The genus Lilium, which includes all true lilies, contains approximately 100 to 120 recognized species that occur naturally around the world. The term “variety” often refers to cultivated plants, and in that context, the number expands to include thousands of human-made hybrids and cultivars. These cultivated forms are the result of decades of selective breeding, designed to produce lilies with specific colors, shapes, and growing habits that are distinct from their wild ancestors.
Defining the True Lily
A true lily belongs exclusively to the genus Lilium. Botanically, Lilium plants grow from a unique underground storage organ: a scaly, tunicless bulb. Unlike the bulbs of tulips or onions, the lily bulb lacks a protective papery outer skin, making it more vulnerable to drying out. The flowers of true lilies are characterized by six petal-like segments, or tepals, which are free from each other rather than fused. This definitive structure is the reason plants like the Daylily (Hemerocallis), Water Lily (Nymphaea), or Lily of the Valley (Convallaria) are not considered true lilies, as they belong to entirely different plant genera.
The shape of the flower also distinguishes the group, with forms ranging from trumpet-shaped, as seen in the Easter lily, to the strongly reflexed tepals of the “Turk’s cap” shape. Many true lilies also produce “stem roots,” which are adventitious roots that grow from the base of the stem above the bulb, helping to anchor the plant and absorb nutrients.
The Nine Horticultural Divisions
To organize the multitude of hybrids created by breeders, the Royal Horticultural Society established a standardized system that divides all true lilies into nine distinct groups. This classification is based on the parentage of the hybrid and the shape of its flowers.
The nine divisions categorize lilies based on their characteristics:
- Division I: Asiatic Hybrids, known for being easy to grow, having upward-facing flowers, and lacking fragrance.
- Division II: Martagon Hybrids, which are often shade-tolerant and display the downward-facing “Turk’s cap” flower shape.
- Division III: Candidum (Euro-Caucasian) Hybrids.
- Division IV: American Hybrids.
- Division V: Longiflorum Hybrids, which includes the traditional Easter Lily.
- Division VI: Trumpet and Aurelian Hybrids, characterized by tall stature and strong, sweet fragrance.
- Division VII: Oriental Hybrids, prized for their very large blooms and intense scent.
- Division VIII: Interdivisional Hybrids, accommodating crosses between established hybrid groups, such as LA hybrids.
- Division IX: The Species themselves, which are the wild, naturally occurring lilies.
The Difference Between Species and Cultivars
The distinction between a species and a cultivar explains the numerical difference between wild lilies and commercial varieties. A species is a population of plants that occurs naturally in the wild, is genetically stable, and can reproduce to create offspring that are consistently similar to the parent plant. These wild lilies are the foundational genetic material for all other types.
A cultivar, which is a shortened term for “cultivated variety,” is a plant selected by humans and maintained through cultivation for specific desirable characteristics. Cultivars are often hybrids, meaning they are the result of cross-pollinating two different species or two different existing hybrids. For example, the creation of A/O hybrids combines the hardiness of Asiatic lilies with the large, fragrant flowers of Oriental lilies, resulting in a plant that would not occur in nature.
Breeders focus on creating new cultivars to achieve traits that improve the plant’s commercial value and garden performance. This includes engineering resistance to common diseases, developing new color combinations, or altering the flower size and stem length. The thousands of named lilies available in garden centers are almost entirely these human-engineered cultivars, each one selected and propagated to retain its unique combination of characteristics. This ongoing process of hybridization is what continuously expands the total number of recognized varieties far beyond the count of the original wild species.