Thistles are a common weed found in pastures throughout South Florida that can have an economic impact on your cattle operation through loss of grazing and reduced forage production which can lead to reduced weaning weights. Thistles are biennials, which means plants grow from seed in the first year and produce seeds the second year. During the first year of growth thistles will grow as a rosette. Rosettes are taproots with a cluster of leaves on or near the soil surface. In the second year of growth a stalk elongates from the rosette. This process is commonly referred to as bolting. After bolting, a thistle will flower, produce seeds, and die.
Flower formation occurs from April to August; therefore, management practices should be implemented prior to flowering for effective thistle control. If left uncontrolled, the chances of higher future thistle population in pastures increases as one plant can produce 4,000 seeds. Thick thistle strands can also lead to loss of forage production and reduced grazing if control measures are not implemented.
The variation of growth of individual thistle plants presents challenges as life stages of thistles influence control effectiveness and options.
Mechanical Control
Rosettes can be removed by hand, cutting the plant below the surface of the soil. This method is time consuming and labor intensive; therefore, it is not practical on large infestations.
Mowing thistles can be an effective strategy; however, timing is critical to control of current and future infestations. Plants mowed in the rosette stage will regrow, therefore, the best timing for mowing thistles is after bolting but prior to flowering. Increased fuel costs should be factored into this decision as multiple mowing treatments may be required for effective control.
Chemical Control
Herbicides are an economical and flexible option for thistle control in pastures. As with mechanical control methods, timing of chemical control of thistles is important. Thistles in the rosette stage are easier to control as the plants are highly sensitive to herbicides. Waiting to apply herbicides after bolting can impact effectiveness especially if thistles have begun flower formation. A table showing control of thistle at three growth stages with common pasture herbicides can be found in “Thistle Control in Pastures” (AG25300.pdf (ufl.edu)).
Thistles are present in the rosette form long before flowers emerge. Scouting of pastures during the winter months while thistles are in the rosette stage provides the best opportunity for thistle control before bolting occurs. Early detection can reduce management costs associated with thistle control as herbicides are more effective when used at rosette stage. Waiting to implement control measures on thistles limits viable options, reduces effectiveness of those options and increases control costs. If control of thistles was not addressed while plants were in the rosette stage this year, chemical or mechanical control should be done prior to flowering to reduce infestation next year.
For more information on control options for pasture weeds contact your local UF/IFAS Extension Agent.
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