You CAN still post to each cultivar thread after it has been moved to the archive forum. Nothing changes except it's location. And those "watching" that forum will know when a new post has been added to any of the cultivar threads.
Edited to add: I am wrong about that .... to know when a new post has been added to a particular cultivar thread, you will have to "watch" that cultivar thread. "Watching" the Daylilies of the Day Archive Forum, only lists it on your home page.
