Hi Nhu, The problem is that Allium neapolitanum also has a 3-sided peduncle with white flowers and is also common in the Bay Area. I think the best indicator is that A. neapolitanum has erect flowers while those of A. triquetrum are drooping. I have also read that the peduncle of A. neapolitanum may be slightly winged on two of its angles. Thanks to all of you who answered. The reason I asked this question is that one (or both) of these onions is a common, though not particularly aggressive weed in my neighborhood, and I wanted to know which species it is. Keying it out in two different keys gave me the two different answers. David E.