Edit: Oops! Found it. It's Lemon Balm, Melissa officinalis. It is a mint, a herb, and a weed—depending on one's personal regard for this invasive escapee. Whether it's "good" or "bad" is given considerable thought in "Dave's Garden", if you scroll down to the section called "Gardeners' Notes": http://davesgarden.com/guides/pf/go/292/ My original post, just for interest: I'm trying again for an ID of this plant. One gardener-friend says it is "just a weed." I think it's a mint and possibly a herb, but I'd be grateful if you happen to know. It has square stems and the leaves are paired, alternating north-south and east-west, as it were, from the stem. The stems are pretty well smooth but they have the very teensiest would-be hairlets—very very tiny—occurring as though they were wannabe thorns that never even began to be the real thing. Hardly noticeable. The plant survived the past Vancouver winter, which was typically mild, the temperature dropping not much below freezing and not a great many times. Even so, I think this is one of our hardiest balcony plants. Thank you.
Looks like lemon balm (Melissa officinalis [ http://www.photosbotanical.com/cat361.htm ]). The whole plant should be strongly aromatic, with a citrus-like scent when crushed. Later edit: sorry, I was looking at the photo and didn't see your added note - you already know it's lemon balm.
