I found this growing in mid August after digging up a wild seed bed. Covered...
This is a species of Brassicaceae (mustard). It could be yellow rocket (Barbarea vulgaris), Indian rocket (Brassica juncea), or a few others. To identify mustards to species, you need to have mature fruit. If you still have the plant and there are enough warm days for it to mature, you could send it to me for identifcation. Be sure to allow the fruit to mature before sending, as fruit length is a characteristic.
If you mail it, include a section of stem, a few leaves from the top, a few leaves from the base, and the mature fruit, and seal in a gallon ziplock bag and place in a box or padded envelope.
Schutter Diagnostic Lab
119 Plant Bioscience Facility
PO Box 173150
Montana State University
Bozeman, MT 59717