Scotch thistle [Onopordum acanthium L. ssp. acanthium][ONRAC] [CDFA list: A] Photographs Map of Distribution Biocontrol

Illyrian thistle [Onopordum illyricum L.][Bayer code: none][CDFA list: A] Photographs Map of Distribution

Taurian thistle [Onopordum tauricum Willd.][Bayer code: none][CDFA list: A] Photographs Map of Distribution

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SYNONYMS:scotch thistle: cotton thistle, woolly thistle, winged thistle, jackass thistle, heraldic thistle

GENERAL DESCRIPTION:Vigorous biennial, or short-lived perennials with coarse, spiny leaves and conspicuous spiny-winged stems. See Comparison of spiny-leaved thistles. Plants typically germinate in fall after the first rains and exist as rosettes throughout the first year until flowering stems develop during the second spring/summer season. Severe infestations can form tall, dense, impenetrable stands, especially in fertile soils. To date, biological control agents have been unsuccessful in the United States. Introduced from Europe and the Mediterranean region.

SEEDLINGS:Cotyledons oval to oblong, gradually tapered at the base, fleshy, ~1.5-2 cm long. Leaves elliptic to oblanceolate and irregularly spiny-toothed.

MATURE PLANT:Stems wings, spiny, continuous, conspicuous. Leaves alternate, spiny.


FLOWERS:Heads spheric to hemispheric, mostly solitary but sometimes in clusters (cymes) of 2-7. Heads consist of numerous spiny-tipped phyllaries in many overlapping rows and numerous disk flowers. Receptacles fleshy, deeply pitted, with pits bordered by membranous extensions of tissue, and lacking bristles (chaff). Phyllary spines less than 5 mm long.

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FRUITS and SEEDS:Achenes narrowly obovate, more or less 4- or 5-angled, glabrous, and mottled brown to blackish. Surface roughened with wavy transverse ridges. Pappus bristles equal, numerous, minutely barbed, fused into a ring at the base which separates as a unit.

POSTSENESCENCE CHARACTERISTICS:Stems can persist into the next season with spiny phyllaries and receptacles attached.

HABITAT:Natural areas, disturbed sites, roadsides, fields, and especially sites with fertile soils.

DISTRIBUTION:Infestations uncommon, but plants can be locally abundant at infested sites.

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PROPAGATION/PHENOLOGY:Reproduces only by seeds. Most seeds germinate in fall after the first rains, but some seeds can germinate year round under favorable moisture and temperature conditions. Buried seed of Scotch and Illyrian thistle can remain viable in the soil seed bank for at least 7 years and possibly to 20 years or more. Yearly seed production and seed dormancy are highly variable depending on environmental conditions.

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MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Survival is discouraged in well-managed perennial grass pastures where gaps are minimized from fall through spring. Unlike sheep and cattle, goats are to known to forage on Illyrian thistle rosettes and flowering stems. Survival is encouraged by overgrazing in grasslands and by low intensity burning of fields that often stimulates a flush of seed germination from the upper soil seed bank.

SIMILAR SPECIES:Onopordum thistles are distinguishable from other genera of thistles with spiny stern-wings and/or leaves by having receptacles that lack bristly chaff and have deep pits surrounded by membranous extensions.

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Prevention: While Scotch thistle is more widely distributed across California, Illyrian and Taurian thistles are confined to only a few areas. Therefore, intensive control efforts may be more fruitful for Illyrian and Taurian thistles. Scotch thistle, however, should not be disregarded. This plant may reach heights of nearly 3 m and form impenetrable stands. All three thistles reduce productivity and strongly compete with native plants for resources. Previous research has demonstrated certain requirements for Scotch thistle seed germination and may have some implications for management. For example, a water soluble inhibitor in the achene coat must be leached prior to germination. In addition, seed germination is much higher when seed ~ soil contact is maximized. Finally, seeds exhibit a germination response to light. These characteristics suggest certain practices may help in scotch thistle management. Since emergence may be greater in wet years, populations should be monitored more frequently for late season germination. Livestock should also be removed from infested areas to reduce seed dispersal and physical trampling of seed into the ground. Finally, any factors that increase light intensity at the soil surface, i.e., overgrazing or soil disturbance should be minimized. A dense canopy of competitive, perennial grasses may be the most effective practice for preventing Onopordum spp. establishment.

Mechanical: Small infestations should be physically removed or cut a few inches below the soil surface. Mowing by early flowering will reduce seed production, but may require repeated treatment because populations typically exhibit a wide range of developmental stages among individual plants. Slashing should be done prior to flowering since seed may mature in the capitula (seed head) after cutting. Plants should not be mowed following seed set, as this increases chances for dispersal.

Biological: There are no biological agents which have been specifically released for scotch thistle control in the United States. However, a specific strain of Rhinocyllus conicus (thistle head weevil) that attacks Italian thistle (Carduus pycnocephalus) has also been shown to feed on scotch thistle. Establishment of this strain in the Pacific Northwest has been unsuccessful. A thistle crown weevil (Trichosirocalus horridus) that feeds on musk, bull, plumeless, Italian, and Canada thistles will also feed on Scotch thistle. In Australia, this insect has been shown to kill scotch thistle rosettes. A thistle flea beetle from Italy (Psylliodes chalcomera) will attack Illyrian thistle, but has not been approved for release in the United States. There are several other potential insects being examined in Australia, but plans for their use in the United States are uncertain.
Studies in Australia indicated that goats prevented seed production in Illyrian thistle, by completely removing the flower heads. However, sheep alone or goats and sheep together did not completely prevent seed production. Goats were also effective in digesting seed as < 1% of the ingested seed was found in the feces, and none was viable.

Chemical: One of the primary difficulties in chemical control of these thistles is their ability to germinate nearly year round. From fall to spring, a range of plant sizes can be found which may result in variable chemical control. These herbicides are all very effective on seedlings and young rosettes, but control becomes more variable with increasing plant age. Onopordum spp. seeds may persist for several years in the soil. Buried seed may persist for up to twenty years, and reinfestation is likely without yearly management. Therefore several years of retreatment may be necessary. Dicamba and 2,4-D will inure or kill other broadleaf plants including legumes. Clopyralid is more selective for controlling plants in the Asteraceae family but will also injure or kill legumes. Table 1 gives specific herbicide rates for Onopordum spp. management.

Table 1. Herbicides used for Onopordum spp. management

Herbicide Rate Timing Remarks
2,4-D 1.5-2.0 lb ae / A Apply fall applications to control rosettes Apply spring applications by early bolting stage Lower rate will control seedlings and rosettes Larger plants require the higher rate
dicamba 0.5-1.0 lb ae / A Same as above Longer residual activity: 12-18 months depending on the rate and environmental conditions
clopyralid 0.09-0.38 lb ae / A May apply to actively growing plants until bud stage Effective on a wide range of thistles, but will also injure legumes

Integrated management: Seedbank longevity is a major factor in managing these thistles. Reestablishing competitive perennial grasses and monitoring infested areas on a yearly basis is critical. Herbicides can successfully be used for reducing thistle populations and giving grasses a competitive advantage. However, they cannot be used as a stand alone solution. These techniques must be linked with good grazing practices in rangeland areas. Otherwise, the thistles will recolonize and rapidly replenish the seed bank to pre-control levels.

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