I haven't been able to identify this weed which is prolific on the gravel (and garden beds) on the rooftop of my 10-storey condo building in downtown Vancouver, BC. Can anyone identify it and let me know what I should do about it? It has shallow roots, hairy red stems, red/green leaves, and little white flowers.
Looks like herb robert to me (geranium family). I remember it taking over parts of my parents' garden in NE USA and that it was a bit of a nuisance. What to do about it depends on how you want to control it. It's an annual/biennial, so individual plants shouldn't last more than two years, but it spreads like crazy via seed. If you don't want to use chemicals, I suggest just keep pulling out plants (which have shallow root systems and are easy to pull), making sure you get any adults before they produce and disperse seed. Could take a few years doing it this way as there's probably a considerable seed bank. Alternatively, look into safe herbicides for a faster result.
It is Geranium robertianum, see Wikipedia page for more information. This common species in the occidental european area is widespread and have a strong variety concerning the leaves shapes and the colors. A lot of scientific publication deals with this question. Generaly it prefears mid-shadow places.