Since I haven't grown corn since living in Ohio years ago...when is the best time to plant corn here in central MD (Howard County)?

I've seen anywhere from late March to May 1 - a BIG range. I know it would depend somewhat on the type of corn, the older "SU" varieties being more cold-soil tolerant...but can anyone with experience chime in?

I also forget the "ideal" soil temps to go for - I usually planted May 10 in Ohio, but farmers always planted earlier...and yet, it seemed my corn caught up in size by late June.

As a side note, has anyone tried laying down clear plastic to prewarm the soil a bit, and/or using a floating row cover to gain either a couple weeks on the front end in spring, or, to just warm them up a couple degrees to get better growth? I realize once it's more than a few inches high, the cover would have to come off...