I could totally be alright with one category--enter your five best photos and be done with it!
But, as a ponder the situation a little more, it occurs to me that the ideas are not mutually exclusive. In that we could still have mini-contests associated with each plant/plant-type week and we could have an annual grande finale wherein it may make sense to have just a few categories based on distance like macro, cameo, tight-scene, and wide-scene... although I do find the concept of the contest that moonhowl describes to be rather compelling
