I slip bobber trees all the time.
6'6" rod, and underhand flip cast. In windy conditions I use a river slip bobber. It is thin from top to bottom and doesn't get dragged around in the wind, current like a traditional slip bobber.
I also put enough weight on the line to make the bobber neutrally buoyant. Only takes a little tug on the line and the bobber goes under. Leave the weight about 18" fromt eh hook. This gives the minnow room to move and adds to the action. Drives Crappie nuts.
As for Crappie, they will be facing the tree into the current. Sometimes with there nose right on the tree. They are staying out of the current, however they will jump out to eat food as it goes by.
HOI Crappie Club
Where family and friends come to compete for a little more than bragging rights.
Quick, someone teach me how to fish so I can win this tournament!!!