The Strybing Arboretum in Golden Gate Park - enter through the main entrance by the library and be treated by the new horticulturally exciting entrance designed by Roger Raiche.
The Japanese Tea Garden in Golden Gate Park
You can easily spend an entire day in the park.

Giradelli Square usually does a nice floral show , its down by Pier 39 - touristville.

If you plan on traveling up north across the golden gate bridge the MECCA for horticultural connoiseurs is Western Hills Nursery in Occidental.
If you are planning to travel south of the city - about 35 minutes south is the famed "Filoli Gardenes and Estate ". ( remember that huge brick mansion on the opening scene of Dallas tv show ? ... thats the one , with 17 acres of formal gardens done in the European style of opulent grandeur.

here is a link that you might find interesting.

Filoli Gardens

You might want to visit the San Francisco Flower Mart
where florists buy their flowers. It starts EARLY!
688 Brannan Street, San Francisco, CA 94102
(415) 362-1520 Retail Hours: Mon-Sat: 9am-3pm
Wholesale services: Mon, Wed, Fri: 4am-3pm