Blender includes an option to use forces for growing hair however as forces can be animated the calculation of the 'growth' of the hair can vary depending on which frame 'use for growing hair' is ticked. Currently blender does not update the 'growth' of the hair as the animation progresses but an update can be forced at any point by tabbing into and out of edit mode - as far as I can tell this isn't particularly useful as if any work needs to be done in edit mode the user then needs to return to the correct frame for growing the hair and tab in and tab back. Either there should be an option to specify a frame number on which forces should be used to grow the hair, and/or the growth should be able to be affected dynamically (the hair is regrown for each frame - probably more useful for manually animating hair styles?).

Thanks,
Josh