I spent $40.00 yesterday on tender plants and the thought came to me, “When is it safe to put these plants in the ground and be fairly safe from frost?”
The answer in the city/county of St.Louis is around May 1st.
This link from the Univ. of Missouri contains all sorts of data about the probability of frost in all the different Missouri counties.
https://ipm.missouri.edu/FrostFreezeGuide/index.cfm
In practical terms, I usually look at the ten day forecast and if if looks safe, I will plant ten days early.
This is obviously for tender plants as some plants will take the cold without much damage.
The video below explains the data.
Beautify Crestwood 