There are hundreds of dahlia varieties, and they fall into more than a dozen classifications based on the size and style of their blooms. Bedding varieties grow 18 inches tall and are appropriate for borders, while cutting varieties grow more than 5 feet tall and have blooms 10 inches wide.
Several dahlia cultivars have layers of rounded petals that curl inward to look like a large pompom. Other varieties have long, thin petals, making them resemble orchids. Tall dahlia varieties require stakes to support their heavy blooms, and pinching off the top of young plants encourages blooms to grow more fully.