ONE thistle plant can produce over 10,000 seeds which are carried to new birthplaces in a light breeze- so it's easy to guess that this noxious weed can take over your fields in very short order. And for the record, yes, Missouri law 263.190 requires landowners to control noxious weeds (including thistle). Read more about the law here.
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Apply herbicides while
musk thistles are in the
rosette stage or pre-bud
stage
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Missouri landowners spend between $750,000 and $1 million (ouch) annually to control musk thistles with herbicides. Chemical control can be effective if properly timed. Apply 2,4-D or other registered herbicides to the musk thistle rosettes in either fall or spring. Applications made after the plant bolts are too late for good control. Although treated plants may appear to be injured, with wilted leaves and twisted stems, the plants will nevertheless flower and produce viable seed. For optimum control, apply herbicides while musk thistles are in the rosette stage or pre-bud stage.
Infestations of musk thistle often occur in areas difficult to reach and may escape notice until after flowering. Farmers are usually busy in the spring and may postpone chemical application until they are caught up with field work. Since musk thistle seeds are long-lived in the soil, it is important to prevent plants from producing seed until new rosettes are no longer produced. The effective eradication of this weed pest from an infested pasture requires dedication and perseverance because of the longevity and viability of seeds (five to seven years) in the soil.
Is mowing it off effective? The best time to mow musk thistle is within two days after the terminal flower head blooms. This is the most effective stage at which to inhibit seed production and prevent rebolting. The problem with mowing musk thistle is that not all the plants in an infestation produce terminal flower heads at the same time. Additionally, mowing on poor soil may actually inhibit the competitive effect of other plants and favor survival of musk thistle seedlings.
Is there any any other way to control it? Specific natural enemies of musk thistle can aid in regulating the spread of this weed. Such natural enemies include the musk thistle rosette weevil and the musk thistle flower head weevil (Figures 3 and 4) and a musk thistle rust fungus. The larvae of the rosette weevil feed on the underside of the leaves of the rosette and on the leaves and stems of bolted plants; larvae of the flower head weevil feed in the base of the developing flower of the thistle. The two weevils are compatible with each other as biological control agents because they do not compete with each other. Natives of Europe, the weevils were studied extensively to ensure that they would not damage economic plants in this country. Entomologists with the USDA Agricultural Research Service Biological Control of Insects Research Laboratory in Columbia, Missouri, released flower head weevils in 1975 near Marshfield in Webster County; rosette weevils were released in 1979-80. In areas of the state where the weevils have been present for the longest period of time, a 70-90 percent reduction in thistle population has occurred. Populations of the musk thistle weevils have increased throughout the state through natural dispersion and through collection and recolonization.
Want to learn more ways to control thistle? The University of Missouri Extension web page offers some additional suggestions here.
