I really like the idea of a foliage category
that could certainly encompass much better variety than a specific plant category.
My favorite thing about the photo contest is actually seeing such a great variety of eye candy all in one spot! 
The biggest downside to the specific plant categories, for me, is wading thru 200 mug shots of essentially the same thing.
No offense intended here to daylilies or lovers of daylilies, but that was by far the worst category for me as far as the boredom aspect of clicking thru the photos knowing that the next would most likely be yet another mug shot of a daylily.
Granted, there were some very nice mug shots to select, but my point is just that the variety categories are so much more fun to view because of the surprise element. (And probably, for that very reason, I really loved the good humor shots like Greg's rose plate in the roses!)
Anyway, all that said; I do think that, as long as we are having specific plant categories, Dahlias would probably be a good one add so that the fine Dahlia photos in the bulb category (which is kinda disconcerting to me because they are not bulbs) can compete directly with all the fine Dahlia photos scattered elsewhere.
The "wrong category" thing, including Daliahs in the bulbs and perennials in the annuals, etc., I find rather humorous. It really doesn't bother me in terms of enjoying the pictures in the contest or voting for them if I like them, although I can see it as a potential credibility issue for the site as a whole having archived photo contests with defined, yet defied categories.
Maybe the easiest approach would be to relax the more apparently confusing definitions somehow. Flowers/flowering plants? could be divided by single, multiple, or mixed groupings or combos? or maybe by bloom season--spring, summer, fall, winter? or even randomly--group 1, 2, and 3 with a 100 or so per group?
//just throwing it out there for some additional things to ponder
