Many landowners and gardeners consider their battles with thistles some of the most challenging. Thistles types, such as milk thistle, grow virulently and invasively, making their ugly presence known. Thistles can be particularly obnoxious when they grow in pastures where livestock must feed. Because thistles usually grow among turf grass that you do not want to eradicate, use a selective herbicide to kill milk thistle. A selective herbicide will kill the broadleaf milk thistle and leave the grass unharmed. Choose a selective herbicide containing 2,4-D (a systemic herbicide).
Mix the herbicide with water (2 tsp. herbicide to 2 quarts of cool water or 8 oz. herbicide to 6 gallons of cool water) in the bucket.
Pour the herbicide mixture into the spray bottle, and seal the bottle tightly.
Apply the herbicide mixture directly onto the undesired milk thistle, saturating the plant generously. Spray the herbicide onto each thistle directly.
Monitor the condition of the milk thistles after you apply the herbicide. If you do not see the milk thistles visibly dying within one week, reapply the herbicide to stubborn milk thistle plants.