How to Grow Dahlias—a Showstopping Flower That Blooms From Summer Through Fall

It's easy to fall in love with dahlias. Their showy flower heads fill gardens with beauty from mid-summer throughout fall—and they look just as beautiful in a vase after you cut and arrange them. Often grown as annuals (but can also be grown as perennials in warmer climates), dahlias come in various sizes, colors, and shapes. From tall varieties with massive blooms to short types that can be grown as a border flower, this plant is as versatile as it is beautiful. To ensure this beloved flower thrives in your landscape, we're sharing everything you need to know about growing dahlias, including how to plant, propagate, prune, and care for them.

When to Plant Dahlias

Dahlias love warm temperatures, so they shouldn't be planted outside until the ground warms up. Typically, dahlias can be planted in late May or June depending on your location. "If you can plant your tomatoes outside, you can plant your dahlias too," says Peggy Anne Montgomery, horticulturist representing Flowerbulb.eu.

How to Plant Dahlias

When planting dahlias, choose a sunny location with very rich, well-drained soil, says Montgomery. Be sure to space the tubers out far enough to allow for proper air circulation.

1. Dig a hole that's 4 to 6 inches deep.
2. Plant the tubers in the hole with the eyes facing upwards.
3. Cover the tubers with 2 inches of soil and water well. Don't water again until the sprout shows above the soil.
4. Place stakes in the soil if you're growing taller dahlias.

Starting Dahlia Seeds Indoors

If you want to get a head start on your dahlia growing season and encourage earlier blooming, consider starting dahlias indoors from seed. "Dahlias can be grown from seed started indoors 4 to 8 weeks before you want to plant them outdoors," says Montgomery. "A temperature of 65 to 70 degrees Fahrenheit is best." Dahlia seeds should germinate in about two weeks.

1. Fill trays or pots with seed starting mix and moisten it.
2. Make 1/2-inch-deep holes in each tray or pot, placing a seed in each pot.
3. Cover the seeds lightly with seed starting mix.
4. Water gently and place the seeds in a warm spot with plenty of light.

How to Care for Dahlias

Dahlias need full sun, well-drained soil, and adequate water to ensure they survive.

Sunlight

Dahlias love sunlight. Generally, these plants like six to eight hours of full sun, but in very hot climates some protection from the hottest afternoon sun is appreciated, says Montgomery.

Water

Water dahlias when they're first planted, then wait to water the tubers again until they sprout above the ground, says Rowe. Once they sprout, water regularly and deeply, being careful to let the soil dry out between waterings. "Continuously wet soil causes tubers to rot," says Montgomery, adding that you should water the soil and not the plant, which can promote foliar diseases.

Soil

Dahlias prefer soil that is well-drained and very rich. Montgomery recommends amending your soil with compost and manure before planting the tubers.

Fertilizer

Fertilize dahlias once a month with an organic fertilizer that is high in potassium to promote flowering, says Montgomery. She recommends using a fertilizer with a nitrogen, phosphorus, and potassium (NPK) ratio of 5-10-10 or 10-20-20.

Types of Dahlias

There are thousands of dahlia varieties to choose from, but these are some common types to grow in your garden.

• Pom Pom dahlias: Round whimsical flowers that are typically 2 to 4 inches across.
• Cactus and semi-cactus dahlias: These varieties have tightly curled petals that give them a distinct look, says Montgomery.
• Border dahlias: Ideal for the front of the border in a garden, these varieties grow to be 12 to 18 inches tall and don't need to be staked.
• Decorative dahlias: This is the largest category of dahlias with the widest variety of colors and styles, says Montgomery.
• Dinnerplate dahlias: Known for their large flower heads, dinnerplate dahlias grow 4 to 5 inches tall and the flowers can be 10 inches across or more, says Montgomery.
• Peony flowered dahlias: The blooms on these dahlias are very full and reminiscent of the peony. They grow to be 2 to 3 feet tall.

How to Prune Dahlias

Pinching your dahlias will increase flowering. Do this when the flowers are about 8 to 12 inches tall or have three sets of leaves. Use sharp, clean pruners and cut off the top 3 to 4 inches of stem, just above a set of leaves, says Montgomery.

In addition to pinching, also remove spent flowers so the plant doesn’t put its energy into producing seeds instead of flowers. "It can also be a good idea to trim off the bottom leaves to promote good air circulation," says Montgomery.

How to Propagate Dahlias

One of the easiest ways to propagate dahlias is through stem cuttings, which you can do during the growing season.

1. When your dahlia plant forms a shoot with two sets of leaves, cut the shoot off at the base with pruners.
2. Remove the lower leaves.
3. Dip the bottom of the cutting in a rooting hormone.
4. Fill a container with drainage holes with potting soil amended with compost.
5. Poke a hole in the soil and plant the cutting in the soil.
6. Water the soil well and place the pot in a warm location with full sun.

Growing Potted Dahlias

Dahlias can also be grown in containers. "If you plan to grow your dahlia to full-size in a container, ensure that it's large," says Kate Rowe, a gardening expert and the former owner of a dahlia farm in Petaluma, Calif. "It should be 15 inches across the top and at least 12 inches deep so your dahlia has space to grow a large root ball."

When planting dahlias in pots, prioritize excellent drainage. "Make sure the container has good drainage holes in the bottom so the water passes through the pot and doesn't collect," says Rowe. The tubers will rot in standing water. Wait to bring potted dahlias outside until your area's last frost has passed.

1. Fill a pot with moist, well-draining potting soil amended with compost.
2. Dig a hole that's about 2 to 3 inches deep.
3. Plant one tuber with the eyes facing upwards per pot.
4. Place your pot in a warm, sunny spot, and don't water until the sprout shows above the soil.

How to Overwinter Dahlias

Dahlias may survive winter in a mild zone 7 and warmer, but they will need to be dug up and overwintered in colder climates. "After the first hard frost cut back all of the stems to about 3 to 4 inches," says Montgomery. Then dig up the tubers, brush off all of the soil, and let them air dry for a few days if it isn’t freezing. "Store them in ventilated cardboard boxes or plastic storage tubs," she says. "Store in a cool (40 to 50 degrees Fahrenheit) dark, humid place like an unheated basement."

How to Harvest Dahlias

Harvesting dahlias for flower arrangements is very easy. "Cut the flowers when they are fully open. Unlike most flowers you cut in bud, dahlia flowers do not open after being cut," says Montgomery. "Cutting in early morning when plants are fully hydrated is best."

Common Problems With Dahlias

Pests can be a common issue you may encounter when growing dahlias. Common insects include snails, slugs, leafhoppers, spider mites, and thrips. Many of these insects can be treated by rubbing the foliage of your dahlias with insecticidal soap, says Montgomery.

Dahlias are also susceptible to some diseases, especially powdery mildew. To avoid this, it's essential to promote good air circulation and keep the foliage as dry as possible, says Montgomery. Also, avoid overwatering your dahlias, which may cause root rot. Water them deeply and then wait for the soil to dry out before watering again.