I only know of one of these trees in the city, and its beautiful wisteria-like flowers are in full bloom right now, at 414 Shrader, near Oak Street in the Haight-Ashbury. The tree is a golden chain tree (Laburnum X watereri), a hybrid cross between Laburnum alpinum and Laburnum anagyroides, the only two species in the Laburnum genus - both native to southern Europe. It's a small deciduous tree (rarely more than 20 feet tall) that blooms in late April/early May.

This tree has been called the "goldilocks of the plant world", preferring a climate that's not too hot, and not too cold - which means it's perfectly suited to San Francisco. To steal a quote from Matt Ritter, this is an "undeservedly rare" tree here in the city, and I'd love to see more of them. But be careful if you plant one - all parts of the plant are poisonous.