I think I might have been over-estimating the entish smut component; the story I was particularly recalling is Vulgarweed's gorgeous but melancholic Lomendánar (Little Love for the Things of My Love) which was written for lbgtfest. She's also written By Root and Twig (Merry/Pippin/Treebeard), which might be more what you are looking for and does indeed feature pollen.

The pioneering sexy-ent writer I recall is Teasel: How Quickbeam got his name, but it's sadly only a drabble. I thought she'd written some more, but I can't find it.