As the name suggests, a mini dahlia is a low-growing dahlia with small flowers. The plants are shorter than standard dahlias, but the main determining factor for a mini dahlia is flower size. Dahlias come in a stunning array of flower shapes, colors and sizes, and mini dahlias are no exception. There are numerous mini dahlia cultivars from which to choose.
Mini dahlias commonly grow between 14 and 20 inches (about 35 to 50 cm) tall, with flowers ranging from 2 to 4 inches (about 5 to 10 cm) in diameter. Dahlias with flowers larger than 4 inches (about 10 cm) are not considered minis. Standard dahlias can grow up to 4 feet (about 1.2 m) tall. The flowers on standard dahlias range from 4 inches (about 10 cm ) in diameter to a huge 10 inches (about 25 cm) diameter, with most falling between 6 and 8 inches (about 15 to 20 cm) around.
Dahlias, and their mini cultivars, are native to Mexico. The plants are frost tender and grow from tuberous root systems. In areas with mild winter frosts, the plants die back with the cold weather but come back the following spring. In frost-free climates, mini dahlias grow year-round. In areas where winter temperatures drop below 10 to 20°F (about -12 to -6°C), cold weather will kill the roots.
For best flowering, mini dahlias should be planted where they will get at least eight hours of direct sun a day. They can tolerate less sun, but flowering will often be diminished. The mini dahlia should only be planted in soil with good drainage. The fleshy tuber root system is prone to root rot when planted in heavy, wet soil.
Though good drainage is critical, sufficient moisture is also important. The mini dahlia will grow and bloom most vigorously when the soil around the roots is kept consistently moist. The plants should be fertilized three to four weeks after planting, and then every three to four weeks throughout the growing season. A 5-10-10 or 10-20-20 fertilizer is appropriate for growing mini dahlias.
Spring, after the soil warms to 60°F (about 15°C) or above is the best time to plant a mini dahlia. The tubers should be planted 1 to 2 inches (about 2.5 to 5 cm) deep in the soil. It is important to hold off on watering until the first green shoots appear above the soil line. In cold climates, the tubers should be lifted from the soil in late fall and stored indoors at 40 to 50°F (about 4 to 10°C).