Our low temperature on the morning of Wednesday, May 28th was 0.014°C at 6:00 am, thus we just managed to stay above freezing.
However, remember that the temperature at the UW weather station is measured at 2 m above the ground (this is the worldwide standard). So your plants, that are most likely closer to the ground, may have some frost damage.
If you did inspect you annuals this morning and can report on the frost damage please leave a comment below.
At the Environment Canada weather station at the Waterloo-Wellington airport they reported a temperature of -1°C at 6:00 am meaning that there is going to be a 3 week difference in our calculations of the last frost day this year.