Questions for Paul and Midwesterners

[gardengalrn]

I have been trying to find a last frost date for my area, as I am itching to get some things started. The 3 choices are Blair and North Platte, NE or Topeka KS. Topeka has a last frost date of April 21. Blair, NE is April 27 and North Platte is May 11. These are roughly equal distances away, as I'm in Republic Co in KS. I figured I would shoot for the first week in May, of course with all the usual disclaimers about unpredictable weather. Does this sound about right? If so, I should be able to get started on peppers soon then tomatoes a few weeks later.
When do you normally plant things like onions, potatoes and peas? I think Dixondale is shipping my onions in the middle of next month and I wondered if that is too soon (this was according to the zip code shipping guide). Kentucky's weather was a lot milder and even then I sometimes jumped the gun on those spring crops.