How to Plant and Grow Dahlia

Dahlias, grown for their beautiful flowers, come in all colors except the elusive blue. They bloom nonstop from summer until frost in a variety of shapes and sizes. As cut flowers, they will last several days, making them excellent for homegrown bouquets. 

A few of the most exciting dahlia types include the cactus form with its needlelike petals and the ball (or pompom) types that have small, spherical blossoms. Dinner plate dahlias have enormous blooms that can measure nearly a foot across. And some cultivars have burgundy foliage that provides a pretty backdrop for the showy flowers. 

Dahlia plants are hardy in warmer regions, but can be saved year after year in colder areas by digging up their large, tuberous roots in the fall and replanted in the spring.

Where to Plant Dahlia

Dahlias can be planted in any sunny location with fertile, moist, well-draining soil with almost neutral pH (6.6 to 7). Because dahlias can range from 1 foot for compact dahlias to more than 3 feet for taller varieties, always consider the mature height of the plant. Also take into consideration that dahlias won’t survive the winter below zone 8 and need to be dug in the fall. If you want to save them for next year, select a location with easy access.

How and When to Plant Dahlias

Dahlias can be planted directly in the ground in late spring when the ground has warmed to at least 60 degrees F and all danger of frost has passed.

To get a head start on the growing season, you can also start them indoors early in the early spring. Plant the bare-root tubers in a pot of well-drained potting soil about six weeks before the last frost. Place the pots in a warm, sunny window. Keep the soil evenly moist, but not wet to avoid rot. Once the foliage emerges and the danger of frost has passed, transplant them in the ground.

Dig a 6- to 8-inch deep hole for each tuber and add some compost or bonemeal. Place the tuber in the hole with the growing points (the “eye” that looks like a potato sprout) facing up. Loosely cover the tuber with 2 to 3 inches of soil. Don’t water it immediately but wait for the sprouts to poke through the soil. As the sprouts grow, gradually add more soil until the hole is filled.

The spacing depends on the variety. Smaller types are fine with 1 foot between them but larger dahlias should be spaced at least 2 feet apart.

Most dahlias need staking. To prevent injuring the tubers, it is best to put the support in the ground before planting the dahlias.

Dahlia Care Tips

Light

Dahlias need full to partial sun. Full sun encourages more upright plants that need less staking, but these plants will still flower in part shade. If planted in shadier areas, the foliage tends to look greener than burgundy on dark-leaved cultivars. Partial shade, especially during the afternoon hours, is a good idea in hot climates.

Soil and Water

The soil needs to be well-drained, rich, and moist with a neutral pH between 6.6 and 7.

Right after planting, dahlias don’t need a lot of water. Overwatering in the early stages can cause the tubers to rot. Once they are established, they need about 1 inch of water per week, more in hot weather. Always water them at the base, never from overhead. After a heavy rain, large flowerheads might be filled up with so much water that they bend over so the plants might need a little help to gently shake it out.

Temperature and Humidity

Dahlias are native to the mountains of southern Mexico and Central America where the summers are warm but not excessively hot. They are not frost-hardy and below zone 8, they can only be grown as annuals. Dahlias tolerate humidity.

Fertilizer

Because you want your dahlias to bloom lushly over an extended period of time, you should regularly feed them a bloom-boosting fertilizer that is low in nitrogen and high in phosphorus. For frequency, follow the product directions but stop fertilizing the plans towards the end of August so the tubers can get ready for dormancy.

Pruning

There are three things you can do to help dahlias grow into strong plants with big flowers up to the first frost:

To encourage a bushier growth habit, pinch off the top 3 to 4 inches of the center branch when the plants are about 1 foot tall. For fewer but larger flowers, you can also remove the two smaller side buds next to the central bud in each flower cluster. And, finally, deadheading the flowers extends the bloom.

Potting and Repotting Dahlia

Dahlias, especially dwarf-size types, grow equally well in the ground or containers. Plant them in pots at least 16 inches in diameter and 12 inches deep so that you can gradually add soil to the tubers as they start growing.

Make sure the container has large drain holes and use a combination of well-draining potting mix and compost.

Like all potted plants, they will need more watering than dahlias in the landscape, as well as more frequent fertilizer applications as the fertilizer washes out of the pot.

Pests and Problems

Slugs and snails can be a problem for young plants. They are especially attracted by wet soil around the plants.

Insects that feed on dahlias include Japanese beetles, aphids, thrips, and earwigs. 

Dahlias can also get a whole range of fungal diseases, including gray mold on the tubers. In the worst case, you’ll have to restart with newly purchased tubers from a reliable source each year. Many pathogens survive in the soil over the winter so to break the disease cycle, make sure that you don’t plant dahlias in the same location for at least three years.

How to Propagate Dahlia

The commonly recommended method to propagate dahlias is from cuttings. But first you need to grow the cuttings from a tuber. In the late winter or early spring, vertically place healthy tubers in a pot filled with damp potting mix. The pot needs to be wide enough to amply fit the length of the tuber but you can fit a few in a single pot. Cover the tubers lightly with potting mix but leave one end (the “neck) of the tubers exposed. Keep the potting medium moist but not soggy in a warm, bright place, ideally under growth lights, until the tuber grows a green shoot with at least two true leaves.

Cut the shoot off the tuber at the base, where it is connected to the tuber (leave the tuber in the pot, as it will hopefully grow more shoots). Dip the cut end in rooting hormone, then plant it in its own pot with potting mix. Keep the cutting well watered at a temperature of 65 to 75 degrees F. Placing the cutting under grow lights for at least 14 hours a day will help its rooting and growth.

Types of Dahlia

Dahlias are classified into 14 groups based on blossom type. Here are some popular varieties:

'Arabian Night' Dahlia

'Arabian Night' offers deep maroon, almost black, blooms that open several to a stem and grows 3 feet tall. Zones 8-10

'Bishop of Llandaff' Dahlia

This variety bears dark fiery-red blooms shaped like small peonies that glow against deep chocolate foliage. This prize-winning dahlia grows 4 feet tall. Zones 8-10

'Radar' Dahlia

Dahlia 'Radar' is a large, informal decorative-type that features deep plum-purple petals tipped in white. It grows 5 feet tall. Zones 8-10

'Chinese Lantern' Dahlia

Dahlia 'Chinese Lantern' bears huge, bittersweet orange blooms that appear abundantly on branching stems from midsummer to fall. It grows 3 to 4 feet tall. Zones 8-10

'Jessica' Dahlia

'Jessica' is a cactus-type dahlia that unfurls butter-yellow petals tipped in flame red. It grows 5 feet tall. Zones 8-10

'Envy' Dahlia

Dahlia 'Envy' offers large, deep red blooms. It grows 5 feet tall. Zones 8-10

'Duet' Dahlia

This type of dahlia features medium-size red blooms tipped in white. It grows 3 feet tall. Zones 8-10

'Penn's Gift' Dahlia

'Penn's Gift' is known for its large pink flowers that can reach more than 1 foot across. It grows 5 feet tall. Zones 8-10

'Pam Howden' Dahlia

Dahlia 'Pam Howden' is an abundantly blooming variety that features 2- to 4-inch-wide water-lily style flowers in an orange-yellow-coral blend. The plant grows 4 feet tall. Zones 8-10

Star Gazer Series Dahlia

Dahlia Star Gazer Series is a dwarf, cactus-flowered dahlia that produces spiky blooms in nine colors, including golds, yellows, deep reds, fuchsia, lavender, and bicolors with white. The multibranching plants grow 16 inches tall. Zones 8-10

'Victory Dwarf' Dahlia

Dahlia 'Victory Dwarf' is a small, single-flowered variety that produces gemlike blooms in red, orange, yellow, and white. It grows to 8 inches tall. Zones 8-10

'Sharon Ann' Dahlia

This variety of Dahlia is a semicactus-type dahlia with spiky light lavender petals unfolding from a creamy white center. It grows 5 feet tall. Zones 8-10

'White Fawn' Dahlia

'White Fawn' offers pristine white blooms, up to 4 inches across, on a plant that grows 4 feet tall. Zones 8-10

'SB's Sunny' Dahlia

Dahlia 'SB's Sunny' is an award-winning variety that features layers of lemon yellow petals tightly clustered on a round, pom-pom flower. It grows 4 feet tall. Zones 8-10

'Suffolk Punch' Dahlia

This type offers cherry-red flowers with an iridescent pink overtone bloom on dark stems. It grows 4 feet tall. Zones 8-10

'Survivor' Dahlia

Dahlia 'Survivor' is a large decorative type that features deep rose-pink blooms that can reach 12 inches across. It grows 5 feet tall. Zones 8-10