Certain tall plants, namely fescue grass and switchgrass, slow the player down as they try to walk through them. While this makes sense, the slowing effect is applied by the plant regardless of its age. This can become slightly irritating when the plant is very young, and barely reaches up to the player's knees, less so than other non-slowing species of grass.

I suggest that the slowing effect is only imparted by grasses which have "part" set to "lower", as this implies that the grass is full-sized and would believably impede movement.