Whitestem distaff thistle [Carthamus leucocaulos Sibth. & Smith][Bayer code: none][CDFA list: A] Photographs Map of Distribution

Smooth distaff thistle [Carthamus baeticus (Boiss. & Reuter) Nyman][Bayer code: none][CDFA list: B] Photographs Map of Distribution

Woolly distaff thistle [Carthamus lanatus L.][Bayer code: none][CDFA list: B] Photographs Map of Distribution

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GENERAL DESCRIPTION:Noxious winter annual composite weeds, with rigidly erect branched stems to 1 m tall. Plants exist as rosettes until spiny-leaved flowering stems are produced in spring/summer. Plants are highly competitive with cereal crops and desirable rangeland species. Because of their spiny nature, distaff thistles can injure the eyes and mouths of livestock forced to graze within dense populations of the weeds. Distaff thistles are closely related to commercial safflower [Carthamus tinctorius L.], precluding the development and release of biocontrol agents in California. Smooth and woolly distaff thistles are difficult to distinguish from one another and are sometimes classified as subspecies of C. lanatus. Currently, they are considered distinct species because gene numbers differ (smooth distaff thistle: 2n = 64, woolly distaff thistle: 2n = 44). Species do not hybridize. Introduced from the Mediterranean region. See Comparison of spiny-leaved thistles.

SEEDLINGS:Woolly and smooth distaff thistle: Cotyledons obovate and tapered at the base. Rosette leaves coarse, 1-pinnately lobed. Lobes spiny-tipped. Size of cotyledons and first rosette leaves sometimes highly variable among populations. Woolly distaff thistle leaves often less deeply lobed near the tips than those of smooth distaff thistle. No information available for whitestem distaff thistle, but this species is probably very similar to woolly and smooth distaff thistles.

MATURE PLANT:Stems mostly branched from upper 2/3 of plant. Stem leaves alternate, coarsely 1-pinnately lobed, stiff, spreading or curved downwards, conspicuously veined, and evenly covered with minute glandular hairs. Lobes few, narrow, mostly opposite, and prominently spiny-tipped. Leaf bases lack petioles and weakly clasp stems. Basal leaves similar to stem leaves, but larger, 1- or 2-pinnately lobed, and often absent at flowering.

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ROOTS and UNDERGROUND STRUCTURES:Taproots slender, elongated, mostly unbranched, and with many fibrous roots.

FLOWERS:Flower heads solitary at stem tips and consisting of several overlapping rows of phyllaries (bracts), numerous disk flowers, and papery, scale-like (chaffy) bracts on dome- to cone-shaped receptacles. Outer phyllaries spiny, rigid, and leaf-like. Inner phyllaries tipped with spiny appendages.

FRUITS and SEEDS:Achenes buff to brown, sometimes mottled with dark brown, oblong or pyramidal, more or less 4-sided, and laterally notched near the base. Apex wavy-margined and broader than the base. Outer achenes often darker, rough-surfaced, and lacking a pappus. Inner achenes mostly smooth-surfaced and with a persistent pappus. Pappus scales brownish, numerous, narrow, and unequal.

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POSTSENESCENCE CHARACTERISTICS:Stems often persist through winter, longer than most other thistles. Phyllaries and seeds remaining in old flower heads can aid with species identification.

HABITAT:Disturbed, open sites of grasslands, pastures, and agricultural lands, especially grain fields. Inhabits many soil types. Prolific in areas with 400-600 mm (16-24 in) annual rainfall.


PROPAGATION/PHENOLOGY:Reproduces by seeds; woolly and smooth distaff thistle: Most seeds (achenes) disperse passively near the parent plant, but some remain in the persistent seed heads. Seed heads can disperse by animals or machinery. Seeds often mature with a high rate of dormancy. Dormant seeds appear to require leaching of germination inhibitors and light to break dormancy. Rates of dormancy can vary among populations at different locations. Most seeds germinate after the first fall rains 1-3 years following maturation, but some seed can remain dormant and viable for up to 8 years under field conditions. Germination of seeds at locations with low rainfall and temperatures can be slow and drawn out. Typically, few seeds germinate after midwinter. Seedlings rarely emerge from soil depths below 5 cm. Optimal emergence occurs at or just below the soil surface. Seeds are susceptible to predation by termites.

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MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Mowing just before the formation of flower buds discourages survival. Mowing earlier can encourage the re-growth of flowering stems. In plants mowed after flower heads have developed, seed can mature in cut flower heads left on the ground. Heavy grazing can encourage survival because livestock selectively graze plants around distaff thistles, thereby reducing competition with other plants for light and nutrients. Distaff thistles are unlikely to establish in well-managed perennial pastures.

SIMILAR SPECIES:The annual blessed thistle [Cnicus benedictus L.] and biennial golden thistle [Scolymus hispanicus L.] are yellow-flowered thistles that may be confused with smooth or woolly distaff thistle. Unlike distaff thistles, blessed thistle has inner phyllaries with long, spiny, pinnate-lobed tips, unlobed outer phyllaries, ~ 20 pappus bristles in 2 series (inner short, outer long), and cylindrical achenes with ~ 20 prominent ribs and pale crown-like teeth on the apical rim. Blessed thistle occurs sporadically on disturbed sites, in fields, and along roadsides in the North Coast Ranges, Central Valley, Central-western region, South Coast, and western Mojave Desert, to 800 m (2600 ft). Introduced from Europe as a medicinal herb. Golden thistle is distinguished by having ligulate flowers, milky sap, conspicuously winged stems, receptacle chaff of long scales enclosing fruits, and fruits with 2-4 stiff, minutely barbed pappus bristles. Unlike other pink to purple-flowered thistles (Carduus, Cirsium, Cynara, and Silybum spp.), whitestem distaff thistle is the only thistle with spiny-lobed outer phyllaries and fruits with short, narrow, unequal pappus scales.

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Prevention: These extremely spiny thistles are very similar in appearance, but there is very little information available regarding either C. leucocaulos or C. baeticus. Therefore, these recommendations are based upon control information for C. lanatus. Range or pasture overgrazing is one of the primary mechanisms of spread for these thistles. Areas with established perennial grasses are less susceptible to invasion by these thistles. Rosettes are prostrate with sharp spines and are not easily grazed. Grazing animals will avoid them for more palatable species, resulting in a rapid increase in thistle populations with concurrent decreases in desirable vegetation. Therefore, maintaining healthy range or pasture conditions will reduce invasion. Grazing animals should be removed from areas where small infestations are present to prevent increased thistle growth. C. lanatus is also a major problem in Australian cereal grain producing areas and may follow suit in California and the Pacific Northwest. Dense infestations of these rigid plants can reduce yields, clog harvesting equipment, and increase seed cleaning costs. Use certified weed free seed and clean combines, drills, and tillage equipment before transporting them to other fields. Areas most susceptible to invasion may be interfaces between cultivated fields and pastures. Seed near the soil surface are viable for two to three years, but buried seed may persist for over six years. Populations in these areas should be monitored for several years since buried seed may be exposed by tillage. C. Lanatus seed germination appears to be dependent upon the leaching of a water soluble inhibitor and wet years may facilitate germination. However, seed germination is usually limited to the fall following rain.

Mechanical: Plants may be controlled by hand pulling or cutting just below the soil surface. Mowing will control infestations, but the timing is critical. Plants should be mowed after bolting but before flower bud formation. Mowing in the early bolting stage may stimulate new flower stem development from the base of the rosette. These thistles produce numerous seeds that fall directly below the plant. However, some seed remain attached in the seed heads into the next year. Therefore, mowing after seed set, or mowing skeleton plants the following year may facilitate seed dispersal and is not recommended.

Biological: There are no biological control strategies currently available for these thistles. One of the main reasons for this is concern of the safety of a related cultivated crop species, Carthamus tinctorius, safflower. Host specificity among these plants has not been found in their native habitat in the Mediterranean or central Europe.

Chemical: Certain herbicides are effective in controlling these thistles. However, an integrated approach is necessary to prevent reinfestations. Plants are most easily controlled when thistles are in the rosette stage and control becomes increasingly difficult as plants bolt and mature. All of these herbicide treatments will have some effect on certain other plants, such as broadleaves in general, or legumes. Consult the herbicide label for specific information regarding susceptible species. Table one summarizes herbicide information for these thistles.

Table 1. Herbicides labeled for use in range settings
Herbicide Rate Timing Remarks
clopyralid 0.25-0.5 lb ae/A Rosette stage Pre-bolting Will injure legumes
2,4-D 1-2 lb ae/A Rosette stage Pre-bolting May injure broadleaves
dicamba 1-2 lb ae/A Rosette stage Pre-bolting May injure or kill broadleaves
glyphosate 0.18 lb ae/A Rosette stage Early-bolting Non-selective treatment
triclopyr 1.5 lb ae/A Rosette stage Early-bolting May injure or kill broadleaves

Integrated Management: In a grazing situation, the spiny nature of these thistles will always shift the competitive balance in their favor. It is essential that restablished grasses be monitored for newly emerging thistle populations. Otherwise, they are likely to rapidly recolonize the area. Monitoring should be conducted in the fall following rain when conditions are optimal for seed germination.

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