSES : 
Musk thistle regenerates solely by seed and is a prolific seed producer
[5,24].  In northeastern Kansas, four large plants averaged 17,750 seeds
per plant [23].  Terminal flowerheads produce the most seeds and average
1,000 seeds per flowerhead; secondary flowerheads average 840 seeds per
flowerhead [26,32].  An estimated one- to two-thirds of the seeds are
well developed and have a high germination potential [24,26].

Seeds are dispersed by wind, small mammals and birds, and water.  Many
intact flowerheads fall to the ground below the parent plant, but some
seeds are dispersed by wind [32].  Smith and Kok [19] studied the
effects of wind speed on musk thistle seed dispersal distance and
pattern.  Seeds traveled a maximum distance of 328 feet (100 m) with
wind speeds up to 12.5 miles per hour (5.62 m/s) in field trials.  A
computer model based on field trials indicates that less than 1 percent
of musk thistle seeds are blown more than 328 feet (100 m), and more
than 80 percent of seeds are deposited in an area less than 131 feet (40
m) from the release point, even at wind speeds of 12.5 miles per hour
(5.6 m/s) [33].

Germination studies have shown that musk thistle has a short, innate
dormancy period [21,23,27,29].  In one study, germination was less than
2 percent immediately after maturity but increased to 50 percent 8 weeks
after maturity.  Ninety percent of 1-year-old seeds germinated within 2
weeks of being placed in a suitable seedbed [21].  Musk thistle seeds
remain dormant for up to 10 years [24].  Seeds buried in the top 0.8
inch (2 cm) of soil remain viable for 2 to 3 years while those buried
deeper than 0.8 inch (2 cm) remain viable longer [29].  Germination is
strongly enhanced by light.  Optimal temperature for germination is
between 59 and 68 degrees Fahrenheit (15-20 deg C) in light and 68 and
86 degrees Fahrenheit (20-30 deg C) in dark [27].

Musk thistle establishment is best on bare soil.  The optimal seedbed
microhabitat is near the soil surface but protected from evaporation.
Small, shallow cracks are ideal.  A light covering of litter after seeds
have fallen reduces evaporation and improves establishment [14].
Populations increase following moist years and decrease during drought [5].

Musk thistle rosettes bolt only after exposure to low temperatures
during winter [28].


SITE CHARACTERISTICS : 
Musk thistle grows where winters are cool and there is adequate
precipitation in either the spring or fall [29].  It occurs in areas
with as little as 10 inches (25 cm) of annual precipitation [24].  In
dry climates such as New Mexico, musk thistle does well near springs and
seeps and in marshy valley bottoms [34].  In cold northern climates,
musk thistle rosettes survive winter only in protected sites such as
gullies that accumulate an insulating layer of snow.  Musk thistle
grows from sea level to about 8,000 feet (2,400 m) elevation [24].

Musk thistle grows best on moist, alluvial soils.  It occurs over a wide
range of conditions from acidic to saline [24].  Musk thistle
distribution is restricted by extremely acidic soils and extremes in
soil water content [5].


SUCCESSIONAL STATUS : 
Musk thistle is intolerant of dense shade [32].

Musk thistle rapidly colonizes disturbed sites [23].  It can form dense
stands with as many as 60,000 plants per acre (150,000/ha).  Stands are
self-perpetuating because the bare soil beneath mature plants is an
ideal seedbed [5].

A site in central Utah was chained in 1982 to remove true pinyon (Pinus
edulis) and Utah juniper (Juniperus osteosperma).  The area was seeded
with perennial grasses and forbs but musk thistle also established.
Musk thistle density increased from 78 per acre (190/ha) in 1983 to
2,700 per acre (6,680/ha) in 1985 [4].


SEASONAL DEVELOPMENT : 
Musk thistle exhibits three life cycles (spring biennial, autumn
biennial, or winter annual) depending in part on time of seed
germination.  The winter annual life cycle is less common than the
biennial cycles [23].  Germination occurs over a period of several
months in both spring and fall [14].  A spring biennial germinates in
spring, forms a large rosette in summer, overwinters, and grows and
flowers the following spring and summer.  An autumn biennial germinates
in fall but does not grow into a large rosette until the following
spring and summer, and then flowers in its second growing season.  A
winter annual germinates in late summer or early fall, early enough that
a large rosette forms before winter dormancy.  It flowers the following
growing season [23].

Musk thistle flowers over a long period, from as early as late May until
killed by frost or dessication [1,12,17,30].  Terminal flowerheads bloom
and set seed first.  In Kentucky, flowering begins in mid-May and
continues until mid-August [21].  In Michigan, musk thistle flowers from
mid-June to mid-August [9].  It flowers in May and June in the central
Midwest, and seeds mature late June to early July [23].  Seeds can
germinate several weeks after maturity if light and moisture are
adequate [23].

Related categories for Species: Carduus nutans | Musk Thistle