BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Centaurea solstitialis | Yellow Starthistle 



GENERAL BOTANICAL CHARACTERISTICS : 
Yellow starthistle is an introduced winter annual with a deep taproot.
The stems, which grow 0.7 to 2.6 feet (0.2-0.8 m) tall, are rigid and
branch from the base.  Flowerhead bracts have 0.2- to 1-inch (0.6-2.5
cm) long, sharp, rigid spines [12,13,14].  Yellow starthistle has two
types of seeds, plumed and plumeless.  A fine, white pappus (plume) is
0.12- to 0.16-inch (3-4 mm) long on seeds in the flowerhead center but
is lacking from those at the margin of the flowerhead [32].


RAUNKIAER LIFE FORM : 
   Therophyte


REGENERATION PROCESSES : 
Yellow starthistle reproduces exclusively by seed.  Vigorous plants may
produce 170,000 seeds per plant, but seed production is generally much
lower when plant densities are high [39].  A site in southeastern
Washington with an average of 16 mature yellow starthistle plants per
square foot (180/sq m) produced an average of 1,890 seeds per square
foot (21,000/sq m).  Individual plants averaged 2 seedheads and 120
seeds per plant [21].

Plumed and plumeless seeds are dispersed at different times, which may
maximize seed survival.  Plumed seeds are dispersed shortly after
maturity.  Plumeless seeds remain in the seedhead until repeated drying
and wetting cause the seedhead to fall apart in the fall or winter [32].

Plumed seeds are dispersed by wind, but yellow starthistle has a small
plume relative to seed size.  Andersen [1] reports that yellow
starthistle had the second highest seed settling velocity of 19 tested
wind-dispersed species in the Asteraceae family.  Ballistic dispersal
may be as important as wind dispersal.  When the plant sways in the
wind, seeds can be launched over distances roughly equal to the height
of the plant [1].  Roche [32] used seed traps to study yellow
starthistle seed dispersal.  Ninety-two percent of seeds were trapped
within 2 feet (0.6 m) of the leeward edge of 1,550 square foot (144 sq
m) stands of yellow starthistle.  No seeds were found farther than 32
feet (9.8 m) from the stand edge.  Gusty winds and dry conditions
maximized seed dispersal [32].

Yellow starthistle seeds are also dispersed by animals, people, and
vehicles.  Seeds may remain viable after passing through the digestive
tract of larger birds such as California quail and ring-necked pheasant.
These birds do not remove the seed hull prior to ingesting yellow
starthistle seed [32].

One hundred percent of plumed and plumeless yellow starthistle seeds
initiated germination within 108 hours after being rinsed with 5 percent
sodium hypochloride solution and placed in dark, moist conditions at 54
to 57 degrees Fahrenheit (12-14 deg C).  Plumed seeds initiated
germination more rapidly than unplumed seeds [36].

Seeds remain dormant in the soil.  At a yellow starthistle site in
southeastern Washington, the seedbank contained 270 yellow starthistle
seeds per square foot (3,000/sq m) [21].  In California, after 6 years
of burial in sandy loam, yellow starthistle seeds averaged 38 percent
germination.  Burial depths greater than 2 inches (5 cm) offered maximum
survival [16].  Joley and others [16] found no difference in total
germination between plumed and plumeless seeds.  However, Callihan and
others [5] reported that buried plumed seeds remained viable for at
least 10 years whereas buried plumeless seeds remained viable only for 6
years.  Burial depths of 1, 2, and 6 inches (2.5, 5, and 15 cm) did not
affect longevity in this southeastern Washington study [5].


SITE CHARACTERISTICS : 
Yellow starthistle requires light on the soil surface for winter rosette
and taproot development, and it requires ample soil moisture in early to
mid-summer [34].  The current northern limit of yellow starthistle is 49
degrees north latitude [33].  In northern Washington, yellow starthistle
is restricted to warm microclimates such as steep, south-facing slopes
[34].  In south-central Washington, which is dry and hot, yellow
starthistle is restricted to deep soils that receive or store adequate
water [34].

Optimal growing conditions in southeastern Washington include deep silt
loam soils, south-facing slopes, and adequate moisture.  Yellow
starthistle cover was positively correlated (P<=0.001) with increasing
soil depth [34].  In California, yellow starthistle occurs below 4,260
feet (<1,300 m) elevation [14].  In Utah, it occurs from 3,000 to 6,200
feet (915-1,900 m) elevation [44].


SUCCESSIONAL STATUS : 
Yellow starthistle is intolerant of shade [33].  Rosettes are
susceptible to shading from overtopping vegetation in the fall and
spring.  Yellow starthistle successfully invaded established perennial
grass stands that were clipped, but did not invade unclipped areas [34].

Root growth during the winter enables yellow starthistle to outcompete
other species for moisture during spring and summer [34].  Yellow
starthistle utilizes moisture below the reach of most competing
vegetation [33].

Yellow starthistle colonizes disturbed sites such as roadsides,
overgrazed rangeland and pasture, orchards, and irrigation ditch banks
[39].  In the absence of major disturbance, yellow starthistle invaded
communities dominated by bluebunch wheatgrass (Pseudoroegneria spicata)
where site conditions were ideal for yellow starthistle growth [34].


SEASONAL DEVELOPMENT : 
Seedlings usually emerge in the fall, form rosettes, and begin growing a
taproot.  Root length after 10 days on a Washington site with cheatgrass
averaged 3.7 inches (9.3 cm) [21].  In a greenhouse study, root length
averaged 6.0 inches (15.2 cm) after 10 days [36].  Root growth continues
throughout the winter.  In early April in southeastern Washington,
taproots of fall germinated yellow starthistle rosettes averaged 11.8
inches (30 cm) in length; the rosettes were less than 2.4 inches (6 cm)
in diameter.  By early July, 10-inch (25 cm) tall plants had taproots
averaging 30 inches (66 cm) in length [34].  Yellow starthistle bolts in
late spring and flowers June through August [33].

Related categories for Species: Centaurea solstitialis | Yellow Starthistle