I live in Boulder Colorado, and this plant has popped up in my yard in a few areas. It is now about 18 inches tall and at the top has a few clusters of very small yellow flowers. What is this?
I don't see the flowers. Crush a flower and see if it has the odor of pineapple. Pineapple weed, Matricaria matricarioides.
Here are more photos. I crushed the flowers, but no pineapple smell. The flowers are very interesting, though my lens could not capture details. There is a long stalk in the middle, about 3-4mm long and thin. The three or four tiny petals seem to be near the base of this stalk, and fall off over time. When first emerging, the flower buds have a pinkish dot at the tip.
This is an umbellifer (carrot relation). Pineapple weed is a composite (Asteraceae) with disc-like flowerheads and a low, bushy habit - there is no undivided central stalk running up through the center.
It's not pineapple weed. The flowers are not as close together as that, and there is no smell in the crushed flowers. If it's an umbillifer, any idea which one?
I think David in LA might be correct. From the photos, I didn't see anything resembling an umbellate inflorescence. Not an Asteraceae, not an Apiaceae, but a Brassicaceae. A closeup of the flowers should cinch it.
I also concur with Dave in LA's identification as Descurainia sophia. The photos on this web page, particularly the close-up of the flowers, looks just like my plants: http://www.missouriplants.com/Yellowalt/Descurainia_sophia_page.html One day I'll get a proper lens to take better close-ups of the flowers. Cheers, Daniel
