This perennial's silvery leaves won't compete with dahlias' massive blooms. Artemisia is a popular companion plant since it behaves oppositely. While dahlias bloom, its soft, cloud-like appearance can be relaxing. Having a natural slug repellant is usually beneficial.
Snapdragon's petite, throated flowers of white, yellow, pink, red, orange, peach, purple, and violet are appreciated in dahlia gardens for their early bloom time. Snapdragons provide color during midsummer, when dahlias are just starting up.