Every Michigan gardener needs to know two dates: the last frost in spring and the first frost in fall. Everything else — when to start seeds, when to transplant, when to harvest, when to panic — flows from those two numbers.
Zone 6a Frost Dates
In Zone 6a, which covers much of central and southern Michigan including Bay City, the average last spring frost falls between May 10 and May 20. The average first fall frost lands between October 5 and October 15. That gives us roughly 145 to 160 frost-free days — enough for tomatoes, peppers, and most warm-season crops, but not a day to waste.
What “Average” Actually Means
An average frost date is not a guarantee. It means that in half of all years, the last frost comes before that date, and in half, it comes after. I have seen frost as late as May 28th in Bay City and as early as September 25th. The averages are a starting point. The weather forecast is the final word.
How I Use Frost Dates
I count backward from the last frost date to determine seed starting times. Tomatoes get started 6 to 8 weeks before last frost — so mid-March. Peppers get 8 to 10 weeks — so late February. Onions get 10 to 12 weeks — so early February. For the full schedule, see When to Start Seeds Indoors in Michigan.
In fall, I count forward from the first frost date to decide when to plant cool-weather crops. Fall lettuce goes in 6 weeks before first frost — so mid-August. Fall radishes go in 4 weeks before — so early September. Kale and spinach can handle light frost and keep going even later.
Microclimates Matter
Frost dates vary even within a single zip code. A garden near water — like mine on the Great Lakes — benefits from the lake’s moderating effect. A garden in a low-lying valley frosts earlier. A garden next to a south-facing brick wall frosts later. Pay attention to your own garden, not just the zone map.
For the complete planting calendar, see the Michigan Zone 6a Garden Planner.
— Chris Izworski, Freighter View Farms, Bay City, Michigan
Chris Izworski is a Michigan gardener, writer, and AI technologist based on Saginaw Bay. He writes at Freighter View Farms about Zone 6a gardening, seed saving, and practical AI in public safety.
Freighter View Farms | Chris Izworski 
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