Best time for taking 5-6 inch cuttings is probably the summer. Cuttings should have three or four leaves on each, which you should not remove, if you have a mist system.

Sand, perlite and sand, perlite and peat would all be good rooting mediums to try.

Let us know how the rooting does for you.
I have a huge hibiscus here and when I find a low branch
I bend it down carefully and notch it where it will be
in the ground. dig a small hole and fill in dirt around
it. Place a rock on it to help hold it down. I've root
several this way. You might try that with the gardenia
if you have some low growing branches.

Pam