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Yellow Star Thistle
Sunflower Family
Yellow star thistle (Centaurea solstitialis) is not currently
established in Sequoia and Kings Canyon National Parks. However, small
populations have been found and eradicated. An estimated
1,400,000 hectares (570,000 acres)
were infested with yellow star thistle in the western states by the
mid 1980's and this highly aggressive plant is a real threat to the
foothill grasslands in the parks.
Yellow star thistle is native to southern Europe. It is considered
a noxious weed and it tends to grow in disturbed, sunny sites at low
to middle elevations. Yellow star thistle's distribution in California
increases rapidly each year. What is of particular concern to park
managers is its ability to successfully reproduce and colonize areas
at higher elevations than many of the other noxious weed species in
California.
Identification
Yellow star thistle is an annual plant that can reach up to one meter
in height. The flowering heads have long tan spines, some of which
exceed 10 millimeters (0.4 inches) in length. The vegetative part of the
plant is spineless.
The stem has obvious leafy wings. In the foothills of Sequoia National
Park yellow star thistle does not flower until late June or July,
later than most other foothill annuals. At higher elevations yellow
star thistle may not flower until September or October.
Look-Alikes
Yellow star thistle is distinguishable from most other thistles in Sequoia
and Kings Canyon by its lack of spines along the leaf margins. Tocolote
(Centaurea melitensis), another introduced species, is the
plant that is most easily confused with yellow star thistle. Tocolote
also lacks spines along its leaf margins, but it is distinguishable from
star thistle by the length
of the spines on its flowering heads. On tocolote the spines are
generally smaller than 10 millimeters (0.4 inches) while on yellow
star thistle the spines
are generally larger than 10 millimeters (0.4 inches).
Natural History
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Long tan spines set yellow star thistle
( Centaurea
solstitialis)
apart from
similar thistles, such as tocolote
( Centaurea
melitensis). Yellow star thistle photo by Jo-Ann
Ordano, © California Academy of Sciences; inset
photo modified from the collection of Michael
Charters.
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Yellow star thistle reproduces entirely by seed. Each plant can produce
up to 10,000 seeds per year. Seeds can remain viable in the soil for
longer than five years. Each flowering head produces seeds with two
distinct forms. One form has fluffy bristles, which allow it to be
wind dispersed a short distance from the parent colony. The other
form has no bristles, and drops directly from the plant, thereby maintaining
the parent population.
Management
About a dozen individuals have been encountered in Sequoia National
Park along the Generals Highway between Ash Mountain and Potwisha
campground in 1998 and 1999. There are numerous persistent populations
in lands adjacent to Sequoia and Kings Canyon National Parks
and they are potential seed sources for introduction into the parks.
Yellow
star thistle is abundant along Highway 198 west of the town of Lemon
Cove. It is very common in Eshom Valley, not far from the North Fork
of the Kaweah River.
It has advanced to mid-elevations along Highway 180, about 10
miles west of the Grant Grove park boundary.
Early detection surveys are conducted in
susceptible habitat on a regular basis in order to provide
immediate detection and control of
this highly threatening species.
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