I'm pretty sure that frost occurs a few degrees above freezing. Thirty seven sticks in my brain, but don't take it to the bank. Of the plants you name, I have found geraniums (pelargoniums) to be somewhat more frost tolerant than the others. They actually bloomed all winter in New Orleans, and yes, we had light frost there occasionally. If your gernaiums are cranesbills, they should be winter hardy in the ground. Sweet potatoes will take a fair amount of cool weather too. I have actually had them return in my yard after a mild winter. That won't happen for you, of course, but we always do get freezes, sometimes down into the mid twenties. Why not just take some cuttings from the SPs and overwinter those? They'll be much less cumbersome. The fountain grass will take a bit of frost, but certainly not a freeze. The impatiens and hibiscus will tolerate little to none. Protect them first. (Some hibiscus are perennials. You'd need to check on the variety you have, and see if they're hardy in your zone.) I have no experience with Angel wings, but it seems that I remember they are perennials, so again, you'd want to check on their hardiness zone. It's sad shutting everything down for the year, isn't it? I have been sticking cuttings of things to overwinter in the basement all day and I'm already wishing for spring. :)

Christie, Your winter sown things should be in their covered containers, so they are protected. Planted out or with out covers on the containers are called unprotected. You do not need to worry about un sprouted seeds. If its a brief frost/light freeze I wouldn't worry so long as the plants aren't touching the plastic cover. If its sustained cold it is iffy on those tender things. If you've already sown them, and they've sprouted you can take your chances on leaving them out and see what happens. If you lose them, there is plenty of time to re sown. If they've sprouted and you are expecting really cold temps longer than over night you can always move them indoors to a garage or really cool room. But only for a short while, other wise they'll get all leggy and weak, then you'll need to harden them off when temps warm back up. You could try to move them to a covered porch and toss a blanket over them for a little while but only while its really cold. (below freezing) This is the reason why we only sow hardy plants early and wait until later to sow the tender stuff. The above is not set in stone, and people in different areas may have other experiences. You'd be surprised how these little sprouts tough out short periods of really cold temps in their little greenhouses. But really tender plants, just can't cope.

I also do hardy annuals early. They tend to be too tough to be bothered by frost. If they sprout early, subsequent frost won't phase them at all in their jugs. Even after planting out into beds, I don't think I've ever had poppies, cornflowers, or larkspur phased by even a hard freeze. Karen

I have been researching myself, but for vegetables, tomatoes, pepper and squash seedlings that I am growing under light. The farmer almanac indicates that this year last frost for me (7a) is April 12th. That means, I can set those plants out right about now. I think I will be conservative to wait another week and set them out next weekend instead.