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It depends where you live andIt depends where you live and your frost dates. We don't plant tomatoes in September in most of North America because we'll have frost before the tomatoes mature. However, in areas in the South that don't have frost, planting can happen in September at temperatures that tomatoes like. If you have October frosts, you'd want to get your transplants in the ground by August 1.  As a rule of thumb: figure out your frost date (see the frost chart on this Web site) and figure out your "days to maturity" for your tomato variety and back out the dates.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?