A few nights of frost? The daylilies should be fine.
The foliage pictures are of daylilies covered in frost during lots of active growth. As long as the daylilies don’t have scapes with buds on them, they should be fine even if they are laying flat out. In 2015 we had 11 nights of May freezing temps including -5.8 C and -4.4 C. The effects that year were some browning of foliage, see flower pics, and shorter than usual scapes. Despite the cold temperatures in May, we had beautiful blooms and increase on plants. The two flower picture show the foliage on some seedlings later in the year.
Of course the effects will depend on duration of freezing as well as temperature.
The winter of 2013-2014 led to the selection of a few super hardy daylilies for zone 3. No tree, snow, or mulch protection -30C plus windchill. All that was left in the open were these six. Winter 2014-2015 should lead to more discoveries as January had 2 weeks of no snow cover and plus temperature.
Karl suggests that when working with our growing conditions, a medium should be considered instead. Using round straw bales as a snow catch would make a huge difference to chances of a daylily living through winter here. Still it is fun to discover which plants are above the rest when it comes to sheer cold/freeze-thaw hardiness. He has a point, and I will likely choose some sort of winter protection if anything is to thrive in the open. At the same time the test for super hardiness in seedlings and cultivars without any winter protection will continue.
There are two that I would consider close to the above in hardiness, but they had other plants near them that may have contributed to a bit of snow catch.
It took 40 years before weather conditions were wet enough for botrytis to affect lilies here at the farm in Lucky Lake, Saskatchewan. Since 2010 botrytis resistance has been an obvious goal. Here are three cultivars that show good resistance. Tetraploid pollen used on Iowa Rose results in easy seed. Poorer results on Morden Butterfly. Not Centurion being a diploid asiatic lily is easy to breed with.