Canada thistle (Cirsium arvense)




















Californian thistle
Canadian thistle
creeping plume thistle
perennial thistle
canada thistle
creeping thistle
field thistle


Carduus arvensis (L.) Robson
Cirsium arvense var. argenteum (Vest) Fiori
Cirsium arvense var. horridum Wimmer & Grab.
Cirsium arvense var. integrifolium Wimmer & Grab.
Cirsium arvense var. mite Wimmer & Grab.
Cirsium arvense var. vestitum Wimmer & Grab.
Cirsium incanum (Gmel.) Fisch.
Cirsium setosum (Willd.) Bess. ex Bieb.
Serratula arvensis L.
Cephalonoplos setosus (Ledeb.) Kitam.
Breea arvensis (L.) Less.
Breea dioica (Cass.) Less.
Breea ochrolepidia (Juz.) Soják
Breea praealta Less.
Carduus arvensis (L.) Robson
Carduus haemorrhoidalis Auct. ex DC.
Carduus neglectus Steud.
Carduus serratuloides Neck.
Carduus setosus Bab.
Cephalonoplos arvense (L.) Fourr.
Cephalonoplos arvensis Fourr.
Cephalonoplos ochrolepidium (Juz.) Juz.
Cirsium albicans Willk.
Cirsium argenteum Peyer ex Vest
Cirsium dioicum Cass.
Cirsium halophilum Turcz. ex Herder
Cirsium horridum (Wimm. & Grab.) Stankov
Cirsium incanum (S.G. Gmel.) Fisch. ex M. Bieb.
Cirsium incanum (S.G.Gmel.) Fisch.
Cirsium macrostylon Rchb.
Cirsium macrostylum (Moretti) Rchb.
Cirsium mutatum Menyh.
Cirsium neglectum Fisch. ex Spreng.
Cirsium ochrolepidium Juz.
Cirsium praealtum Cass.
Cirsium ruthenicum Fisch.
Cirsium sordidum Wallr.
Cirsium stocksii Boiss.
Cirsium × celakovskianum Knaf
Cnicus arvensis (L.) G.Gaertn. & al.
Cnicus arvensis (L.) Hoffm.
Cnicus lanatus Willd.
Cnicus macrostylus Moretti
Cnicus neglectus Parish ex Greene
Cnicus ruthenicus
Cynara repens Stokes
Serratula incana S.G. Gmel.
Serratula setosa Willd.
Serratula spinosa Gilib.


CONFIRMATION STATUS: Pending confirmation.


TAXONOMY: The currently accepted scientific name for Canada thistle
is Cirsium arvense (L.) Scop. Canada thistle is extremely variable with
regard to leaf division and vestiture, and it has been treated as several
species, numerous varieties, or as a single highly polymorphic species.
Several authors recognize different varieties based primarily on differ-

ences in leaf morphology. Other botanists are doubtful that the variety

designations are meaningful, and while others state that contemporary

European botanists do not consider described variants of the species to

be taxonomically significant.


NATIVE STATUS: Introduced, United States and Canada.




Habit: This adventive plant is an obligatory biennial that forms a rosette

of leaves during the first year, and bolts upward during the second year to

produce flowers. It forms occasional side stems, but remains erect in stat-

ure, reaching 3-6' in height. The stout stems are light green, somewhat

angular, and covered with dense white hairs.


Leaves: The alternate leaves are up to 7" long and 2" across. In outline,

they are lanceolate in shape, but deeply pinnatifid. The widely spaced

lobes narrow into points that are individually armed with a pale yellow

spine. Some of the upper leaves near the flowerheads may be lanceolate

or linear, but remain unlobed. Across the surface of the leaves, there are

short white hairs and scattered small spines. The upper surface is dark

green, while the lower surface is light green. At the base of each leaf,

there are a pair of narrow wings that are dark green and decurrent against

the stem. These wings are extensions of the leaves and arm the stems with

spines up to ¼" long.


Flowers: The upper stems terminate in flowerheads of purplish pink to

purple flowers. Each flowerhead is 1½–2" across and contains numerous

disk florets, but no ray florets. Each disk floret is long and tubular, but

becomes divided into 5 long thread-like lobes. This provides the flower-

head with a showy hair-like appearance. At the base of each flowerhead,

are numerous green bracts that taper into stiff points that curl outward and

are quite prickly. Among these bracts are cobwebby hairs in the back-



Fruit/Seeds: The blooming period occurs from mid- to late
summer and lasts about 3-4 weeks, after which the florets wither away.
They are replaced by achenes with large tufts of white hair. These achenes
are long, slender, and slightly ribbed. They are dispersed by the wind.


Roots: The root system consists of a stout taproot that runs deep into the

ground. This plants spreads by reseeding itself and occasionally forms

colonies. While allelopathy has not been conclusively demonstrated for

Canada thistle, this species may produce phytotoxins that inhibit the grow-

th of other plants. Fructan metabolism in Canada thistle adds to its compet-

itive advantages by allowing it to grow at relatively cool temperatures.


REGENERATION PROCESS: Because of its invasive nature and wide
distribution, a great of research has been conducted concerning the regen-

eration process of Canada thistle. Canada thistle reproduces both sexually

by seed and vegetatively by creeping roots. Generally, vegetative repro-

duction contributes to local spread and seeding to long distance dispersal.

Introduction into new areas is mostly by wind- or water-borne seed, or by

seed in contaminated crop seed, hay or machinery. Canada thistle allocates

most of its reproductive energy to vegetative propagation, and a patch can

spread rapidly by vegetative means under favorable conditions. However,

he contribution of sexual reproduction to the survival and spread of Canada

thistle may be underestimated and may be an important mechanism for

initiating continued genetic diversity in a clonal population.


Shoot elongation and flowering in Canada thistle are induced by 15-hour
day length, therefore flowering and seed production will be limited or
prevented in regions with shorter summer days. A typical Canada thistle
shoot may produce 32 to 69 flowerheads per shoot (1-5 per branch) on
average, but can produce as many as 100 flowerheads in a season. Canada
thistle is "imperfectly dioecious", with male and female flowers occurring
on separate plants. Up to 26% of "male" plants are actually self-fertile
hermaphrodites or subhermaphrodites that occasionally produce seed.


Canada thistle is insect pollinated, primarily by honeybees. Male and female
plants must be located within a few hundred yards of each other for insect
pollination and seed set to occur. Seed set is highest when male and female
plants are intermixed and decreases when female plants are more than 164
feet (50 m) from male plants. Since Canada thistle can grow in large
patches, it is not uncommon to find sterile heads of female flowers. Canada
thistle has a reputation for producing few viable seeds, but the literature
gives a wide range of estimates for seed production with numbers ranging
from 0 to 40,000 seeds per stem. Reports of average seed-set per flower-
head range from 21-93. Other reports indicate that females produce an
average of 30 to 70 seeds/flowerhead and males average 2 to 10 seeds
head. The number of flowerheads per stem reported ranges from 0 to 100.
Inefficient pollination and genetic variability may contribute to poor seed
yields. Seeds of Canada thistle are subject to predation by insects before
dispersal, but information is more qualitative than quantitative. Weather
extremes (cool and moist or hot and dry) can interfere with pollination, so
some years even female plants do not produce much seed.


Canada thistle seeds are released about 2-3 weeks after pollination. They
are equipped with a pappus, loosely attached to the seed tip, that enables
wind dispersal, and have good aerodynamic efficiency. Canada thistle seeds
have been observed windborne on the prairie several hundred meters from
the nearest source population. Evidence from seed rain studies on Mount
St. Helens, Washington suggests that Canada thistle seeds can travel
several kilometers. This dispersal mechanism accounts for the numerous
examples of Canada thistle seedling establishment after disturbance in
natural areas, especially after fire. However, wind dispersal has not been
considered a major factor in its spread, since the pappus readily breaks off,
leaving the achenes within the seedheads . In developed areas, seeds are
more commonly spread by animals, in hay, contaminated crop seed,
machinery, and irrigation water. Observations in Rocky Mountain Nation-

al Park  indicate that trails, especially those used by horses, are major
invasion pathways for Canada thistle. Livestock consuming unprocessed
hay before entering national forests will likely spread more Canada thistle
seeds than those consuming feed pellets, since pellet manufacturing
destroys 99% of viable Canadian thistle seed when it includes grinding
and screening.


Canada thistle seeds mature quickly and most are capable of germinating 8
to 11 days after the flowers open, even if the plants are cut when flowering.
Research indicates that almost all Canada thistle seed can germinate upon
dispersal, although germination is extremely variable (0-95%). Viability
of seeds during the 1st season after dispersal may be as high as 90%. Most
seeds germinate in the spring after the year in which they are produced,
with some seeds producing basal leaves before winter and emerging to
flower the next spring. Germination may be affected by ecotype, tempera-

ture, day length, depth of seed burial, substrate stratification, and seed

freshness. Seeds from "male" plants are smaller and percent germination

is lower. Canada thistle seeds germinate best in warm temperatures (68 to

104 degrees Fahrenheit (20-40 °C)), with alternating light and dark periods.

Germination in Canada thistle was best after 0.5 to 16 days at 88 to 108

degrees Fahrenheit (31-42 °C). At lower temperatures germination is aid-

ed by high light intensity. Germination at higher temperatures can help en-

sure that maximum germination takes place during warmer periods of the

year. Canada thistle seeds are somewhat tolerant of heat. Canada thistle

seeds germinate over a wide range of soil moisture. Canada thistle seeds

can germinate on the soil surface, but that germination is best when seeds

are buried 0.2 to 0.6 inch (0.5-1.5 cm) deep. Emergence as deep as 6 cm

in some soil types has been reported. The soil seed bank does not usually

contain large numbers of Canada thistle seeds. Length of survival is relat-

ed to depth of burial, with seeds surviving up to 22 years when they are

buried more than 8 inches (20 cm) deep. Under more natural conditions

of shallower burial and periodic soil disturbance, Canada thistle seeds are

more short lived (<5 years), with most seed being lost from the soil seed

bank by germination during the 1st year. Seeds that have been in water for

several months can still be viable.


Canada thistle seedlings usually start growing slowly and are sensitive to
competition and shading. Seedlings grow poorly in very moist, poorly
aerated soils and do not tolerate drought stress. Before seedlings become
perennial, they are also highly susceptible to tillage.


Vegetative spread of Canada thistle can occur from horizontal extension
of the root system, from root fragments, or from subterranean stem tissue.
Spread can be rapid when there is little competition, with 13 to 20 feet
(4-6 m) of horizontal root growth possible in one season. Canada thistle
can develop new aerial shoots at any location along the root length, from
the original vertical root, or from buds on lateral roots. Within a few weeks
of germination, a Canada thistle seedling with at least 4 true leaves can
begin producing root buds that can eventually produce new shoots. Buds
on lateral roots may form new adventitious shoots as frequently as 0.3 to
1-inch (0.8 to 2.4 cm) intervals, although the number of root buds is likely
to vary from place to place and year to year. A single Canada thistle plant
can potentially produce 26 adventitious shoots, 154 adventitious root buds,
and 364 feet (111 m) of roots after 18 weeks of growth. It is possible that
a colony of male plants would maintain itself regardless of whether it
produced fruits.


Root buds are inhibited by the presence of the main shoot, primarily due
to a competition for water, and new root bud growth is highest during late
fall and winter months following death of aerial shoots. When the main
shoot is removed (e.g. as by mowing) the root buds are released, and new
shoots emerge rapidly, especially when humidity is high. Wilson found

that some 19-day old plants were capable of regenerating top-growth after
clipping, and that 40-day old plants could produce 2 or 3 shoots after
clipping. Root fragments as short as 0.2 inch (6 mm) and more than 6
weeks but less than 2 years old can regenerate entire plants, regardless
of whether they have identifiable root buds at the time. In one experiment,
researchers observed that an 18-week-old plant had the potential of pro-

ducing 930 shoots if its root system was cut into 10-cm-long pieces.


Vegetative spread of Canada thistle may also occur from subterranean
stem tissue that can produce shoot buds and adventitious roots at each
node. Partially buried stem sections from the postbloom stage survived
and produced adventitious roots that over wintered and produced new
infestations the following spring. Similarly, Canada thistle can survive
disturbance to be part of the early successional community in natural
areas by resprouting from buried root and stem fragments.


HABITAT TYPES: Contrary to the common name, Canada thistle is
originally from Eurasia and has existed in the North America probably

since the 1600s. It may have been introduced as a contaminant of crop

seed and/or ship's ballast.. It is probably the most widespread of all

thistle species.


Canada thistle is adaptable to a wide range of habitats. Typical habitats

include cropland, abandoned fields, fence rows, areas along roads and

railroads, vacant lots, weedy meadows, streambanks, ditches, lakeshores,

seashores, sand dunes and other open sandy areas, in clearcuts and forest

openings, and in wet and wet-mesic grasslands, overgrazed pastures, till-

ed fields or open waste places, campgrounds, and after logging, road

building, fire and landslides in natural areas.


Canada thistle invasion is enhanced by heavy grazing by cattle and bison,

areas left barren during planting operations, and on earth mounds made by

pocket gophers and badgers prairie potholes and degraded prairies. This

plant can invade lawns that are not mowed regularly, and it is aggressive

enough to invade many natural habitats although it is not common in high

quality natural habitats.


SITE CHARACTERISTICS: Canada thistle grows best in open sunny

sites. Because Canada thistle is relatively shade intolerant, it may be found

growing along the edges of woods (both deciduous and coniferous), but is

rarely found under forest canopy, in undisturbed prairies, good to excellent

pastures, or woodland or sites that are shaded most of the day. The wide

distribution of Canada thistle suggests that it is adaptable to many soil

types. It grows on all but waterlogged, poorly aerated, and peat soils, in-

cluding clay, clay loam, silt loam, sandy loam, sandy clay, sand dunes,

gravel, limestone, and chalk. Canada thistle also occurs over a wide range

of elevations from sea level to elevations in excess of 8,000 feet (2,500 m).


SEASONAL DEVELOPMENT: The flowering period varies from place

to place and year to year, but occurs sometime between May and October

in North America. The blooming period is longer in northern locales than

in the south.


GENERAL DISTRIBUTION: Canada thistle is found throughout most

of the United States and Canada, but is less common or absent from the

lower southeastern states and much of the Gulf coast states west to Texas.


Canada thistle is native to southeastern Europe and the eastern Medi-

terranean area, and was probably introduced to North America in the

1600s as a contaminant of crop seed and/or ship's ballast. It is probably

the most widespread of all thistle species. In addition to North America,

Canada thistle is invasive in northern and southern Africa, the Middle

East, Japan, India, New Zealand, Australia, and South America. It infests

at least 27 crops in 37 countries and thrives in temperate regions of the

northern hemisphere.


Canada thistle has been identified as a management problem in many
national parks and on The Nature Conservancy preserves in the upper
Midwest, the Great Plains states, and the Pacific Northwest. It is an
invader in Mesa Verde National Park, Colorado, Yellowstone Nation-

al Park, Wyoming, Wood Buffalo National Park, Northwest Territor-

ies, Canada, Theodore Roosevelt National Park, North Dakota, and the
Camas Swale Research Natural Area in the Willamette Valley, Oregon.

Although Canada thistle is not usually found in undisturbed forested

areas, it has the potential to colonize a wide variety of forest habitats

within its range following overstory removal and soil disturbance.




IMPORTANCE AND USES: Primarily butterflies, skippers, long-
tongued bees, including bumblebees, large leaf-cutting bees, miner
bees, and Epeoline cuckoo bees, and tiny black beetles visit the flow-

ers for nectar. The flower nectar also attracts butterflies (especially

swallowtails), skippers, and bee flies. Green metallic bees and other

Halictid bees may collect pollen from the flowers, but they are non-

pollinating.The caterpillars of the butterfly Vanessa cardui (painted

lady) feed on the foliage, as do the caterpillars of many moth species.

Canada thistle supplies an early source of food for goldfinches and

the clay-colored sparrow as it blooms earlier than most thistle species.

Studies have found that rabbits eat bull thistle leaves, especially in

winter and early spring. Bull thistle seeds are eaten by mice and voles.

Bull thistle is included in a list of known grizzly bear food plants.

Bull thistle is eaten by Mazama pocket gophers in south-central Ore-

gon, and high bull thistle densities were observed in Yosemite Nation-

al Park in sites of intense pocket gopher digging. Pocket gophers

consume taproots from below, and their digging provides sites for

further thistle establishment, so that they are effectively "farming"

thistles. Because of the thorns, mammalian herbivores usually avoid

eating this plant. Even in overgrazed pastures where cattle and sheep

have little to eat, the bull thistle is one of the few plants that is gener-

ally left alone. However, livestock will eat this plant when little else is

available, and can have problems with irritation of mouthparts and the

digestive tract as a result.


The thistles have long been associated with humans. Thistles have been
used medicinally as well as for food. However, they are also notoriously
associated with unkempt agricultural land. While this may be true of
invasive thistles that are not native to North America, the vast majority
of native thistles fill specific ecological niches and have traits useful to
humans. The young stems and roots of bull thistle are edible. Native
North Americans used the roots and young leaves and newly bolted
stems of Cirsium species for food. Cirsium roots have been sold commer-
cially for use as rabbit bait in Australia. It is suggested that bull thistle
may be easily processed for rubber using standard equipment.



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