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I would not recommend this practice. While it does acidify the water, too much can hurt the plant. Vinegar is often used as a homemade weed killer as it is very acidic. Instead, stick with an acid-loving fertilizer (appropriate for azaleas and camellias) for your gardenia. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/fertilizing-gardenias-in-your-garden.htm If you're a coffee drinker, you might also consider using the grounds to feed your gardenia. Here is more information: https://www.gardeningknowhow.com/composting/ingredients/coffee-grounds-gardening.htm

Lichen normally does not cause any problems for plants, other than making them somewhat unattractive. For more info on lichen, and how to get rid of it, try this article: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm

Yellow leaves normally signal that the plant is under some type of stress. In order to remedy the problem, you must first try to pinpoint the issue. This article should help you with that: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/helping-a-gardenia-bush-with-yellow-leaves.htm This article may also help: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

These articles will help: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/propagating-gardenias.htm, https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm

You can cut the plants back this spring; however, you should be aware that the buds may have been damaged, so the likelihood of getting flowers this year is minimal. These articles will help you as well: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm, https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm, https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm For info on protecting plants from freezes/frosts, these articles should help: https://www.gardeningknowhow.com/plant-problems/environmental/protect-plants-in-freeze.htm, https://www.gardeningknowhow.com/plant-problems/environmental/protecting-plants-winter.htm This article will tell you how to care for gardenia plants: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm

You can give it some fertilizer now but I would only do so at half strength. Once it has established in its new surroundings, you can fertilize as usual. This article will help with fertlilizing your gardenia: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/fertilizing-gardenias-in-your-garden.htm