As far as I can tell (and from some information from researchers) photoperiod by itself does not appear to have much effect in daylilies. As a simple test I have brought daylilies inside during late December and early January and placed them where the light they received was from outside. They broke dormancy and grew throughout the rest of the winter even though they were experiencing very short days and long nights. The amount of light the plants receive is different between short days and long days and more light does allow plants to grow more (within reason) but the same effect can be produced by higher intensity light (within reason).
Stout indicated that for daylily foliage types (as he defined them) evergreen was dominant to dormant. That would mean that sometimes two evergreens crossed together could produce some dormant seedlings (depending on the genetic backgrounds of the particular evergreen plants) but that two dormants crossed together should not produce any evergreen seedlings. The catch is that foliage types change depending on the winter weather (nearly all cultivars registered as evergreens appear to be simple 'dormants' in my zone 4 climate) . It is also very unlikely that foliage types or growth characteristics (the alternative phenotype to evergreen is deciduous; the alternative phenotype to dormant is ever-growing or non-dormant) are inherited as simple single Mendelian genes.
