Daylily
'Arctic Angel' (Salter-E.H., 1986)
scape height 16 inches
bloom size 2.75 inches
bloom season Midseason
ploidy Diploid
foliage type Evergreen
bloom habit Diurnal
http://www.daylilies.org/Dayli...
Found that one on the AHS search, this is a link for the search, you can put in height and bloom size, ect, but unfortunately cannot search by color.
This is the only one I came up with that is white when searching 18 inches