Sherardia arvensis (Field Madder or Spurwort) is the tiniest wildflower I have ever photographed. This small annual’s flowers are only 3 mm. I’ve tried in the past to photograph this tiny wildflower but didn’t have any luck till I upgraded my phone/camera.
Here’s another photo to give you a better idea of their size:
You can see more of my tiny flower photographs in my Flickr Gallery.