I'm stumped, folks. This one isn't in any of my books, nothing even remotely similar. However, it is in my front garden! My initial impression based on the flower shape is that it's a Dratted White Aster of some description, but the leaves say Lamiaceae, and so does the stem, which is square and pubescent. The whole thing is about 50-60 cm tall, with multiple branches; flowers are about 1.5 cm across and borne on petioles of about 3 cm, in clusters of three or four; buds resemble those of Asteraceae. The largest leaves are about 5-7 cm from petiole to tip, and about 2-4 cm broad at the broadest point, and broadly lanceolate with serration. The pic with my hand in it should give some sense of scale. There is no discernable scent other than chlorophyll when a leaf is crushed, and the plant doesn't urticate when touched - in fact, it's sort of pleasantly soft. Help! Once I know what it is, I can decide whether it stays or goes.
Hi Lorax, Square stem and opposite leaves is not something unique to Lamiaceae only. Galinsoga has opposite leaves (and flowers very similar to those of your plant). Bidens has opposite leaves and often has rectangular stem.
Reminds me some kind of Tanacetum. Compare, for example, to this: http://www.rcplondon.ac.uk/garden/2007-07/Tanacetum-parthenium.jpg Maybe some South American species. You can try InfoJardin forum (http://www.infojardin.com/). Lots of discussions there. Maybe someone knows.
I'd be tempted to go with Galinsoga because the flowers are so similar, but I can't find a reference for one with pinnate leaves like the plant in my garden has... I'll check out InfoJardin, Andrey, thanks! Tyrlych - I know what Bidens look like here, though - we've just got the one species and it has yellow flowers and a round stem, and very distinctive foliage.
Hmmmm. Today in the garden I found some actual Galinsoga parviflora, and while the flowers on my mystery plant are definitely similar (in fact, nearly identical), the real Galinsoga is much much much tinier (see attached pic). Curiouser and curiouser.... I think I'll keep the G. parviflora, though - it has tasty greens. The jury's still out on the giant NOID.
OK, but which species? I can only find record for three, and all are recorded as being very small plants, which mine obviously isn't.
