Pollen will deffinatly effect lift produced.
I have been in 300 where blades have got coated in a layer of pollen and she has been using a bit of power to hover.
By simply cleaning them and with out changing the weight I have gone back out and used 1 to 2 inches less.
Long grass will effect your cushion but it will not suddenly make it dissapear after hovering in the same place for 20 minutes. If you look at the pollen on the leading edge when it builds up it has about the same dimentions as a good hoar frost.
Anything rough on your leading edge will effect lift thereby increasing the power needed.