If you want to be successful with a gardenia in winter, provide it lots of light and very cool temps. I actually keep mine on my 3 season unheated porch and keep them down into the 40's all winter long. They are kept in a well draining mix, not fed, just allowed to sort of rest and they stay green and even open buds at that times.

I find the heat, too little winter sunshine, even artificial light causes their demise along wit lots of pest issues.

I have three I carried over from last two winters thriving. I will takes pics if you would like to see)

Also , if you want to prevent mites, spray them with a good horticultural oil once a month and you will never see a mite again.

Mike