Al,
You are quite right, Cellular Absorption would be present the moment you came to a hover over the long grass.
I've never heard of pollen making such a significant change.
I don't doubt what you say, I just find it hard to imagine such a build up of pollen that would alter the aerodynamics of a blade so quickly. I would expect such a build up of any foreign matter to be apparent elsewhere and noticeable, eg. on the perspex or fuselage, visible recirculation.
I wonder if bees have the same problem.