I have a gardenia plant and the leaves are curled and the color has turned from dark green to light green.

Question

The leaves are crinkly and they have turned from dark green to a light green.

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Answer 1

Inconsistent watering could be the culprit, causing drought stress. If you think that is a possibility then water it heavily right away and you may save it.

Another possibility is spider mites. They are hard to see but you should see webbing among the leaves. This article tells more:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-leaf-curl.htm

This article and a soil moisture meter can help to take the guess work out of when to water:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm