Question
I live in the 94565 area and have 2 plumerias in pots and just purchased a Tiare plant.  Can I plant them in the ground or do I need to keep them in pots and bring inside in the winter.  I would prefer to plant outside if they can survive.  I have heavy clay soil.

Answer
You are in a USDA zone 9B, but that just isn't enough information for your tropicals.  

As your area of the SF bay/Contra Costa is effected by fog and inland heat, and the zip code you gave me covers a lot of real estate, I would suggest you consult  your Sunset Western Garden Book for your specific location...

It has maps in the front, and your location falls within one of the three SUNSET zones 14, 15 & 17, depending on how high up the hill you are from the Bay.  Not to be confused with the USDA zones, that cover a broader area.

Plumarias, zones vary by the individual species, and there is a chart within Sunset to hep you determine IF you can plant it outdoors in your zone (YES for some species / NO for others).

For the Tiare, or Tahitian Gardenia, is an aromatic tropical flower, used to make Leis...

http://davesgarden.com/guides/pf/go/58376/

It is a Zone 10 (USDA), so on the surface, I'd say, "No way" for sticking it outside year-round, but maybe you've got a hardy variety. I'm not intimately familiar with it.  

You are on the boarder of the 10 zone, at 9B (USDA).  It isn't in my Sunset version (1995), so my guess is that it is a new introduction.  Gardenias in general are fussy.

Ask a local nursery if they've had sucess with it in the Antioch/Pittsburg area.

~M