Dahlias are popular additions to many gardens, known for their diverse forms, vibrant colors, and varying sizes. These tuberous perennial plants, native to Mexico and Central America, bloom from midsummer until the first frost. The genus Dahlia belongs to the Asteraceae family, which includes sunflowers and daisies, and boasts nearly 50,000 named varieties. This rich diversity offers an impressive array of shapes and hues, making dahlias a captivating choice for gardeners.
Choosing and Planting Dahlias
Selecting the right dahlia varieties is key for a successful display. Dahlias come in many types, such as dinnerplate, pompom, and cactus forms, each with a unique aesthetic. Consider the mature size and form when choosing tubers, as varieties vary significantly in height and bloom diameter.
Dahlias need at least six to eight hours of direct sunlight daily, ideally with morning sun. Well-draining soil is important for healthy growth, as tubers rot in waterlogged conditions. Amend heavy clay soils with organic matter like compost or well-rotted manure to improve drainage and fertility, loosening the soil to a depth of at least 10-12 inches.
Plant in spring after the last frost and when soil temperature warms to at least 60°F (15°C). A common guideline is to plant dahlias when tomatoes are planted in your region. Dig a hole 4-6 inches deep, placing the tuber horizontally with the “eye” or growth bud facing upwards. Space smaller varieties about 12 inches apart, and larger types 18-36 inches, for adequate air circulation and growth. After planting, water thoroughly to settle the soil, but avoid further watering until green shoots emerge within a few weeks.
Dahlia Care and Maintenance
Consistent watering is important for dahlias once they have sprouted and are actively growing. After shoots appear, water deeply once or twice a week, increasing frequency during dry or hot periods. The soil should remain moist but not waterlogged; check by hand, rewatering when the top 2-3 inches are dry.
Dahlias benefit from fertilization. Use a low-nitrogen fertilizer, such as a 5-10-5 NPK ratio, as excessive nitrogen promotes leafy growth over flower production. Apply fertilizer according to package directions once or twice during the growing season after plants are established.
Taller dahlia varieties, especially those over three feet, require staking or support to prevent heavy blooms and stems from breaking. Install stakes, such as hardwood or bamboo poles, at planting time or when plants are young to avoid damaging tubers. As the plant grows, gently tie stems to the support, allowing some slack.
Pinching and deadheading encourage bushier plants and continuous flowering. Pinching involves removing the central growing tip when the plant is 8-12 inches tall with at least three to four sets of leaves. This redirects energy to lateral buds, resulting in more stems and blooms. Deadheading, the removal of spent blooms, signals the plant to produce more flowers rather than diverting energy into seed production. Maintaining a weed-free area around dahlias helps reduce competition for water and nutrients.
Common Challenges and Solutions
Dahlias can encounter pests like slugs, earwigs, aphids, and spider mites. Slugs and earwigs target young foliage, while aphids and spider mites infest leaves and stems, causing discoloration and stunted growth. Organic control methods, such as hand-picking slugs or using insecticidal soap for aphids, are effective.
Fungal diseases, such as powdery mildew, may appear in humid conditions or with poor air circulation. Proper plant spacing and watering at the base help prevent these issues. If powdery mildew develops, fungicidal sprays for ornamental plants can provide treatment.
A lack of blooms often stems from insufficient sunlight, as dahlias need at least six hours daily for optimal flowering. Over-fertilization with high-nitrogen products can also lead to lush foliage but few flowers. Additionally, improper watering, including both overwatering (which can cause tuber rot) and underwatering, can hinder bloom production.
Overwintering and Storing Dahlia Tubers
In climates where winter temperatures drop below freezing (USDA Hardiness Zone 7 and colder), dahlia tubers are not hardy and must be dug up and stored. Dig after the first hard frost blackens the foliage, signaling dormancy. Allowing a week or two after the frost gives tubers time to gather energy.
Carefully dig up tubers using a digging fork to avoid damage. Loosen soil around the plant, then lift the clump. Remove excess soil and allow tubers to dry or “cure” for a few days in a cool, dark, dry location with good airflow. This curing toughens the skin and prevents rot during storage.
Store tubers in a cool, dark, frost-free environment, ideally between 40-50°F (4-10°C) with 75-90% humidity. Basements, root cellars, or temperature-controlled spaces are suitable. Pack cured tubers in a medium like peat moss, vermiculite, or wood shavings, ensuring they do not touch to prevent rot spread. Periodically check stored tubers throughout winter, removing any showing signs of rot or shriveling.