It's that time of year again. Landscapers and home gardeners alike are wondering when that final freeze of the winter/spring season will occur. Why? So sprouting flowers and plants can grow without worry and the growing season can officially begin. As seen on the map below, the Midwest typically dips down to the freezing mark for the last time in late April. Indianapolis, IN is usually the earliest to observe its final freeze, while Rockford, IL is still susceptible to damaging cold until nearly the end of the month. It's important to keep in mind that even if the air temperature is above freezing, clear and calm spring nights could still pose a threat to vulnerable vegetation. If the ground radiates enough energy, temperatures near the surface could fall to freezing, if not a few degrees below. That's why frost develops on plants some mornings, even if overnight lows only fell into the mid 30s.
Of course, the dates above are just an average based on climatology. To gain a better idea of the date range that the final freeze can be observed, see the chart below. It lists the all-time latest final freeze dates at each of these major Midwest cities.