Source: California Invasive Plant Council


URL of this page: http://www.cal-ipc.org/ip/management/ipcw/pages/detailreport.cfm@usernumber=38&surveynumber=182.php

Invasive Plants of California's Wildland

Cynara cardunculus
Scientific name   Cynara cardunculus
Common name artichoke thistle, cardoon, desert artichoke, wild artichoke
Synonymous scientific names none known
Closely related California natives 0
Closely related California non-natives: Cynara scolymus
Listed CalEPPC List A-1,CDFA B
By: Michael Kelly
Distribution
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HOW DO I RECOGNIZE IT?
Distinctive features:

Artichoke thistle (Cynara cardunculus) is a spiny thistle of the sunflower family, head high, occasionally taller, crowned by a cluster of showy, bright purple thistle flowerheads that are two to three inches in diameter from April through July. Rarely, patches of white-flowering plants are found (Munz 1963, Kelly pers. observation). One to several stout flower stalks rise from a bushy rosette up to five feet in diameter. A spray of basal leaves, each deeply lobed and gray-green, to four feet in length, arches gracefully up and out from the base. Smaller leaves, otherwise similar in appearance, grow from the flower stalk as it extends upwards. Stout spines on the leaves, stems, and bracts around the flowerheads make it easy to recognize.

 

Description:
Compositae. Perennial herb. Stems: leafy, branched, stout, and generally erect unless heavy flowerheads cause bending, to 5 ft (1.5 m) in height. Leaves: basal until bolting, at which time cauline leaves appear off inflorescence stalk, alternate, 1-4 ft (3-12 dm), pinnately lobed to divided, very spiny. Gray-green upper surface loosely cobwebby, lower surface densely gray-tomentose. Rubbing leaf surface easily removes cobwebby material.
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Inflorescence: lower heads typical of aster family, discoid, large, with one to several per cyme in a loose cluster at top of stalk. Typically, the uppermost flower opens before lower or peripheral flowers on any given axis. Involucres: ovoid or hemispheric, 1-2.25 in (3-6 cm) in height, 1.5-2.75 in (4-7 cm) diameter, not including tips, tending to narrow above. Phyllaries surrounding flowerhead easy to identify, with stout spines at tips, overlapping in series, generally ovate, leathery, entire, glabrous, receptacle flat, fleshy, bristly. (Spiny tips on phyllaries on immature flowerhead easily differentiate this plant from the closely related Cynara scolymus, the common agricultural globe artichoke.) Flowers: striking, with purple, sometimes blue corollas, +/- 2 in (5 cm), tube very slender, throat widened abruptly, lobes linear; anther bases long-sagittate, tips oblong; style appendage long, cyclindric, minutely papillate, tip barely notched. Fruits cylindric to obconic, =/- 4-angled or +/- compressed, glabrous, attached at base; pappus of many stiff bristles, 1-1.5 in (2.5-4 cm), in several series, white or brownish, plumose below, fused and falling together (Hickman 1993). Corollas occasionally white (Munz 1963, Kelly pers. observation).

 

WHERE WOULD I FIND IT?

Artichoke thistle is found in disturbed places, to 1,650 feet (<500 m), throughout the state, except deserts (Hickman 1993). It is common in annual rangelands, especially with a coastal influence, but also is found inland in disturbed grasslands or abandoned agricultural fields and is associated with overgrazing (Thomsen et al. 1986). It was one of the worst pests on California rangelands by the 1930s, invading over 150,000 acres in thirty-one counties and requiring prodigious and expensive efforts to eradicate or control it. By the 1980s the worst concentrations of the plant were found in Orange, Solano, and Contra Costa counties, with locally dense populations elsewhere in the Coast Ranges, Central Valley, and Sierra Nevada foothills (Barbe 1990). Placement on the California Department of Food and Agriculture’s B List reflects the fact that it became too widespread and difficult to eradicate in many areas, with the authorities opting for preventing its spread, and control when feasible.

Artichoke thistle has been observed colonizing riparian woodlands and natural openings in chaparral and coastal sage scrub, growing under willow, mulefat, and sycamore, as well as in native grasslands (Pepper and Kelly 1994). It does well in soils with a heavy clay content (Thomsen et al. 1986), which helps explain its invasion of grassland habitat occupied by endangered San Diego thornmint (Acanthomintha ilicifolia) (Kelly 1996).

 

WHERE DID IT COME FROM AND HOW IS IT SPREAD?

Native to the Mediterranean (Hickman 1993), artichoke thistle became widespread on over 150,000 acres of California rangeland and also in Australia, New Zealand, and South America on grazing lands, especially the Argentine pampas (Thomsen et al. 1986). It is now recognized as the wild form of the cultivated globe artichoke, Cynara scolymus L. When grown from divisions of the perennial crown, globe artichoke will reliably produce the spineless, edible flowerhead and plant known to agriculture, but grown from seed it often reverts to a wild form, producing the inch-long spines around the flowerhead normally found on C. cardunculus (Thomsen et al. 1986).

Artichoke thistle was known as the “edible thistle” and mentioned in literary references dating back to several centuries before Christ. Cultivation of the edible wild form appears to have spread from Naples, Italy, into the broader Mediterranean during the fifteenth century (Warren 1996). Cultivated forms apparently were developed from artichoke thistle in monastery gardens during the Medieval period (Thomsen et al.1986). Emigrants from the Mediterranean region apparently carried one or more forms of the plant to other countries. During his voyage on the Beagle, Darwin (1989) found artichoke thistle had already reached the Argentine pampas and escaped cultivation: “Very many, probably several hundred square miles are covered by one mass of these prickly plants and are impenetrable by man or beast. Over the undulating plains where these great beds occur, nothing else can now live.”

Botanical surveys in California from 1860 to 1864 reported the globe artichoke variety as having escaped cultivation. Artichoke thistle was already reported as having established itself outside cultivation in a pasture in San Diego County in 1897. Its appearance in California rangelands can be traced to its introduction for ornamental and culinary uses (Thomsen et al. 1986).

Although cultivated vegetatively from crown division, artichoke thistle in the wild spreads only by seed. The large seeds are dispersed by a variety of mechanisms. Bristles from the receptacle and the pappus (thistledown) remain attached to the seeds when they are released, aiding in dispersal. Seeds separate easily and quickly from the thistledown and, because of their large size and weight, they usually are not carried more than sixty-six feet (20 m) from the parent plant. Strong winds, however, may pile up great banks of thistledown against fence lines and road margins. However limited this dispersal mechanism may be, over a few years wind can significantly expand the patch size of a local infestation.

Most seeds seem to fall close to the parent plant. Birds feeding on the seed heads probably knock some seed to the ground and occasionally move it greater distances. Hillside patterns suggest that water and gravity carry seeds short distances on slopes. Seeds may attach to cattle and other mammals, and the plant is known to spread along game trails in coastal sage scrub in southern California. Its distribution along utility roads in some areas of San Diego County suggests that vehicle tires are transporting seeds.

 

WHAT PROBLEMS DOES IT CAUSE?

Artichoke thistle is an important rangeland problem because it reduces forage production and limits movement of livestock (Thomsen et al. 1986). The stout, upright yet spreading nature of the plant, its formidable spines, and high densities make wildlife movement through it difficult. The arching leaves shade a considerable area. Combined with its aggressive root system, artichoke thistle outcompetes native vegetation for light, water, and nutrients. At high densities it becomes a monoculture that excludes shrubs, herbaceous plants, and even annual grasses. For example, mature broom baccharis (Baccharis sarothroides) declines in vigor when in close proximity to artichoke thistle. The thistle also is a threat to the endangered San Diego thornmint.

There appears to be no alteration of soil chemistry or allelopathy, since other species, including those originally displaced, readily recolonize the site once artichoke thistle is dead. It is unlikely that the plant alters fire cycles, since it mainly displaces annual exotic grasses, which are highly combustible. However, one 4,000-acre infestation in San Diego was first colonized by this thistle after a grassland fire in the same area in the mid-1980s (Dumka 1997).

 

HOW DOES IT GROW AND REPRODUCE?

Artichoke thistle flowers as early as April in southern California and into July throughout its range. In a year of average rainfall in San Diego of nine inches (22.5 cm), a mature plant can produce more than a dozen flowerheads with as many as 200 seeds per head (Kelly 1996). In drier years most established plants sprout, flower, and produce seed, but these plants are usually smaller than plants growing during wetter years. Some established plants will sprout and then wither before flowering.

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Bees, a common visitor to the flower, are probably the most important pollinator. Mature plants resprout repeatedly when cut or when herbicide is applied too early or in too small amounts (Pepper and Kelly 1994).

(click on photos to view larger image)

cynara-small2

In southern California artichoke thistle germinates as early as December and as late as July. Seven years of control efforts with careful record keeping in Peñasquitos Canyon Preserve (San Diego) suggests that a sizable seedbank with an average duration of five years can be expected (Kelly 1991). In drier years seedling mortality is high. Growth is rapid during cool and wet winter months in coastal and southern California. Plants usually flower the first year if rainfall is equal to or greater than average, but flowering may be delayed until the second summer in drier years. Late-germinating plants barely reaching a foot in height are capable of producing flowers and setting seed. Plants form multi-stalked clumps after the first year and the clumps enlarge thereafter. A deep taproot develops during the first year (Parsons 1992). The roots can eventually reach eight feet in depth. Rosette leaves usually die over the summer. The plant tends to produce single-species stands that can reach densities as high as 22,000 plants per acre (Thomsen et al. 1986). With a large, expanding underground taproot and tuber, a single plant quickly becomes a many-stemmed clump. Some populations become dense enough to restrict growth, resulting in tall, spindly plants rather than the robust, broad clumps typical of plants not under competitive pressures.

 

HOW CAN I GET RID OF IT?

Because of artichoke thistle’s ability to resprout after chemical spraying and to build up a seedbank that lasts five years or more, yearly monitoring and repeat eradication are necessary. Eradication is most effective when mature plants are bolting, generally in early to mid-April in southern California and late April or May farther north. Chemical control efforts have proved successful on sizable populations in several open-space parks and on military lands managed for their natural resources in San Diego County. Seedbanks in some areas have been exhausted, and maintenance is minimal and routine (Kelly pers. observation).

Historically, artichoke thistle was controlled by hand grubbing, root-plowing by tractors equipped with a specially designed blade, displacement planting, and applying the herbidide 2,4-D from airplanes and helicopters. These methods often proved too labor-intensive or expensive to be used successfully in larger infestations. Costs of control efforts sometimes exceeded the value of the land (Thomsen et al.1986). Workers often wear chainsaw chaps as protection when working to control it.

 

Physical control:

Manual methods: Grubbing is practical when only scattered plants are present, but much of the taproot must be removed or new growth will develop from the cut surface. Cutting and removing seed heads can stop seed production in small populations where timely eradication of the plant is not possible (Pepper 1994).
Mechanical methods: Discing or plowing larger infestations in wildlands is impractical and not advised. Although it is theoretically possible to exhaust the carbohydrate reserves of the plant’s tuberous roots, this would require many years of continued effort and several carefully timed passes each season because artichoke thistle can resprout repeatedly. Discing and plowing also disturbs the soil, opening it up to reinfestation by this species or other invaders. The deep root, reaching eight feet (3 m) in depth, also makes mechanical removal difficult.

Prescribed burning: This could be helpful in removing the above-ground biomass, making access for chemical control easier and more efficient. Burning would not be expected to kill the plant, given its perennial underground storage reserves, but might kill the surface layer of the seedbank. Applying herbicide to resprouts four to six weeks after they emerge would likely be most effective for eradicating this thistle from the burned area.

 

Biological control:

Insects and fungi: The plant is a food source for invertebrates, including earwigs, ants and their aphid consorts, harlequin beetles, and bees; the latter being found frequently on the flowers. No USDA approved biocontrol agents exist for this thistle in California. Given the close relationship of the wild artichoke to the cultivated crop artichoke, the purposeful introduction of a biocontrol fungus or insect is not likely. However, Terellia fuscicornis (artichoke fly), an exotic fly from the Mediterranean Basin, has been discovered, identified, and collected in 1994 from San Joaquin, Sonoma, and Madera counties. It has subsequently been identified in wild artichoke populations in a number of other counties. This exotic fly has been found on both Cynara cardunculus and C. skolymus and feeds on flowers and seed heads. Since it is a seed-eater, it does not currently threaten commercial artichoke production. Given the fly’s widespread distribution and its potential as a biocontrol agent on artichoke thistle, a CDFA B rated weed, attempted extirpation is precluded under agricultural regulations, which is potentially good news for wildlands (Penrose 1994). By March 1997 the artichoke fly had been downgraded from a Q to a C listing by the California Department of Agriculture. The Q listing is a quarantine listing; it cannot be moved around. The C listing, is a conditional listing, allowing it to be moved if certain conditions are met with permits between counties (Darling 1997). Until the arrival of the artichoke fly, herbivory by insects or mammals, including cattle, offered little hope of controlling artichoke thistle.

Grazing: The spiny nature of the plant deters cattle and sheep from grazing heavy infestations, but hungry animals will eat the leaves (Parsons 1992). Several species of birds feed on the seeds, but not to any great extent. Herbivory by deer is not apparent, probably because of the spines. Rabbits occasionally eat it at the cotyledon stage, but not beyond. The occasional dead plant encountered in the wild appears to be the work of gophers, but their impact on the population is insignificant.

 

Chemical control:

Glyphosate is effective in killing artichoke thistle. Cut stump applications were reported as effective at any stage of growth in nature preserves in the Santa Monica Mountains (Pepper and Kelly 1994). Subsequent cut stump application on thousands of plants in Peñasquitos Canyon Preserve in San Diego have confirmed this method (Kelly pers. observation). The cut-stump method involves cutting the plants as close to the base as possible with a machete, loppers, or a brush cutter and applying a solution of 25 percent glyphosate (as Roundup®) to the stump. This method is useful for isolated plants or remote populations where spray equipment is impractical or when it is in close proximity to sensitive species and foliar spray is not advised. The cut-stump method has been successfully used to eliminate artichoke thistle from patches of the endangered San Diego thornmint with no harm to the latter.

A foliar spray of 2 percent glyphosate (as Roundup®) can achieve a kill of 95 to 98 percent on mature, bolting plants. Spraying plants that had gone to seed also achieved a similar high kill rate. Spraying plants in earlier stages of growth before the plant sends up its flower stalk kills the above-ground vegetative structures, but often does not kill all of the roots. In such cases the plant dies back, but up to 75 percent of sprayed mature plants resprout in the same season. Transport of fluids is generally up to the stems and leaves and less down to the roots in pre-bolting plants, probably preventing sufficient herbicide from reaching the roots. However, spraying seedlings with 2 percent glyphosate is effective. Cutting down dense patches with power tools or a tractor is a useful prelude to chemical treatment. It allows workers to penetrate patches with less damage from spines on standing dead plants and reduces the amount of herbicide needed.

A new herbicide, clopyrlid (as Transline®) appears to be effective when sprayed on this thistle at the rosette stage, but less effective on mature, bolting plants (Carrithers 1997).