Single dahlias, according to American Dahlia Society standards, are open dahlias with a single row of uniform, evenly spaced ray florets in a flat plane surrounding the disc flowers. Dahlias of the single class are two inches or more in diameter. Mignon single dahlias are the minis of the species. The standards are the same as for the singles, and the mignon singles have been a separate ADS classification since 1977. Since it was developed in Holland in the late 19th Century, the mignon single has been variously called miniature, Tom Thumb, Lilliput, top mix, dwarf single, bedding dahlia and dwarf mignon.
Early single hybrids originated with Dahlia coccinea varieties crossed with one or two other dahlia species. These dahlias were called "simplex" and were described as two- to three-inch blooms with long, tapered rays that gave the flower a starry appearance on bushes only two or three feet high. All such varieties in the first decades of the 1800s were red, crimson or scarlet; only when growers added Dahlia crocea and D. barkerae to the gene pool toward the middle of the century did the hybrids that resulted show a range of colors more like those we expect in dahlias today, and that color range was incorporated into both the single and double dahlias that were developed by the thousands as dahlias grew in popularity.
An example of color variation in single dahlia flowers is Union Jack, a British introduction hybridized in the late 1800s and registered early in the 20th century. It is a great garden dahlia but unlike many of its class, has poor stems for cutting and arranging. Popular at Lynch Creek Farm is Kelly, a single blend of pink and red with two- to four-inch flowers on a three-foot plant.
Pretty mignon singles include Baby Yellow, a bright yellow dwarf dahlia that grows just a foot high and makes a great pot plant, and its counterparts Baby Red and salmon-orange Inflammation. Another tiny mignon single dahlia is Lupin Tori, a white dahlia with tapered ray florets and dense foot-high plants that look like they're spangled with stars.
By the later decades of the 1800s many growers referred to "garden dahlias," the single dahlia flowers, and "show dahlias," the doubles. Although some single dahlias like Union Jack were popular since their development, many American Dahlia Society shows didn't have a single class until the 1970s. Now the single forms are popular for floral design and border gardens, and single dahlias are showcased in many exhibitions and flower shows.