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Here is information on pruning.

https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm

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https://www.gardeningknowhow.com/.../gardenia/when-to-prune-a-gardenia. htm

This is called sooty mold. It won't directly hurt your plants but it attacks insects that are eating your plant. check for bumps down the middle (scales) and random white blobs(White flies) and treat as necessary.

These are usually sold as mature as they can get in these containers. It is best to re pot it, but it can survive for a short time in that container. Nurseries usually maintain root, and top growth while they are in those containers, but this is not advised if you are new to these. Incorrect root pruning will kill it.

This article will give you more information on the plant: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm

This will have to be an acid loving mix. You can fertilize once per month. This article will give you more information: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/fertilizing-gardenias-in-your-garden.htm

Most gardenias are hardy in zones 8-11, but there are a few varieties that are also hardy in zone 7, including the "Kleim's Hardy", "Frostproof", and "Chuck Hayes" varieties.