Thank you for contacting the Denton Co. Master Gardeners Help Desk. I'm pasting below the link for the advice on gardenia care, by Dr. Jerry Parsons of Texas A&M. As you will see in this Q&A, gardenias only bloom successfully in a narrow range of temperatures.

https://aggie-horticulture.tamu.edu/parsons/search.php?category=Gardenia

Just like bluebonnets, gardenias have their season, and our hot summer temps in Texas are not conducive to long bloom times, but the foliage is very attractive and makes a lovely display. Growing them in containers is the idea cultural practice since they do not do well in our alkaline soil. Use a fertilizer for acid loving plants, often labeled for azaleas or camellias, and follow directions as to amount and frequency. If you see any sign of sunburn, it would be best to move them so that they get less direct rays of the sun, even though it is somewhat shady under your pergola.

I am also pasting below, for your future reference should you need it, a page on gardenias from the Texas A&M AgriLife Extension's Plant Disease Handbook:  https://plantdiseasehandbook.tamu.edu/landscaping/shrubs/gardenia-cape-jasmine/

Clemson University has an excellent page on general gardenia care. Gardenias should be brought inside should the low temperatures fall to the region of 15 degrees.  All plants in containers are more vulnerable to cold damage than those planted in the ground:  https://hgic.clemson.edu/factsheet/gardenia/

Please contact us directly at our Help Desk email address, should you need further information:  <personal data hidden>

Pam S.