Can I cross diploids with tetraploids? For some reason I thought that is not doable, but while searching Tinkster I found some cultivars that are children of TET and DIP.

But I am also getting contradicting information. The site below said you can only cross TET with TET and DIP with DIP.

http://www.dallasdaylilies.com/hybridize.html

It also says that "daylilies do not self pollinate themselves. In order words, if you have cultivar Red Volunteer, you cannot produce another Red Volunteer by pollinating the flower back onto itself." Is this true?