Posted: 03/03/2014 at 13:56
Bittercress is an annual, as you say, but it can have several generations in one year, which is why it's important to get rid of it before it can flower and go to seed. Hoeing is OK at this stage. Once it's gone to seed you can't touch it without the seed pods exploding and firing the tiny seeds everywhere - it's a very efficient mechanism to keep the species going!
Incidentally, there are two similar species of bittercress, Hairy and Wavy (Cardamine hirsuta and C. flexuosa). It isn't possible to tell the difference at the seedling stage, but they are both equally undesirable in gardens.