Dahlias are tall plants that stand out in the garden; some varieties have gigantic blooms reminiscent of sunflowers. If you're in the market for dahlias, wait until after the last frost to purchase them so you can immediately plant them outdoors. If you live in United States Department of Agriculture (USDA) hardiness zones 8 or warmer, most dahlias are winter hardy in your climate. In colder climates, you'll have to dig up the underground root stems, called tubers, in the fall and store them during the winter months for planting the following spring.
Choose which kind of dahlias you wish to purchase. Some dahlia varieties are around 2 to 3 feet tall, while others are as tall as 6 feet. Some blooms are only 2 inches in diameter while others are 12 inche. Decide if you're willing to go to the trouble of staking them. The taller and larger the blooms, the more likely the stems will bend as they grow and will need staking.
Decide how many dahlias you'll need to fill your space. Even though dahlias vary in height and bloom size, most are typically spaced about 2 to 3 feet apart.
Decide if you want to buy the dahlias as established plants or as tubers (their underground root stems). Tubers are less expensive than established plants and offer more variety. Established plants are nursery grown and may be in bloom, or ready to bloom, when you buy them. Conversely, tubers will take two to three weeks to grow and do not bloom until summer.
Purchase your dahlias from either a local nursery or purchase them online or through a catalog. In person, choose established plants that look healthy and have leaves that are not wilted, brown or spotted. Inspect that the dahlias' stems. Plants should be strong and erect. Select tubers that are firm, not rotted and not shriveled. If you order dahlias online or through a catalog, check to see if there is a warranty that comes with them.