Right now is the best time. The absolute best kill is a 50/50 concentrate mix of Round-up and Imazapyr, but Round-up alone will work pretty good. Yes, mix the contrate 50/50 with water, and one hack and one squirt for every 2-3 inches of tree diameter. Use a Windex squirt bottle for squirting. Each squirt is about 1 mm and 2 mm per hack is about right (two squirts per hack).