Scotch Thistle (Cotton Thistle)
Family Asteraceae
Onopardum acanthium
Designation/Status
Arizona Noxious Weed List -
Prohibited
Federal Noxious Weed List – Not Listed
Origins
Scotch Thistle is native to
regions of Europe and Eastern Asia.
Plant Characteristics
Life Cycle: An aggressive biennial.
Plant lifecycle is not bound by strict photoperiod (day length) or
temperature requirements. Seed production per flowering head
ranges from 110 to 140. Scotch Thistle seeds contain a water
soluble germination inhibitor that allows the seed to germinate
only when buried in the soil or under ground litter. Seeds are
spread by water, wind, wildlife, and human activity, with most
seeds falling nearby the parent plant.
Visual
Appearance: Scotch Thistle ranges in height from 2-12feet at
maturity. Stems have broad, spiny wings formed by leaf bases.
Rosette leaves are large (up to 2 feet long and 1 foot wide),
spiny, and covered with a dense mat of hairs that give the plant a
gray color. Stem leaves are hairy, alternate, and coarsely lobed
violet to reddish flowers look like a shaving brush up to 2 inches
in diameter. Spiny bracts surround the flower head. Fruits are
about 3/16 inch long, tipped with slender bristles.
Habitat
Found in most western states and occupies sites
characterized by high soil moisture, especially in dry climates,
such as waste areas and roadsides.
Control Measures
Mechanical and Cultural: Tillage, hoeing, or even hand pulling should be successful. Scotch Thistle does not reproduce by root, so any mechanical or
physical method that severs the root below the soil surface will
kill it.
Biological: Seed Head Weevil (Rhinocyllus conicus)
and Mealy Bug (Trichosirocalus horridus) have been used.
Chemical: Clopyralid, Dicamba, MCPA, Picloram, 2,4-D,
Metsulfuron, and Chlorsulfuron will control Scotch Thistle. Most
of these chemicals are most effective on Scotch Thistle when
applied to the plant's rosettes in the spring or fall. It has
been found that 0.3 oz of Metsulfuron per acre can eliminate
viable seed production.
Other Points of Interest
On one historic occasion,
Scotch thistle saved the Scottish from being attacked by the
Norse. Introduced to the Eastern United States in the late 1800s
