Provenance may be part of the difficulty those on the fringes of hardiness have experienced before. If one digs a shrub (of any kind, not just Gardenia) from way down south and puts it in the ground at the north edge of hardiness, it's much less likely to survive than a plant propagated from material already surviving nearby. Try to find those produced locally, not trucked up from the south, when considering anything of marginal hardiness. Then I'm a firm believer in a good cover (pile) of leaves if deciduous, just a covering at the base if evergreen. Will help moderate soil temp, moisture, increase drainage after it's been there for a while. A couple large, dark-colored paver/walkway bricks on the south side by the base could help also, to warm the ground when the sun hits them.
Wish I knew what kind of Gardenia is in the yard here to give credit where credit is due. They are in almost every yard in the neighborhood, heck, in the whole area (where drainage is generally excellent, virtually instant.) Blooms heavily in early spring, then sporadically at any time, like yesterday, this one bloom showed up. The more compost, trimmed grass, general organic matter I put around the base of it, the more and more happy it looks, even growing some bigger leaves. Full, total sun. I don't prune it much, except when a part is sticking out in the path. A few years ago, I took all of the smaller branches off a bigger limb that looked sick, intending to ask DH to saw the whole thing off at the base soon. Forgot about it, and it's covered itself with great new growth by now. The best thing in this yard besides the trees, that was already here before me.
