Oh, my - that is SO PRETTY!

So many issues with this post, it is hard to know where to start!! First, soil pH skewing one way or the other a point or two is not going to significantly affect rate of growth. Could have other implications but not reduced or zero growth. More likely to be cultural issues like root congestion, lack of required nutrients or inadequate watering. Second, directions for lime use to increase soil pH is given for square footage, not volume. Based on Al's 5-1-1 mix recipes, pH is neutralized with approx. 4 Tbsp lime per 5 gallons of mix. Since you don't want to neutralize but only increase slightly, I'd suggest you start with less and test. (would be way easier to use a acid planting mix for this purpose. It's what I use for all my containerized ericaceous plants). Finally, water can and does work very well to alter soil pH. Gardeners in more arid climates with a hard or alkaline water supply typically will acidify their irrigation water with vinegar or sulfuric acid.

There are certainly some varieties with increased cold tolerance - Frost Proof, Summer Snow, Kleim's Hardy. But that is not the only factor involved - you must have appropriately acidic soil, good drainage, protection from winter winds and shade from afternoon sun. And gardenias are not the slightest bit drought tolerant, which could surely be an issue with our hotter and much drier summers. Would not be my first choice for a hedge in full sun. Just a bit too fussy to keep happy.

Ok so little update My "Celestial Star" (to the right) and "Double Mint" (to the left) are blooming around the same time- I found some stamens and pollen on "Double Mint" - "Celestial Star" seems to only have stigmas so far. I took out some stamens out of Double Mint bloom to brush it on Celestial Star stigma. I also used pollen from my unknown pot gardenia ( once bloomer, seems to be hardy to 15F, medium blooms) and from my Gardenia "Magnifica" (probably repeat blooming much less hardy, around as hardy as "Aimee"). So far probably so good, cause peduncles are still attached. I noticed that in order to be more certain of pod forming a spent bloosom still have to attached like this; (this one is like 2 weeks) I hope it won't abort it. In case of my potted gardenias "Magnifica" completely is unable to be a seed parent aborting all peducles as soon as blooms fade. Gardenias have high germination rate and if everything should end all right I should obtain at least few pods. So I will use my garden and local pine forest in some hidden location to test it. I really need to have much more varieties but it looks like this project will span at least few years.