Lilac

First, let's define what kind of lilac.  East Coast tree-type lilac.  Not ceanothus -- prevelant & practical for Native American soap making.  No full-fragrant & purple in nature.

So, what's the connection between lilac and poetry?   Purple passages.  A full fragrance which isn't cloying and evokes the finest of memories.  

Where in San Francisco?  The Arboretum, of course.  Across from the native California section.  Go quickly -- she's in bloom.  Wander through thenative California section and take in the tightly curcled ceanothus bushes.  Enjoy a fully purple day.