When should you plant dahlias for the most beautiful flowers, according to garden experts?

If you plant at the wrong time, the tubers can rot.

Dahlias are real eye-catchers in the garden and it is easy to understand why. Their unique petal arrangements come in many colors, shapes and sizes and provide a spectacular display from summer until the first fall 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.

This 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 sun and well-drained soil with a near neutral PH of 6.6 to 7. If you plan to grow this flower in Growing Your Garden These expert-approved guidelines on when and how to plant dahlias will set you up for success.

Meet the expert

Determining the last frost date in your area

Dahlias are tender annuals, which means you must wait until the danger of frost has passed before planting outdoors. “The last frost date is the average date of the last light frost in the spring,” says Caitlin Meredith, owner of Quietside Flower Farm in Downeast Maine. “A light freeze is typically a temperature ranging from 29 to 32 degrees Fahrenheit, which will kill sensitive annual plants such as dahlias.”

There are plenty of resources available to help you determine the last frost date for your region, including your local extension office, the Old farmer’s almanacthe NOAA National Centers for Environmental InformationAnd our helpful guide.

Tips

Local gardeners know the microclimate well. Ask local growers for the next expected frost date to ensure a safer start to the planting season.

When should you plant dahlias from seed?

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

When should you plant dahlia tubers?

Dahlia tubers can be planted in your garden once all danger of frost has passed and the soil temperature is at least 55 to 60 degrees Fahrenheit. “Dahlia tubers remain dormant until soil temperatures rise above 55 degrees Fahrenheit and are at increased risk of rot 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 on earlier flowering.” If you do this, start the tubers four weeks before planting them outdoors.

Tips

If you expect a particularly wet period, wait before planting tubers. “Until they begin to germinate, the tubers are susceptible to rot from too much moisture,” says Mary Turner, gardener and owner of Salt Farm Flowers.

How to Plant Dahlias by Seed

Growing dahlias from seed is a good way to get a head start on the growing season. But be aware 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, it must be grown from a tuber,” says Meredith.

Moisten your potting soil mix or seedling soil mix thoroughly before planting.
Fill a large container with the soil. Let the soil sit for several minutes while the water sinks in. Mix it with your hands and let it sit for a few more minutes.
Fill a six-pack or plug tray with your moistened soil. Make gentle depressions with your fingertips to create recesses ¼ inch deep.
Place one seed per turf in the tray by hand or with a seeder.
Cover the seeds by smoothing the soil over the top again.
Water your seeds again with a very gentle spray to allow the soil to settle. (A spray that is too harsh will loosen or even wash away the seeds.)
Store the seeds in an area of your home where the temperature is between 65 and 70 degrees Fahrenheit. This is most easily accomplished with a heat mat or grow light.
Water seedlings once or twice a day until they germinate.
Once the seeds have germinated, move them from the heat mats to a sunny location. How often you need to water depends on the location and climate of your seedlings. You should water at least once a day.
Once your seedlings have developed one or two sets of leaves and some roots, they can be transplanted into 4-inch pots.
When the seedlings have two to three sets of leaves and you are at least two to three weeks after the last frost date, move them outside to acclimate to the environment for a few days and then transplant them into the ground.

Warning

Dahlias are toxic to dogs and cats So keep this in mind when choosing your growing location.

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 end. “The body and eyes are connected by a slender neck, so it is important not to disconnect the two or your dahlia will not grow,” says Sideris.

Dig a hole where you want your dahlia to grow. Moisten the soil.
Place the stake you will use to support your dahlia before planting the tuber to ensure you don’t accidentally puncture the tuber.
Plant the tubers directly in the ground at a planting depth of about 4 inches. Place 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.)
Cover your tuber with soil and do not water it. Once growth emerges from the ground, begin watering your plants deeply once a 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 a week.

Tips

Dahlia tubers initially have no roots and therefore cannot absorb water. “For this reason, it is important not to overwater, otherwise you risk rotting the tuber,” says Sideris. As your plant grows and develops roots, increase the amount and frequency of watering depending on your climate.

Frequently asked questions

Should I soak dahlia tubers before planting?

Do not soak dahlias before planting them. “They will most likely turn to mush shortly after you do that,” says Turner.

Is May too late to plant dahlia tubers?

May is a great month to plant dahlias for anyone in growing zones 3 through 6 because the soil is warmer at this time, Turner says.

Do dahlias come back every year?

Dahlias do not grow back in growing zones 6 and below, which is almost half of the US. “Those areas can have soil that freezes in the winter and turns the tubers to mush,” says Turner. “They don’t tolerate being frozen.” If you have a winter where the ground does not freeze, you can overwinter them.