Sabrina,
Your temperatures are better than fine - a night as warm as 65 degrees here is a rarity, and daylily seedlings thrive. Granted, they won't grow as fast as yours will, because when the nights cool off to 55 at dusk, a good part of the next morning is spent getting the plants up to growing temperature. When the weather warms up in late summer and the nights are 60-62 until 11:00 pm, I see improved growth in all plants.
Daylilies evolved in temperate zones, and, unlike tropicals, a fluctuation between day and night probably does them good, as does a change in season. I think if you can figure out some simple method of fortification that will prevent animal damage, there's no reason to bring them back inside, ever. My fall-planted seedlings, even in a mild year, will see plenty of light frost in the form of 30-32 degree nights from late December through February, and there are few, if any losses. Properly handled and grown, given lots of light, your winters are no challenge to a daylily at practically any stage of growth. I've seen volunteer daylily seedlings sprouting in the garden in the middle of winter, and they just grow very slowly until spring comes. The most rudimentary of cold frames, set near the foundation on the south or southeast side of a building would be fine. Your biggest threat might be from excess moisture if you get a lot of rain in the winter - particularly if the soil mix is water-retentive.
Sue,
The last time I shopped Miracle-Gro, their standard mix was 15-30-15, but I was able to find their tomato and vegetable formulation, and the nutrient ratio was similar to the numbers you gave.
My seedlings are definitely kaput - they were in full sun and received the full brunt of our Indian Summer. Also, they were in a 50-50 coir/perlite mixture that I was trying out. They were up off the ground, so it was as if they were in an oven, and the foliage turned that ugly gray-green and collapsed in rot. I checked - there's nothing under the soil but cooked roots. The weather and conditions just caught me off-balance. The whole flat didn't die off, maybe 30% of the seedlings. The fact that the survivors are probably tough-as-nails is my consolation.
Ken
East S.F. Bay Area
USDA Zone 9 - mediterranean
