Well, I'm in zone 9b so my requirements are a little different. I think what Peggy may be referring to is the latest news that got blown out of proportion about Tropical Milkweed. (A. curassavica). There has been much to do about it lately thanks to a study that came out about the plant and its connection to the OE spore that infects and kills Monarchs.
OE is a naturally occurring spore that can and does make Monarchs sick and will kill them. It's most obvious when you have a chrysalis that turns black and never emerges or when you have a Monarch that gets "stuck" while emerging from the chrysalis. But some Monarchs do live and they spread it when they lay their eggs. The spores also exist on the plants so the caterpillars and eggs can become infected just by being there.
Now, the problem is that Tropical Milkweed is one of the most widely available milkweeds. It's generally the only one I see for sale in Home Depot and Lowe's. It also grows pretty much year round unless we get a freeze. Even then, I've fed the caterpillars wilted, damaged leaves and they will accept them in captivity.
Well, not long ago a study came out that said Tropical Milkweed was evil because it doesn't go dormant like the native milkweeds. This, the study said, kept Monarchs from migrating like they were supposed to. It encouraged them to stop and breed when they normally would just keep flying on to Mexico. And because the plant never goes dormant, the OE spores were on the plant all the time! They never went away! And so the study showed that Monarchs on Tropical Milkweed were infested at a much higher rate than those that had fed on other milkweeds.
The press kind of blew this one out of proportion and scared a lot of gardeners who thought they were doing good. Now, personally I did tear up all my Tropical Milkweed last year because I am trying to go as native as I can. That's a personal choice. I've had A. tuberosa for years and yes, the Monarchs nearly always picked the Tropical Milkweed instead. However, this spring I have not been able to keep them off the A. tuberosa. I think, if given the choice, they will go for the natives. I am also growing Giant Milkweed (Calotropis gigantea) because the Monarchs rarely lay eggs on it and it has made for a good back-up food source. They've been laying on it this year and I'll probably get rid of it once I have enough native milkweeds growing.
The only people who really need to be concerned are people who live where Tropical Milkweed doesn't die back. If they still want to grow it, Monarch Watch is suggesting they just keep it cut back in the winter to discourage any egg-laying. They feel planting any milkweed is better than planting none. Of course, I think planting natives are best if you can find them. It's always easier to plant natives in your garden because they require less care. I also think butterflies are more apt to choose the plants they evolved alongside for millions of years. Kent gave some really good suggestions for specific species above. One source I have bought seeds from is Everwilde Farms: www.everwilde.com Each plant has a little map that shows you where it's native so you don't buy the wrong one. They do carry some of the species Kent mentioned but they're out of stock on some right now. Sometimes I just Google the Latin name and see what vendors come up. Finding native plants is getting somewhat easier than it used to be, but it can still be a challenge.
So that was a really long post! I'll just sum up by saying that I have fed Monarchs native and non-native species and haven't noticed any difference in survival rates. Going native is generally best. And I probably should have said this first, but