Cardamine hirsuta Hairy Bittercress
This pretty little plant has become such a pest in it's native Europe and Asia. It is now present in north America too. It is an annual and is edible as a bitter herb, though I have never tried it. It is usually small and never grows any bigger than 30cms, 12 inches. It flowers from January to May and is often found in bought plants from Garden Centres or Nurseries. As soon as you touch the tall slender seed pod it will burst, scattering 100's of seeds.