4oz. Per gallon 24db can successful kill till mid Aug. Around 20th and your to late. Don't get discouraged if it appears at first you haven't killed it. Key signs are to look for leaf cupping. Ideally you want to start next year around 2nd week of may while its small. Seed head forming is what you don't want. Mint seed is tiny and carried by the wind and on muddy wet feet. I'd kill all I could on your property not just ya plots for better control. Back pack sprayer some ice water and foot work will do more than ya think.