Gardenia and jasmine need separate summer checks, even when both are being grown for fragrant flowers in containers. Look at the light, drainage and root-zone moisture before deciding whether either plant needs a change.
Check the pots separately
Gardenia and jasmine may share a sunny patio, but their containers can dry at different rates. A small pot in wind may need attention before a larger sheltered one. Lift each container and check below the surface instead of doubling a winter watering interval by guesswork.
Keep the root zone working
Check the growing medium and root condition before adding any treatment. An arbitrary amount of vinegar is not a reliable way to manage an individual container's acidity.
Change exposure gradually
When moving plants outdoors, introduce stronger light in stages and watch the foliage. Check the plant's identity and local temperatures before leaving it out overnight. If flowering is poor, review light, recent pruning and root health before increasing feed. Suitable care follows the plant and the pot, not a single summer trick.
Related reading: Gardenia and Bai Chan: Sorting Out Overlapping Names.