Garlic is a problem in peppermint fields because it is difficult to control. The key to controlling this weed is to not plant a peppermint field in the fall after a garlic crop is harvested. Wait until spring and cultivate the field to destroy any emerged garlic plants; then use greenhouse transplants to establish the mint field, or wait until fall and plant the field with dug roots. If garlic is present in a mint field, it may be hand-rogued or killed with a spot treatment of glyphosate (Roundup Ultra).
Perennial Clover and Alfalfa
Perennial clover and alfalfa often appear in low-lying areas of the field where drainage is poor. Control this weed in the preceding crop, level the field, and provide good drainage. Clopyralid provides control of clovers and alfalfa.
Field bindweed is a perennial weed that aggressively competes with mint and can shorten the life of a stand. Begin managing this weed at least 1 year before planting by applying multiple herbicide treatments. Early fall treatment works best because at this time the bindweed is moving energy reserves into the roots for next year's growth. Long-term control also involves rotation with crops such as strawberry transplants.
Mayweed Chamomile and Pigweed
Mayweed chamomile and pigweed both annual weeds, are very destructive to the quality of peppermint oil but are easily controlled with currently registered herbicides. In young mint fields, bentazon controls pigweed and clopyralid controls mayweed chamomile. In established mint, a fall application of diuron in combination with oxyfluorfen controls both fall-germinating chamomile and spring-germinating pigweed.
Common salsify is very troublesome in peppermint stands. It reduces oil quality and germinates in fall and winter. Control this weed at early growth stages with low rates of clopyralid, because high rates of clopyralid cause injury to mint. Hand-rogue all remaining plants to prevent seed production and accidental harvest with the mint crop. The preemergent herbicides flumioxazin or oxyfluorfen applied in fall give partial control.