Althea, also known as Rose of Sharon, is indeed a member of the Hibiscus family. For best bloom, it wants lots of water,although it will tolerate drought. Full sun is fine, afternoon shade may be better.

It is listed as an invasive because it self-sows prolifically. However, if it is not near a woodland, it should be fine.

This is a good plant data sheet-

Here is a link that might be useful: UFL plant fact sheet

As for your gardenia, sunlight might be the problem. Though they do best with some protection from the sun in the hottest part of the day, they need plenty of early sunshine in order to produce those incredible blooms.

Thanks:)
Jen

Not all Altheas self seed. I have 5 in my yard and never a seedling. However, mine are doubles and probably hybrids.