Emperor Dahlia, a towering and magnificent plant, offers a dramatic presence to any garden. Its impressive height and large, captivating blooms make it a unique specimen.
Unique Characteristics
Emperor Dahlia (Dahlia imperialis) stands out with its remarkable height, typically reaching 8 to 20 feet tall. Its blooms, often 3 to 6 inches across, can reach up to 8 inches. Flowers commonly present in shades of lavender, mauvish-pink, or white, often with a single row of petals and a central disc. Unlike many dahlias, they are late-season bloomers, flowering from late summer into autumn or early winter, depending on climate. Their distinctive cane-like, four-sided stems resemble bamboo.
Planting and Site Selection
Proper site selection is key for Emperor Dahlias. They thrive in full sun, requiring at least 6 to 8 hours of direct sunlight daily for optimal growth and flowering. Well-draining soil is crucial; tubers are susceptible to rot in waterlogged conditions. Amending soil with organic matter, such as compost or well-rotted manure, improves drainage and fertility.
Plant tubers in spring, after the last frost and when soil temperatures consistently reach 60°F (15°C). Dig holes 4 to 6 inches deep, placing the tuber horizontally with any “eyes” facing upwards. Space tubers generously, 28 to 40 inches apart, to allow for expansive growth and ensure good air circulation. Due to their height, install sturdy stakes or support systems at planting time to prevent wind damage.
Ongoing Care for Emperor Dahlias
Consistent moisture is important, especially during dry spells; they require 1 to 2 inches of water per week. Water deeply at the base of the plant, avoiding overhead watering which can encourage fungal issues. Avoid watering immediately after planting tubers, as this can lead to rot before roots develop. Regular watering can commence once sprouts appear.
Fertilization supports vigorous growth and abundant blooms. After sprouting, apply a low-nitrogen liquid fertilizer (e.g., 5-10-10 or 10-20-20 blend) every 3 to 4 weeks until mid-summer. From July to September, switch to a high-potash fertilizer to promote flower development. Robust staking is essential; use 5 to 6-foot stakes and tie stems every 12 to 18 inches as the plant grows to maintain upright support.
Pinching the plant at 12 to 16 inches tall encourages bushier growth and more blooms. Snip off the top of the central stem just above a set of leaves. Deadheading spent flowers regularly encourages more blooms by redirecting energy from seed production. Remove faded blooms back to a leaf joint or side shoot to maintain plant vigor and appearance.
Overwintering Emperor Dahlia Tubers
In climates where winter temperatures consistently drop below freezing (USDA Zones 3-6), Emperor Dahlia tubers must be lifted and stored indoors. In milder zones (Zone 7), tubers may survive with thick mulch, but indoor storage is still recommended. Gardeners in zones 8-10 can leave tubers in the ground year-round.
After the first frost blackens foliage, allow tubers to remain in the ground for one to two weeks to toughen their skin for storage. Cut stalks back to 4 inches above ground level before digging up tuber clumps with a garden fork. Brush off excess soil from tubers; washing is optional but if done, ensure they dry thoroughly. Place cleaned tubers upside down for a few days in a cool, dry location to allow remaining moisture to drain from stems.
Store tubers in a cool, dark, frost-free environment, ideally between 40°F and 50°F (4°C and 10°C). Pack them in materials like peat moss, vermiculite, or dry sand in ventilated containers to prevent desiccation while allowing air circulation. Periodically check stored tubers for signs of rot or shriveling; remove affected tubers and adjust storage conditions as needed to maintain appropriate humidity.
Common Challenges and Tips
Emperor Dahlias can encounter challenges, many manageable with attentive care. Pests like slugs and snails can damage young foliage; aphids, thrips, spider mites, and earwigs may affect leaves and flowers. Regularly inspecting plants and applying appropriate organic pest controls or insecticidal soaps can mitigate these issues.
Fungal diseases like powdery mildew and gray mold can occur, especially in humid conditions or with poor air circulation. Ensuring proper plant spacing and avoiding overhead watering prevents these problems. Rot of tubers and stems is linked to overly wet or poorly draining soil. Improving soil drainage and managing watering can prevent this condition. Their immense height makes them susceptible to wind damage, reinforcing the importance of robust, well-maintained staking systems.