Your zone completely throws me, "Where summer is Winter", I thought zone ten would be where summer is hotter than... well very warm. Six inch pots for daylilies for over a few months I would think would be stretching it. Small pots in zone 10 , I would think, would need watering twice a day in warm weather. I was also confused by the mention of planting them in the yard for fear the lawn mower would run over them. I have never seen daylilies planted in the "yard" is that meaning "lawn" that you would normally mow? Or by "yard" did you mean garden? I would love to see photos of those 5 year old plants.
I think Polymerous gave good advice, and I would pop at least one of those plants out of the pots and examine the roots, if those pots are not just jammed packed full of roots those plants have to be severely undernourished.
