Young sweet corn plants are highly susceptible to damage from sub-freezing temperatures, so do not plant seeds until the last spring frost is behind you. Wait until the soil has warmed to at least 55 degrees F.
The exact time to plant sweet corn depends on what part of Pennsylvania you live in. Pennsylvania State University recommends planting sweet corn in early to mid-April in the southern part of the state, and in mid- to late-May in the northern part. (
Continue to plant corn as late as July 1 for a crop that can be harvested successively over the season. Corn responds to heat, so seeds planted later will mature faster.