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Got Thistles?

Rosette Stage

Background
Thistles can quickly get out of hand if left alone.  Each Thistle is capable of producing 4,000 seeds during its life cycle.  In Florida, there are nine species of Thistle, but control recommendations are the same for each species.  Being proactive in scouting for this spiny plant from late fall through the spring is necessary to prevent infestations in your pastures.  

Life Cycle
Three stages of growth are important to understand for Thistle management.  First, is the rosette stage which takes place mostly during the winter months.  At this stage, Thistles are circular and growing flat or close to the ground.  Thistles are most sensitive to herbicides during the rosette stage.  Second, is the bolting stage which takes place from late January – July.  The stalk of the Thistle starts to grow upright from the rosette during this stage and herbicide effectiveness starts to decline.  Third, is the flowering stage which takes place form April – August.  During this stage, Thistles reach maturity and few herbicide options are recommended.

Bolting Stage

 

Control
Timing is crucial for Thistle control.  When Thistles are caught early, there are several options for herbicide treatments that provide great control.  As the plants mature many herbicides become less effective.  Mowing Thistles in the late bolting stage, just before the flowering stage, can be effective, but multiple mowing treatments are often needed.  Mowing during the rosette stage is not effective because the plants will regrow.  If Thistles are few in numbers then hand removal or spot spraying may be justifiable.  Below is a chart displaying the effectiveness of several herbicides and the different stages of Thistle growth.

Control of thistle at three growth stages with common pasture herbicides:

Herbicide Rate $/Aa Thistle Growth Stage
Rosetteb Boltingc Flowering
2,4-D 2 qt/A 6 90 85 40
Metsulfurond 0.3 oz/A 4 90 40 40
Weedmaster 2 pt/A 5 95 90 55
Remedy Ultra 2 pt/A 15 95 90 75
Pasturegard HL 1.5 pt/A 18 95 90 70
GrazonNext HL 1.0 pt/A 8 99 95 90
aApproximate herbicide costs.
bThe rosette stage is when the thistle forms a low-growing ring of leaves.
cThe bolting stage is when the thistle forms a stalk and prepares to flower.
dFor use in bermudagrass only.

For more information on Thistles: http://edis.ifas.ufl.edu/ag253