Overwintering Cuphea hyssopifolia/Mexican Heather
Last year, in the Spring, I put in some Mexican Heather (Cuphea hyssopifolia) as an accent around some hollies in my yard. I had heard that they are not hardy here near Atlanta (I'm in 7a or b depending upon whose hardiness map you look at) so last September I dug 3 of them up and potted them to overwinter them in the house. Seeing that they had a woody stem, I wondered if they would make a decent bonsai-esque plant. I pruned them back, and about 3 weeks later brought them indoors before frost. At first they were fine indoors, but then the leaves started drying out even though the soil was moist. Not a one made it through the winter. They were in a sunroom that has south and east exposure, near the east windows.
Any hints as to how to overwinter the new ones I bought this spring? Any thoughts on bonsaiing this?