since you know what the plant is ... then the botanists job is done .... so you might try the growers.. in the perennial forum ... and you might find someone who knows the cultivar name.. rather than the larger group name .. you never know.. where you will find the answer .. so dont just give up here ... ken

You can't divide one big root, so unless your Dianthus has rooted itself along the stems, you must take cuttings. After bloom, just cut some stems, strip off the leaves, and stick them in a pot (at least 3 inches of the stem). Keep in bright light but not sun and keep moist. When you see new growth, you'll know it has rooted. Dianthus are extremely easy to root this way.

If by Mexican petunias you mean Ruellia species, I have 2, a tall one and a short one which have both been in the ground for 2 years now. They seem to do fine. I am near Landrum, but far enough from the foothills that I don't get the thermal belt effects - it gets COLD! The dianthus may live and may not, they can be spotty, but it's not, seemingly, a matter of hardiness, just of persnicketiness of individual plants - I have seen them thrive in the most unfriendly places, fail in good places, and also do well - take a chance! You can always divide the surviving plants as you dig them up in the spring, if any do die over the winter. If you have a fairly sheltered spot to heel them into, then I would put say half there, and then put the rest where you can. For the dianthus, since they are evergreen, do keep an eye on water needs - the ruellia will die back to the ground, and won't need water. Don't forget to label them all as you stick them in the ground, both for colors and to aid in locating them - as I recall, the ruellias die totally back, and you think they are gone for good, until they begin to grow.

I'd advise no mulch - if it's perennial, especially one of the ones with bluish leaves, a mulch over the crown may well kill it. At best, you'll end up with long stringy stems with tufts on the end and no foliage closer to the crown. I grew various species dianthus and their cultivars for years, and never mulched them. When they were accidentally "mulched" by leaves from other plants, they always looked awful afterwards, and I usually had to take cuttings and reestablish the plants to have a decent looking mat of foliage. I don't actually know of a dianthus which is NOT winterhardy in Virginia zone 7, but there are a number which don't like our summers. Something that came in a pot as a gift, and survived the winter, sounds like one of the miniature carnations. They're a bit touchy here, but given good drainage and air circulation, are sufficiently hardy to make it.