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Yellow Starthistle (St.
Barnaby's thistle, cotton-tip thistle)
Asteraceae (=Compositae), the aster
family
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Background
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 Yellow starthistle originated in the Mediterranean area
and Asia. It spreads exclusively by seed, which may lie dormant for as long
as 10 years. It causes "chewing disease" and death in horses. Yellow
starthistle will grow wherever downy brome (cheat-grass) grows.
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Description
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Yellow starthistle is a winter
annual maturing from 2 to
72 inches tall. A rosette of deeply lobed leaves up to 8 inches long forms
after seed germination in the fall. Stem leaves up to 4 inches long develop
in early spring, their blades forming fringelike extensions on the stem.
Yellow flower heads develop at the tips of branched stems from late spring
until fall. Flower head bracts bear stiff, sharp thorns 3/4 inch long. Seeds
are tan with white and brown mottling, 1/8 inch long; both
plumed and unplumed seeds develop in each flower head. Plumed seeds are not highly
windborne; unplumed seeds not at all.
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Distribution
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Y ellow starthistle is widely scattered throughout the
U.S., but is a severe problem only in the West. |
Control
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Three seed head weevils and 2 seed head flies have been good to excellent
biological control agents in Idaho.
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© 1999 University of Idaho:
Text and photographs for these pages from Idaho's Noxious Weeds, by
Robert H. Callihan and Timothy W. Miller (revised by Don W. Morishita
and Larry W. Lass).
Please contact: Ag Publishing, University of Idaho, Moscow,
Idaho 83844-2240; 208 882-7982 visit the Resources for Idaho website at
http://info.ag.uidaho.edu, for more information about this or other
publications.
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