Dahlias appear in a sea of blooms in myriad shapes, sizes, and colors. Available in innumerable hybrid species, dahlias are tender perennials that grow from tuberous roots. Dahlias need to be dug up and stored over the winter in the Upper and Middle South (USDA Zones 6 and 7), but they can stay in the year-round ground further south.
Dahlia fanciers divide them into many classes based on flower form. Blooms come in all colors except true blue, range from two to 12 inches wide, and are great for cutting. Plants grow one to seven feet tall. This non-invasive flower is toxic to animals and harmful to people if consumed.
- Common Name: Dahlia, Garden Dahlia
- Botanical Name: Dahlia pinnata
- Family: Asteraceae
- Plant Type: Perennial, Tuber
- Mature Size: 1-6 ft. tall, 1-3 ft. wide
- Sun Exposure: Full
- Soil Type: Well-drained, Loamy
- Soil pH: Acidic to Neutral (6.5-7.0)
- Bloom Time: Summer, Fall
- Flower Color: Red, Pink, Orange, Yellow, Green, Purple, Black, White
- Hardiness Zones: Zones 8-10 (USDA)
- Native Area: North America, Central America
- Toxicity: toxic to pets
Dahlias, grown from tubers, need warm soil and full sunlight to grow. Specific varieties of showy blooms can spread up to three feet wide, so give these flowers enough space when planting. Adding compost to the plants on new growth will help produce the best blooms. The more you feed dahlias, the more blooms they will produce. These flowers grow in gardens and containers and should have regular watering. Dahlia varieties with full, tight blooms (such as formal decorative, ball, and pompon types) make excellent, long-lasting cut flowers.
Dahlias wilt in the hot sun and often stop blooming when the temperature tops 90°F. Big-flowered kinds are the most heat sensitive. In very sunny climates, try shielding dahlias during the summer beneath a polyethylene shade cloth, which blocks half of the sunlight—beach umbrellas also help to shade plants at midday. Dahlias grow best in light afternoon shade.
A dahlia plant prefers moist, well-drained soil that contains lots of organic matter. Loamy and sandy soils are also good for growing dahlias. In January and February, combine leaves and mushroom compost with the ground. Plants growing in good soil don’t need very much fertilizer, but soils that don’t have high enough levels of phosphorus need more fertilization. The more feeding dahlias receive, the more showy and abundant the blooms will be.
Don’t water dormant roots after planting until sprouts show aboveground, or the roots will probably rot. After that, water the plant deeply only when plants look wilted in the early morning before the sun hits them, usually once or twice a week. The soil should never be dry.
Temperature and Humidity
Dahlias dislike long, hot summers, making them challenging to grow in the Lower South (Zone 8), but they struggle to grow in the Coastal and Tropical South (Zones 9 and 10). Wait until the final spring frost passes to plant tubers. If moving dahlias indoors for the winter, keep the plants in a humid area that never drops below 60°F.
Use a fertilizer within the first 30 days of planting tubers to promote growth. The more you fertilize dahlias, the more showy and numerous blooms. Stop fertilizing plants when temperatures drop if you plan to keep dahlias over the winter. Since fertilization promotes growth, you shouldn’t force new flowers during dormancy. Use a fertilizer low in nitrogen and high in phosphorus—Water frequently.
Types of Dahlias
Dahlias are available in single blooms, cactus, anemone-flowered, pompon and ball, and numerous hybrid variations. Some popular dahlia varieties include:
- Café Au Lait Dahlia (Verbena bonariensis): This dahlia blooms around six to ten inches wide in a creamy, peach, and rose color. This decorative dahlia is popular for cuttings and features broad, flat-tipped petals.
- Cornel Dahlia (Dahlia «Cornel»): A pompon and ball dahlia that features deep, rich, perfectly symmetrically shaped petals. These plants grow well in containers.
- Cactus Dahlia (Dahlia «Pianella»): This cactus dahlia, known for its spiky, pointed petals, features bold blooms in mid-summer. This dahlia is susceptible to cold weather, so move it indoors in early autumn.
Dahlias need specific pruning to encourage new blooms. Pinching the flower tops after at least three sets of branches appear. During the growing season, dahlias produce many flowers, which will help create a fuller plant. Dahlias need deadheading after blooms fade to help encourage new growth. Use a sharp tool to make clean cuts, or use your fingers.
After blooms emerge, restricting growth to three or five flowers for every branch will help every flower get the necessary nutrients. However, you can allow up to 10 flowers if you want smaller flowers.
There are a few ways to propagate dahlias. The most common methods include cuttings and dividing tubers. Here is how to propagate dahlias from cuttings:
- After dahlias show at least three inches of growth in the spring, use a sharp knife to cut below the sprout and partially into the tuber.
- Remove leaves on the lower portion of the branch.
- Dip the end of branches into a rooting hormone and place stems into a container filled with potting soil. Aerate the soil by poking a few holes in the top of the container.
- Water deeply and allow it to drain completely. Keep the soil wet and place the plant in full sunlight for two to three weeks.
- After new growth emerges (at least three inches tall), you can transplant the cuttings to their final growing location as long as the temperatures are above 60°F and the last frost passes.
A second way to propagate dahlias is by dividing the tubers. Here is how:
- During a dahlias» dormancy, dig up tubers by encircling the plant and not damaging the roots.
- Place tubers with eyes (small bumps signaling next year’s growth) in a warm, moist area for a few days. After, the eyes should begin to sprout.
- Arrange a few containers filled with potting soil. Supplement the soil with vermiculite to help retain moisture and increase drainage. Add water and mix to create moist soil (not too wet or dry).
- Place tubers with sprouting or swelling eyes in a container. The eyes should face upwards. Cover with a least one inch of soil and place near a sunny window.
- Water, so the soil is never dry. Continue until dahlias sprout with at least three sets of branches. During the spring, plant dahlias in their final location.
How to Grow Dahlias From Seed
It is possible to grow dahlias from seeds you collect from the previous year’s blooms in the fall after they dry up. Inside the old blooms, seed pods are available around one month before the first frost. Additionally, dahlias seeds are available at garden stores for purchase.
Grow dahlias by seed using a seed-starting tray. Start planting seeds in late winter or around two months before the final frost.
Keep the seed-starting tray in a sunny location and the soil moist until the seeds begin to sprout.
After a few sets of leaves form, dahlias can move to containers where they can continue to grow until it’s time to plant them in the spring.
Potting and Repotting Dahlias
April and May are good months. Plant the roots about one foot deep, spacing tall selections (over four feet tall) four to five feet apart and shorter ones one to two feet apart. Containers at least one foot tall and wide should provide enough room for dahlias to grow without needing to move them to a new pot. Use garden stakes, driven to the bottom of the container, to help tall plants stay upright.
At the sign of the first frost, it is time to move dahlias indoors to keep them alive over the winter. Cut branches to around six inches and gently room tubers from the ground. Clear the tubers with rotten roots using a clean, sharp knife. If the tubers are withered, these are typically dead and need removing, but be careful cleaning viable tubers. Leave tubers to dry for a few weeks before storing them in a cool, dry area for the winter. Occasionally, spray tubers with water throughout their dormancy and use compost to help them from completely drying out. Once the final frost passes, you can move the tubers back outside.
Common Pests & Plant Diseases
Dahlias are relatively susceptible to pests. The most common infestations include slugs, aphids, earwigs, caterpillars, mealybugs, spider mites, and leafhoppers. Slugs are most destructive to dahlias during the early growing season because the leaves are more tender. Dry soil helps prevent slugs from attacking healthy leaves. Pruning and maintaining healthy blooms will deter earwigs, while you can manage leafhoppers with insecticidal soap or spray.
Some diseases that impact dahlias include stem rot, Botrytis blight, and powdery mildew. Maintaining the correct amount of water and balancing soil and sunlight helps prevent these diseases from forming. Powdery mildew grows on leaves when too wet, so watering in the morning will help the plants have enough time to dry.
How to Get Dahlias to Bloom
The best way to get dahlias to bloom is by giving them proper light and water. Additionally, since some dahlias grow very tall, using garden stakes will help them to stay upright. Dahlias require a lot of fertilizer to continue growing, so adding a compost fertilizer every few weeks will promote new growth. Finally, remember to deadhead wilted or faded blooms so they don’t take away nutrients from healthy flowers.
Common Problems With Dahlias
Plants Not Flowering
One reason dahlias might not bloom is because of a leafhopper infestation. Leafhoppers cause Aster yellows, a disease that forms on the leaves instead of flower buds. There is no way to cure this disease, so prevention is best.
Leaves Turning Brown
If you notice that your dahlia leaves are turning brown, this usually signifies a disease is present. Remove diseased branches with a sharp pruning tool. The first sign is wilting and browning leaves, so try to prevent it from spreading to the plant’s roots so you don’t need to remove the entire plant.