Artichoke thistle [Cynara cardunculus L.][Bayer code: none][CalEPPC: A-1][CDFA list: B] Photographs Map of Distribution


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[
SYNONYMS] [GENERAL DESCRIPTION] [SEEDLINGS] [MATURE PLANT] [ROOTS and UNDERGROUND STRUCTURES] [FLOWERS] [FRUITS and SEEDS] [POSTSENESCENCE CHARACTERISTICS] [HABITAT] [DISTRIBUTION] [PROPAGATION/PHENOLOGY] [MANAGEMENT FAVORING/DISCOURAGING SURVIVAL] [SIMILAR SPECIES] [CONTROL METHODS]

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 fly’s impact on artichoke thistle populations is unknown. Larvae feed only on mature flower heads, thus commercial artichokes are not significantly affected.

SEEDLINGS: Cotyledons 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 fall.

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MATURE PLANT: 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.

ROOTS and UNDERGROUND STRUCTURES: Tapoot thick, fleshy, can penetrate soil to depths up to 2 m. Roots fragments often generate new shoots.

FLOWERS: April-July. 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.

FRUITS 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.

POSTSENESCENCE CHARACTERISTICS: Stems typically die after flowering and can remain standing for several months. Old flower heads may persist on the stems.

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HABITAT: Disturbed, 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.

DISTRIBUTION: Throughout low elevation areas of California, except Great Basin and desert regions. To 500 m (1650 ft).

PROPAGATION/PHENOLOGY: Reproduces 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 years.

MANAGEMENT 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.

SIMILAR SPECIES: 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.

CONTROL METHODS:

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.

References
Hillyard, D. 1985. Artichoke thistle. Fremontia 12:21-22.
Parsons, W.T. and E. Cuthbertson. 1992. Noxious Weeds of Australia. Inkata Press. Melbourne, Australia.
Thomsen C. D., G. D. Barbe, W. A. Williams and M.R. George. 1986. Escaped artichokes are troublesome pests. California Agriculture 40:7-9.

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