Use Flip Tube Direction to switch the direction of the current brush between Along Path (best for canvas painting of plants) and Along Normal (best for scene painting of plants).
There is a Force Tube Direction to be Along Path checkbox in the Canvas section of the Paint Effects Globals. Force Tube Direction to be Along Path causes brushes with tubes whose elevationMin is greater than 0.5 to be drawn along the path rather than along the normal in Canvas mode. This has the effect of making plant brushes draw in the expected direction when painting in the canvas.