I'm sure you are correct. *I have a friend who narrowed down to the family Brassicaceae (Mustard) today. *Seeing your post now reconfirms it. *Thank you very much for solving this mystery.
BTW, my bittercress plant only had roots about 1 inch long, and it came up quite easily. *On the other hand, the D. capensis had 5 inch long roots and the entire medium lifted up when I yanked/transplanted it out. *Which one is the bigger weed?!