When should seedlings be started in Pennsylvania?

You can use your last frost date for your area, look up your planting zone(PA ranges from 5 in the north to 6 in the south I believe) and that will tell you when seedlings should be safe to plant outside. Then look at the back of your seed packet and it will tell you how long it takes them to grow.

For example say your a zone 6 and last frost date is May 15 and your seeds take 6 weeks. You would count 6 weeks backward and start them April 1st so they could be planted out May 15.