All three plants belong to the family Asphodelaceae (or Xanthorrhoeaceae in some older classifications), but they are members of distinctly different genera. This is the most fundamental difference from a botanical perspective. True lilies, both Asiatic and Oriental, are from the genus Lilium. They grow from true bulbs, which are modified underground stems with overlapping scales. Daylilies, however, belong to the genus Hemerocallis. They grow from a mass of fleshy, thickened roots called rhizomes or sometimes "tubers," not from a true bulb. This difference in their underground storage structure is a key identifier.
The growth habit and foliage of these plants provide clear visual distinctions. Lilium species (Asiatic and Oriental lilies) produce a single, central, upright stem from their bulb. This stem is surrounded by narrow, lance-shaped leaves that are arranged in whorls or spirals along the entire length of the stem. In contrast, Hemerocallis (daylilies) grow in a clumping habit from their crown. Their leaves are long, arching, and straplike, growing from the base of the plant in a fountain-like mound. The flower scapes (stems) of daylilies are leafless and rise distinctly above this basal foliage clump.
The flowers, while sometimes similar in form to the untrained eye, have significant structural differences. True lilies (Lilium) have flowers with six petals and prominent, pollen-producing anthers. The flowers can be upward-facing, outward-facing, or pendant. Asiatic lilies typically bloom earlier (early to midsummer) with vibrant, often spotless flowers, while fragrant Oriental lilies bloom later (mid to late summer) with larger, often highly fragrant, outward-facing blooms. Daylilies (Hemerocallis) get their common name from the lifespan of each individual flower, which typically lasts only one day. Each flower has three petals and three sepals (collectively called tepals), and they are borne on branched scapes with multiple flower buds. A single daylily scape will produce many flowers over an extended period.
The reproductive strategies further highlight their taxonomic separation. True lilies (Lilium) reproduce primarily through their scaly bulbs, which can be divided, and also by producing bulbils (small aerial bulbs) in the leaf axils of some species. They also produce seed. Daylilies (Hemerocallis) reproduce and spread vegetatively by division of their crown and fleshy roots. They form large, dense clumps over time and can also be propagated by small plantlets (proliferations) that sometimes form on the spent flower scapes, in addition to seed.
