|I got patio trees from Home Depot a month ago, one hibiscus
|Yellow leaves on your hibiscus may be just a natural leaf drop. Hibiscus sheds its older leaves (usually lower down and on the insides of the branches), in favor of developing healthy new leaves. The foliage is a favorite of aphids, though, so check the undersides to make sure they are not present and their feeding is not causing the leaf yellowing. Wilty leaves can indicate a need for more or less water, or could simply be a reaction to hot summer winds. Try hosing the plant off when temperaures are really hot (do this in the morning, not in hot sunlight) and water thoroughly when the top of the soil feels dry. Apply water slowly so it has a chance to soak in.
Aphids can be controlled on your gardenia with Insecticidal Soap. Repeat sprays may be necessary; be sure to spray the undersides of the leaves and the stems as well as the tops of the leaves.
Best wishes with your plants!