How to Grow and Care for Dahlia Flowers

Dahlias are celebrated for their diversity in color and form, with a bloom season that stretches from midsummer until the first autumn frost. Their ability to produce an abundance of flowers makes them a favorite for many garden designs, from formal beds to casual cottage-style plantings. When given proper attention, they are a reliable perennial performer.

Dahlia Varieties and Forms

Dahlia classifications recognize numerous forms, distinguished by the arrangement and shape of their petals, or ray florets.

• Formal and Informal Decorative: Formal types have broad, flat petals in a regular pattern. Informal types have a more relaxed look, with petals that are often twisted or wavy.
• Cactus and Semi-Cactus: These offer a spikier aesthetic. Straight Cactus varieties have petals rolled back for more than half their length, creating a starburst effect.
• Laciniated: These take the cactus form a step further, with petals that are split at the tips for a fringed appearance.
• Ball and Pompon: These provide spherical blooms. Ball dahlias are larger, globe-shaped flowers, while Pompons are smaller, more compact versions with tightly quilled petals.
• Waterlily: These have fully double, symmetrical blooms that are broad and flat, resembling their aquatic namesake.
• Anemone and Collarette: Anemone-flowered dahlias have outer petals surrounding a dense center of colorful disc florets. Collarette dahlias have a single row of outer petals with a smaller ring of petaloids, called a collar, around the central disc.
• Other Forms: Simpler, open-faced Single dahlias and uniquely shaped Stellar and Orchid types are also available.

Planting Dahlias

The timing of planting is determined by the last frost date. Dahlia tubers are sensitive to cold and should be planted only after all danger of frost has passed and the soil has warmed to at least 60°F (15°C). Planting too early risks frost damage, while waiting too long shortens the blooming season.

Dahlias require a location with at least six to eight hours of direct sunlight daily for vigorous growth and flowering. The site should also offer protection from strong winds, which can damage tall stems. Well-draining soil is necessary, as waterlogged conditions can lead to tuber rot.

Prepare the soil by amending the bed with compost or well-rotted manure to improve structure and fertility. Dig a hole four to six inches deep and place the tuber horizontally with the “eye” facing upward. Space tubers 12 to 18 inches apart for good air circulation.

Cover the tuber with soil but do not water until the first green shoots emerge to prevent rot.

Seasonal Dahlia Care

Dahlias require deep, consistent moisture to support their foliage and large blooms. A thorough soaking once or twice a week is better than shallow, daily sprinklings, as this encourages the roots to grow deeper into the soil. Using soaker hoses or drip irrigation is an effective way to deliver water to the root zone while keeping foliage dry, which helps prevent fungal diseases.

Dahlias benefit from a fertilizer low in nitrogen and higher in phosphorus and potassium, as too much nitrogen encourages leaf growth at the expense of flowers. Apply a low-nitrogen fertilizer every three to four weeks starting from midsummer. This schedule supports strong root development and prolific blooming.

Taller varieties require physical support to prevent their stems from snapping under the weight of the flowers or during windy weather. Staking should be done early, often at planting time, to avoid damaging the tuber clump later. A simple stake for each plant or a corral system of posts and twine for larger plantings can be effective.

Another technique is “pinching,” which involves snipping off the central growing tip when the plant is about a foot tall. This encourages the dahlia to branch out from the base, resulting in a bushier, sturdier plant with more blooms.

Overwintering Dahlia Tubers

In climates where the ground freezes, dahlia tubers must be dug up and stored for winter. This process begins after the first hard frost blackens the foliage, signaling the plant to enter dormancy. After the frost, cut the stalks to a few inches above the ground and wait one to two weeks before digging. This period allows the eyes, or growth points, on the tubers to mature for the following season.

Use a garden fork to carefully dig around and lift the tuber clump, then shake off loose dirt. The tubers then need to cure for a few days to a week in a dry, well-ventilated, and frost-free location like a garage or shed. Curing toughens the skin and heals minor cuts, which prevents rot during storage.

Once cured, the clumps can be divided with a sharp knife to multiply your stock; ensure each new tuber has at least one eye attached to its neck. For storage, pack the tubers in a medium like peat moss or wood shavings inside a box or crate. This keeps the tubers from touching and maintains slight humidity.

Store the container in a cool, dark, frost-free location where temperatures remain between 40-50°F (4-10°C). The tubers can be stored here until it is time to plant again in the spring.

Common Dahlia Pests and Diseases

Slugs and snails are fond of the tender new shoots in the spring and can be managed by keeping the area clean or using bait and physical barriers. As plants mature, sap-sucking insects like aphids and spider mites may appear. Spider mites thrive in hot, dry conditions, causing leaves to yellow and develop a stippled appearance.

Powdery mildew is a fungal disease that appears as a white coating on leaves, especially in humid weather with poor air circulation. Prevent its spread by properly spacing plants and removing some lower leaves to improve airflow. Existing infections can be treated with fungicidal sprays or horticultural oils.