The
key to controlling musk and Scotch thistle is to prevent seed production
since that is the only way these plants reproduce. Each plant is capable
of producing up to 20,000 seeds.
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Scotch
Thistle after the herbicide "clopyralid" was applied
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Chemical: Herbicides are highly effective in controlling musk and Scotch
thistle. Applications should be made in late spring/early summer
and again in the fall.
Herbicides
that are recommended for use on musk and Scotch thistle, either
independently or in combination, are: picloram(Tordon 22K), clopyralid(Transline,
Stinger), clopyralid plus 2,4-D(Curtail), chlorsulfuron(Telar),
glyphosate(Roundup/Rodeo), dicamba(Banvel), and 2,4-D. Always
read and follow label instructions when making any application. Consult your local pesticide dealer, County Weed Supervisor, or
Extension Agent for specific recommendations.
Mechanical: Continual mowing or chopping is most effective when musk and Scotch
thistle plants are just beginning to flower. Unfortunately, these
weeds do not bloom as individual plants at the same time; thus,
epeat treatments are necessary. As biennials, these weeds can also
be effectively controlled by grubbing, or digging below the crown
level. Doing so at the rosette stage is the easiest. Severing the
tap root below the soil surface with a shovel can also effectively
kill the plant. This should be done in areas where there are only
a few plants though as this option is quite labor intensive and
time consuming.
Bio-Controls: One insect control agent for musk thistle is the seed head weevil
(Rhinocyllus conicus), which is well distributed in the county and
spreads on its own. The larval stage of this weevil devours most of
the seeds in a head that it infests. This insect alone does not affect
total control of musk thistle infestations. The crown weevil (Trichosirocalus
horridus) attacks the crown and reduces the flowering potential of
musk thistle. When used together, the crown and seedhead weevils provide
fair to good control. No insect species effectively controls Scotch
thistle. Good grazing practices stimulate grass growth which
results in healthy pastures and rangeland. Overgrazed sites result
in bare spots which are prime targets for noxious weed infestations.
