Hi Sparky
I would say mid May for hardy stuff and end May first week in June for half hardy.
Although the past couple of years have been somewhat earlier.
Years back I considered the same problem and tried to do something about it.
If you want to see my results go to my website (click on world disc under my logo) and then click on the
Zones button at the top of the index page.
Your opinion /s would be appreciated ( good or bad)