When to Plant Dahlias for the Most Beautiful Blooms, According to Gardening Experts

Dahlias are true showstoppers in the garden, and it's easy to see why. Their unique petal arrangements come in many colors, shapes, and sizes, providing a spectacular display from summer until the first autumn frost. Closely related to sunflowers, daisies, chrysanthemums, and zinnias, the dahlia plant produces single-stem flowers that range from 2 inches to 1 foot in diameter.

These easy-to-grow flowers are perennial in warmer climates and are considered hardy annuals in areas below USDA hardiness zone 8. Dahlias thrive in full sunlight and well-drained soil with an almost neutral PH of 6.6 to 7. If you’re planning on growing this flower in your garden, these expert-approved guidances on when and how to plant dahlias will set you up for success. 

Determining the Last Frost Date in Your Areas

Dahlias are tender annuals, meaning you’ll need to wait until the danger of frost has passed before you can start planting outside. “The last frost date is the average date of the last light freeze in the spring,” says Caitlin Meredith, owner of Quietside Flower Farm in Downeast Maine. “A light freeze is typically temperatures ranging from 29 to 32 degrees Fahrenheit, which will kill tender annuals like dahlias.”

There are plenty of resources available to help you determine the last frost date for your area, including your local extension office, the Old Farmer’s Almanac, the NOAA National Centers for Environmental Information, and our helpful guide.

When to Plant Dahlias by Seed

If you’d like to give your Dahlia seeds a head start to the growing season, sow them indoors before spring. “Dahlias can be started indoors from seed six to 10 weeks before your desired planting date,” says Marina Sideris, co-owner of Dooryard Farm, an organic farm in Camden, Maine. Alternatively, you can sow dahlia seeds directly outside after the risk of frost has passed in spring. 

When to Plant Dahlia Tubers

Dahlia tubers can be planted in your garden once all danger of frost has passed, and soil temperatures are at least 55 to 60 degrees Fahrenheit. “Dahlia tubers will remain dormant until soil temperatures are above 55 degrees Fahrenheit and have an increased risk of rotting if planted in soil that is too cool,” says Meredith. “Some gardeners like to start their tubers in pots indoors to give their dahlias a head start for earlier blooms.” If you do this, start the tubers four weeks before you plan to plant them outdoors.

How to Plant Dahlias by Seed

Growing dahlias from seed is a good way to get a head start on the growing season. But know that dahlias grown from seed are not true to their parent plant due to cross-pollination. “If you want to grow a clone of a specific cultivar, then it needs to be grown from a tuber,” says Meredith.

1. Moisten your potting soil mix or seedling soil mix thoroughly before planting. 
2. Fill a large bin with the soil. Let the soil sit for several minutes while the water sinks in. Mix it up with your hands, and let it sit for several more minutes.
3. Fill a six-pack or plug tray with your moistened soil. Make gentle depressions with your fingertips to create ¼-inch-deep divots.  
4. Place one seed per divot in the tray by hand or with a seeder.
5. Cover the seeds by smoothing the soil back over the top.
6. Water your seeds once more using a very gentle spray so that the soil will settle. (Too hard a spray will dislodge or even wash away the seeds.)
7. Keep the seeds in an area of your home where the temperature is between 65 to 70 degrees Fahrenheit. This can be most easily accomplished with a heat mat or grow light.
8. Water seedlings once or twice per day until germination occurs. 
9. Once the seeds have germinated, move them off the heat mats into a sunny location. How often you need to water depends on your seedlings' location and climate. You’ll need to water at least once daily.
10. Once your seedlings have developed one or two sets of leaves and some roots, they can be transplanted into 4-inch pots.  
11. When the seedlings have two to three sets of leaves, and you are at least two to three weeks past your last frost date, move them outside to acclimate to the environment for a few days, then transplant them into the ground.  

How to Plant Dahlia Tubers

Dahlia tubers have a bulbous body at one end, and a small crown with future growth points called eyes at the other. “The body and eyes are connected by a slender neck, so it’s important not to disconnect the two, or your dahlia will not grow," says Sideris.

1. Dig a hole where you wish to grow your dahlia. Moisten the soil.
2. Place the stake that you will use to support your dahlia before planting the tuber to ensure you don’t accidentally puncture the tuber.
3. Plant tubers directly in the ground at a planting depth of about 4 inches. Position the tuber on its side with the eye closest to where you will stake the plant. (Placing the tuber at a 45-degree angle with the body down and the eye up works fine.)
4. Cover your tuber with soil, and do not water it. Once growth emerges from the soil, begin watering your plants deeply once per week (or less during periods of rain). Water until the plants are at least 8 inches tall with 3 or 4 sets of leaves. If you live in a hot, dry climate, you may need to water more than once per week.