Certified GKH Gardening Expert
Typically, I would recommend systemics for licenced applicators, only. Many can be harsh, and carry legal consequences, if used incorrectly.
I don't, normally, recommend specific products, but in this case I will. Since choosing a safe product is paramount in this situation, I see it necessary to recommend a safe way to use a systemic control.
SNS 209 from Sierra Natural Science is what I use. It is organic, and works well without being a risk to the environment or other plants in the area.
Here are some articles that will help you with the care of Gardenia and Hydrangea: