Dogs eating grass is completely normal. They are omnivores and in the wild they would graze naturally. The spring grass is especially tasty to them. Your dog may eat grass more if there was a deficiency in it's diet or if it was feeling unwell in its gut, but lots of dogs just love to graze anyway, its totally natural.
They actually do eat course/couch grass as a purgative, both for vomiting to rid excess bile/nausea and for bowel clearing. It passes through the gut and out within two hours. As for flowers, beware many common flowers and weeds are toxic to dogs ie sweet peas/foxgloves.