Artichoke thistle [Cynara
cardunculus L.][Bayer code: none][CalEPPC: A-1][CDFA list: B] Photographs
Map of Distribution
SYNONYMS: cardoon, wild
or desert artichoke
GENERAL DESCRIPTION: Large spiny perennial to 2.5 m tall. Artichoke thistle
primarily invades disturbed grasslands, especially those in coastal regions.
Dense colonies displace desirable vegetation and wildlife and can exclude
livestock. It is a progenitor of the commercially cultivated, spineless globe
artichoke [Cynara scolymus L.]. Some taxonomists consider globe
artichoke and artichoke thistle to be the same species,
C. cardunculus L. The two species readily hybridize, and a few spiny
wild types often develop among globe artichoke seedlings. See Comparison
of spiny-leaved thistles. Artichoke thistle was introduced
from the Mediterranean region as a vegetable and ornamental. The artichoke
fly (Terellia fuscicornis) was accidentally introduced into California,
but is not a CDFA approved biocontrol agent. Preliminary studies suggest that
some native thistles (Cirsium spp.) may be vulnerable to attack. At
publication time, the flys impact on artichoke thistle populations is
unknown. Larvae feed only on mature flower heads, thus commercial artichokes
are not significantly affected.
obovate, 3-5 cm long, bases gradually long-tapered, tips rounded,
glabrous or with scattered short woolly hairs. First and subsequent
few leaves elliptic, ~ 3-20 cm long, tapered to a long stalk,
+/- weakly toothed, teeth tipped with a fine yellowish spine ~
0.5-3 mm long, +/- covered with short white woolly hairs. Seedlings
develop a deep taproot during the first year. Rosette leaves often
die during the first summer and re-grow when rains commence in
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Stems erect, thick, branched near the top, ribbed. Basal leaves
1-2-pinnately lobed or divided, often appear +/- compound,
up to 2 m long, lobes tipped with stiff, yellowish to pale
orange spines 0.5-2 cm long. Upper surfaces loosely and lower
surfaces densely covered with white to gray woolly hairs. Stem
leaves alternate, resemble basal leaves but smaller and extend
down the stem ~ 1-3 cm at the base (decurrent), forming short
spiny wings. Cultivated types may lack or have weak spines.
and UNDERGROUND STRUCTURES: Tapoot thick, fleshy, can penetrate soil to depths
up to 2 m. Roots fragments often generate new shoots.
Heads solitary at stem tips, ovoid to hemispheric, 3-15 cm
in diameter, consist of numerous bluish to purple or rarely
white disc flowers ~ 5 cm long. Phyllaries ovate, overlapping
in several series, tapered to a stout point. Receptacle fleshy,
covered with bristles. Insect-pollinated.
and SEEDS: Achenes conical to cylindrical, slightly compressed
to +/- 4-angled, 6-8 mm long, with an attachment scar at the narrow
base, glabrous, dark brown to tan, sometimes with black, brown,
or dark green longitudinal striations. Pappus bristles feathery
(plumose), 2.5-4 cm long, fused into a ring at the base, tan,
attached slightly off-center, deciduous.
CHARACTERISTICS: Stems typically die after flowering and can remain
standing for several months. Old flower heads may persist on the
open sites in grasslands, chaparral, coastal sage scrub, and riparian
areas; abandoned agricultural fields. Often associated with overgrazing.
Grows best on deep clay soils. Does not tolerate heavy shade.
low elevation areas of California, except Great Basin and desert
regions. To 500 m (1650 ft).
by seed. Most seeds fall near the parent plant or disperse
up to ~ 20 m (66 ft) with wind. Some seeds disperse to greater
distances with water, soil movement, animals, and human activities.
Most seeds germinate after the first rains in fall, but some germination
can occur year round under favorable conditions. Field observations
suggest that most seeds survive about 5 years under field conditions.
One-year old plants sometimes flower, but most plants do not flower
until their second year. Individual plants often live for many
FAVORING/DISCOURAGING SURVIVAL: A large portion of the taproot of hand-pulled plants
must be removed, otherwise plants will regenerate. Burning does
not kill taproots. Cutting flower stems before maturity or browsing
by goats can reduce seed production. On agricultural land, repeated
cultivation can eventually eliminate an infestation.
Unlike artichoke thistle, other weedy thistles lack a fleshy receptacle,
have smaller, less deeply lobed leaves, and all except Cirsium species
have a pappus of scales or minutely barbed bristles.
Prevention: Wild artichoke thistle only spreads by seeds,
which are large and heavy and generally fall near the parent plants. However,
the bristly pappus may attach to animal hair or fur as the plant is known to
spread along game trails. Seeds may also be blown along roadsides or possibly
be carried by vehicle tires. Populations on hillsides also may move down slope
as the heavy seeds move by gravity. Infestations along roads or paths or up
slope should be quickly targeted for removal to prevent spread to new areas.
Mechanical: The typically large size and spiny nature of
artichoke thistle plants make physical removal very challenging. Grubbing is
possible when plant densities are low but is impractical on large infestations.
New shoots will emerge from the tap root and must be repeated removed. Completely
digging out the tap root is extremely difficult, since it may extend to a depth
of eight feet. An alternative is cutting and removing the seed heads to prevent
seed production when complete plant removal is not possible. Large scale mechanical
removal requires repeated plowing or cultivation and is not generally recommended
on wildlands. Heavy leather gloves and chainsaw chaps are generally recommended
when doing hand removal.
Biological: There are no registered biological agents for
artichoke thistle, due to it close relation to globe artichoke. However, the
artichoke fly (Terrelia fusicornis) has been found on artichoke thistle throughout
the state. Native to the Mediterranean, the fly feeds on the flowers and seedheads
of both artichoke species. The full impact of the fly on artichoke thistle populations
is still unclear. However its potential to reduce seed production is promising.
Prescribed burning: The role of prescribed fire for artichoke
thistle is uncertain. Fire may remove some top growth and possibly kill some
seed on the soil surface. However, artichoke thistle has also been observed
to be one of the first colonizers to arrive following wildfire.
Grazing: Most herbivores generally are deterred from feeding
on artichoke thistle due to the spiny nature of the plant. Cattle may selectively
overgraze surrounding vegetation, thereby favoring the thistle. Additionally,
cattle and deer may completely avoid dense stands of the thistle due to its
spiny nature. Gopher activity may occasionally cause the death of individual
plants. However, the population impact appears minimal.
Chemical: Glyphosate has been a very effective tool for artichoke thistle control.
A cut stump treatment involves removing the top growth as closely to the ground
as possible and then quickly applying a 25% glyphosate solution to the stump.
This methods works very well where foliar applications may damage surrounding
vegetation or where remote populations limit equipment access. Glyphosate may
also be effective when foliar applied as a 2% v/v solution to mature bolting
plants. Earlier applications result in rapid resprouting. Clopyralid may also
be effective when applied at the seedling to rosette stages, but is less effective
or older plants.
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