Bitterwort, the mature plants in fall can survive a light but killing frost.
I would leave my petunia starts out if temps fell to mid-thirties but wouldn't
take a chance lower than that. By the time mine are potted up and a bit more
mature, they take up so much room that I'm forced to leave them out if we
get a late frost. I'm fortunate in that we have a south-facing garage door situation with a blacktop driveway. Our house is built into a slope (we live
in the rolling hills of Chester County) so there's some protection on the sides.
Mine are still in a 72 cell flat and I'm a little overdue to potting them up into
4" pots-something I'll take care of starting tomorrow. From then on, if we get
a light frost scare, they'll be moved to the micro-climate in front of the garage doors. If we should get an usually cold prediction, like 25 degrees, I'd cover
with a blanket. (I'd also try to squeeze as many into the garage as possible!)
Tonight will be the coldest (no frost here, though) night in the weekly forecast.
That'll put us late into April and we're likely to have only a very light frost.
So far, I've never lost young spring petunia plants even though we've had
a couple of very light frosts and have had to cover tomato plants (mid- May,
it's unusually but it can happen that late here).
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