Dahlias are prized by gardeners for their incredible diversity in shape, color, and size. The plant produces continuous, vibrant color from mid-summer until the first frost. This spectacular variation makes the question of “how many types” exist complex, as classification relies on the structure of the flower head, not a count of distinct botanical species.
The Foundation of Dahlia Classification
Nearly all cultivated dahlias are complex hybrids derived primarily from a handful of original species. These plants are categorized based on their physical appearance, specifically the shape and arrangement of their ray florets, commonly called petals.
Horticultural organizations, such as the American Dahlia Society (ADS) and the Royal Horticultural Society (RHS), maintain the official classification system. This system groups dahlias into categories defined entirely by the flower’s form, meaning the term “type” refers to these standardized horticultural groupings.
The Major Flower Forms
The classification system recognizes approximately 14 to 15 distinct flower forms, defined by how the petals are shaped, rolled, or arranged. These forms range from simple, daisy-like blossoms to dense, spherical heads.
The Single form is the simplest, featuring a single, open ring of flat petals surrounding a central disc. These are often preferred by pollinators. The Mignon Single is a variation with the same structure but is limited to blooms under two inches in diameter.
Anemone-flowered dahlias feature one or more rings of flat, outer petals surrounding a dense cluster of elongated, tubular central florets. This creates a prominent, cushioned dome. Collarette dahlias are similar but include a prominent “collar” of short, ruffled petals situated between the outer petals and the central disc.
Waterlily dahlias are characterized by shallow, saucer-shaped blooms with multiple rows of broad, flat, slightly cupped petals. The uniform arrangement creates a delicate, formal appearance. Peony-flowered dahlias feature open centers with two or more rows of ray petals that are typically irregular and slightly twisted, giving the flower a fuller but less structured look.
The Decorative dahlias are divided into Formal and Informal categories. Formal Decorative dahlias have flat, broad petals arranged with regularity, often slightly recurving toward the stem. Informal Decorative dahlias have petals that are curled, twisted, or wavy, creating a lush, shaggy, and less structured appearance.
Cactus dahlias are identified by narrow, pointed petals that are rolled (involute) for more than half their length, giving the bloom a spiky, quill-like texture. Semi-Cactus dahlias have petals rolled for less than half their length, making them appear softer and less sharply pointed. The Laciniated dahlia is a highly-textured form where the petal tips are split or notched, creating a fringed or frilly effect.
The Ball and Pompon dahlias are geometrically precise, fully double, and spherical forms. Ball dahlias have blunt-tipped petals that are rolled inward (involute) for at least three-quarters of their length and arranged in a tight spiral. Pompon dahlias are miniature versions, generally under two inches in diameter, featuring tightly quilled petals that create a perfectly round, button-like sphere.
The Stellar form features double blooms with long, narrow, partially involute petals, giving them a slightly star-like or spiky silhouette. Novelty forms are reserved for flowers with distinct characteristics that do not fit neatly into the other established groups.
Distinctions Based on Size and Growth Habit
Dahlias are also sorted by their physical dimensions, measured across the bloom’s widest point. There are six standard size categories:
- Giant (AA): Ten inches or more in diameter, often called “dinnerplate” dahlias.
- Large (A): Eight to ten inches.
- Medium (B): Six to eight inches across.
- Small (BB): Four to six inches.
- Miniature (M): Two to four inches.
- Micro (MC): Under two inches.
These size designations are applied to nearly every flower form.
Dahlias are also grouped by their overall growth habit, which relates to plant height. Dwarf or Bedding dahlias grow under two feet tall, suitable for containers or the front of a border. Border dahlias reach between two and four feet, while Tall dahlias can grow to five feet or more, often requiring staking.
Types Versus Named Cultivars
It is important to distinguish between the standardized types or forms and the number of individual dahlias available. The 14 to 15 forms serve as broad categories for organizing the plants. The actual number of distinct dahlias is exponentially higher due to tens of thousands of individual named varieties.
Each named variety is known as a cultivar, a plant bred and maintained for specific, unique characteristics. For instance, “Formal Decorative” is a type, but ‘Kelvin Floodlight’ is a Formal Decorative cultivar known for its giant, yellow blooms. There are currently over 57,000 registered dahlia cultivars worldwide.