Whitestem distaff thistle [Carthamus leucocaulos Sibth. & Smith][Bayer code: none][CDFA
list: A] Photographs Map
Smooth distaff thistle [Carthamus baeticus (Boiss. & Reuter) Nyman][Bayer code:
none][CDFA list: B] Photographs Map
Woolly distaff thistle [Carthamus lanatus L.][Bayer code: none][CDFA list: B] Photographs
Map of Distribution
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.
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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and UNDERGROUND STRUCTURES:Taproots slender, elongated, mostly unbranched,
and with many fibrous roots.
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.
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.
CHARACTERISTICS:Stems often persist through winter, longer than
most other thistles. Phyllaries and seeds remaining in old flower
heads can aid with species identification.
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.
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
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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.
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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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
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
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