This can be a confusing topic. You can ask a lot of people and get a lot of different answers. There are many reason this is a difficult topic for many reasons: what is a frost? Microclimates? Plant Hardiness? ETC.
With a little research, you will come up with two different dates. One with a probability of 50% of a frost after the given date and one with a 10% chance. Which one do you use?
The answer depends on your local area (microclimate) and what you are planting.
Here are a couple of rules I follow:
1)Never think of planting anything thing that says “after last frost” before the date given in the 50% charts. The period of time is too great to predict accurate temperature ranges.
2) Start watching the weather after the 50% number to get an idea of weather trends.
3) If you start your own transplants, stager the dates a little to give yourself some extra insurance.
4) If the forecast looks good 7 days leading up to the 10% number, get ready and plant. You will be talking about the head start you got all season.
5) If you do get a freeze after planting, do not feel bad. You are back to the same point as everyone else.