As far as I know, the blooming tree is a fairly new tradition because it's difficult to find storks in bigger cities. In the past you absolutely had to see a stork (or a swallow) in order to hang your martenitsa on a tree or hide it under a rock. Also, when you see a stork for the first time, you have to hang the martenitsa on a fruit bearing tree, such as an apple or peach tree. Such trees are hard to come by in Sofia :) It had to do with fertility, I guess. In case you stow the martenitsa under a rock, you have to check at some point if there are any insects or larvae crawling there. These are supposed to determine whether the year ahead will be good and successful for you.  So if you see plenty of ants or worms crawling on your martenitsa you can sign in relief :)
Apart from Bulgaria, there are similar traditions in Romania and Moldova. Maybe a few other countries  as well but not quite sure where exactly and how the tradition has evolved throughout time in those places.