For the period of record, 1881 through 2018, the average last frost in Fargo-Moorhead is May 13. The current 30-year average (1981 through 2010) is May 8. Last year, the final frost came on today's date: May 11.
Except for a few years when the final frost has been in late April or in June, May tends to be the time of year when we observe the last freezing temperature of the season.
The fact is a final frost just about any time in May would be considered “normal.” This makes it difficult to know when it is safe to plant a garden. One could wait until Independence Day to plant a garden but then one would not get any flowers or vegetables.
In our region of short summers, it is necessary to take on a certain amount of frost risk in order to have a garden. So plant in May when you feel comfortable. But watch the weather. You might need to cover!