Earlier hybrid daylilies are all "diploid" with 2 sets of chromosomes. "Tetraploid" hybrids have 4 sets of chromosomes. Both groups have beautiful varieties... I've heard some people claim that tets are more robust, but I've also noticed some of the still-around early hybrids are very robust, so your mileage may vary,.
I think the only reason to pay attention to "dip" vs. "tet" is if you are crossing your daylilies to produce seeds... you can't successfully cross a dip with a tet. Daylilies are only fertile with others that have the same number of chromosome sets.
