Planting at the wrong time can cause tubers to rot.

Dahlias are true showpieces 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 frost of fall. Closely related to sunflowers, daisies, chrysanthemums and zinnias, the dahlia plant produces single-stemmed flowers ranging from 2 inches to 1 foot in diameter.

These easy to grow flowers are perennials in warmer climates and are considered hardy annuals in areas below USDA plant hardiness zone 8. Dahlias thrive in full sun and well-drained soil with a near-neutral pH of 6.6 to 7. If you’re considering growing this flower in your garden, these expert-approved tips on when and how to plant dahlias will set you up for success.

Meet the expert

Related: How to Grow Dahlias, a Stunning Flower That Blooms from Summer to Fall

Determine the date of the last frost in your areas

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

There are many resources available to help you determine the last frost date in your area, including your local extension office, Old Farmer’s AlmanacNOAA National Centers for Environmental InformationAnd our useful guide.

Advice

Local gardeners are familiar with the microclimate. Ask local growers about the next expected frost date to ensure a safer start to the planting season.

When to plant dahlias by seeds

If you want to give your Dahlia seeds a head start for the growing season, sow them indoors before spring. “Dahlias can be started 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 once the danger 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 the soil temperature is at least 55 to 60 degrees Fahrenheit. “Dahlia tubers will remain dormant until soil temperatures exceed 55 degrees Fahrenheit and are at increased risk of rot if planted in soil that is too cool,” says Meredith. “Some gardeners like to plant their tubers in pots indoors to give their dahlias a head start for earlier flowering.” If you do this, start the tubers four weeks before planting them outdoors.

Advice

If you are expecting a particularly wet period, avoid planting tubers. “Until they start to sprout, tubers tend to rot if there’s too much moisture,” says Mary Turner, master gardener and owner of Salt Farm Flowers.

How to plant dahlias by seeds

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, then it needs to be grown from a tuber,” says Meredith.

1. Thoroughly moisten your potting soil or seedling soil mixture before planting.

2. Fill a large tray with soil. Let the soil sit for several minutes while the water soaks in. Mix it with your hands and let it sit for several more minutes.

3. Fill a six-pack or plug tray with your moist soil. Make slight depressions with your fingertips to create ¼-inch-deep hollows.

4. Place one seed per plug in the tray by hand or with a seeder.

5. Cover the seeds by smoothing the soil over them.

6. Water your seeds again using a very gentle spray so that the soil settles. (A spray that is too strong will dislodge or even wash away the seeds.)

7. Store the seeds in an area of ​​your home where the temperature is between 65 and 70 degrees Fahrenheit. This can be achieved most easily with a heat mat or grow light.

8. Water the plants once or twice daily until germination occurs.

9. Once the seeds have germinated, remove them from the heat mats and place them in a sunny location. How often you should water depends on the location and climate of your plants. You will need to water at least once a day.

10. Once your plants have developed one or two sets of leaves and a few roots, they can be transplanted into 4-inch pots.

11. When the plants have two to three sets of leaves and you have 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 in the ground.

Warning

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

How to plant dahlia tubers

Dahlia tubers have a bulbous body on one end and a small crown with future growing points called eyes on the other. “The body and eyes are connected by a thin neck, so it’s important not to disconnect the two, otherwise your dahlia won’t grow,” says Sideris.

1. Dig a hole where you want your dahlia to grow. Moisten the soil.

2. Position the stake you will use to support your dahlia before planting the tuber to ensure you do not accidentally pierce the tuber.

3. Plant the tubers directly into the soil at a planting depth of about 4 inches. Place the tuber on the side with the eye closest to where you will plant the plant. (Placing the tuber at a 45 degree angle with the body down and the eye up works great.)

4. Cover your tuber with soil and do not water it. Once growth emerges from the soil, begin watering your plants thoroughly once a week (or less during rainy periods). 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.

Advice

Dahlia tubers do not have roots to begin with and therefore cannot absorb water. “For this reason, it is important not to overwater, otherwise you risk tuber rot,” says Sideris. As your plant grows and develops roots, increase the amount and frequency of watering according to the needs of your climate.

Frequently Asked Questions

Should I soak dahlia tubers before planting?

Do not soak dahlias before planting. “They’ll probably turn to mush shortly afterward,” says Turner.

May it be too late to plant dahlia tubers?

May is an ideal month to plant dahlias for anyone living in growing zones 3 to 6 because the soil is warmer during this time, Turner says.

Do dahlias come back every year?

Dahlias will not return to growing zones 6 and lower, almost half of the United States. “These areas can have soil that freezes during the winter and turns the tubers into mush,” Turner says. “They don’t tolerate being frozen.” If you have a winter where the ground doesn’t freeze, you can overwinter them.