I'm in CA and have a couple planted in the ground. They take light frosts for short periods of time before they start showing damage. They totally fried last year in our "big freeze" when night temps. got down to 25 for a week.
From my experience (in the ground), I would say around 30F is the limit without protection.

wanda

Iochromas are always the first things to show any freeze damage in my own garden, and I seldom get frost. I would say if you are already getting high 30'sF as your lows, you should bring them in now. In my garden, they start showing damage right at 32F, but mostly if it is a sudden drop in temperature and lasts for more than a few hours.

I don't have experience with trying to overwinter them indoors, but you can also easily start them from new cuttings, and I would think they could be whacked back to the ground and kept semidormant for the winter in an above freezing garage. They grow so fast in height, I wouldn't be too concerned about really cutting them back.

I've had my iochromas for about 5 years. I bring them in before the first frost and grow them under HID lights. They do well and most continue to bloom throughout the winter. They don't go dormant like brugs. I have to cut several of them way back to get them inside and it doesn't seem to bother them in the least. I've had rotten luck with cuttings. I don't get a high percentage to root and those that do will grow just fine for a few months then they'll just die for unknown reasons. The same thing has happened with cuttings to a couple others that I know. I do have very good results starting them from seed. I have had to deal with mealy bugs and scale while wintering them over.
Karyn

Many thanks, everybody, for your advice! I'm probably going to have to do something with the iochromas today, since the weather's supposed to be quite cold Sunday night. I'm beginning to realize that I probably should have kept them in large pots, rather than planting them directly into the ground.

I use flourescent grow-lights which, although they do a fairly good job of keeping plants alive over the winter, do generally require that those plants not be too tall. So I'm probably going to have to cut the iochromas back drastically. Snipping off those buds, however, is going to be painful! Thanks again.

Audrey we are suppoded to get a dramatic drop in temps Sunday night also and I'm getting ready to move everything this weekend. I know what you mean about wishing that plants weren't inground. I have several that need to be dug and it's been raining for the past 3 days. This is going to be such a mess! We desperately needed the rain and haven't had any significant precip since Apr but I wish it had come at a different time! I'd try to root some of the branches that you trim. Maybe you'll have good results.
Karyn

I find they root readily in perlite with rooting hormone and temps on the cool side, (60'sF). It also helps to cut off all flower buds and cut large leafs in half to reduce water loss. Maybe tent them to keep humidity up, and they should root within 3 to 4 weeks time.