Dahlia Care: Growing Show-Stopping Summer Blooms
Dahlias are the divas of the summer garden, producing an incredible diversity of flower forms, sizes, and colors from mid-summer until the first frost. From petite border types to towering "dinner plate" varieties, dahlias offer unmatched floral impact. While they require a bit more effort than some perennials, particularly regarding winter storage in colder climates, the reward is a spectacular, long-lasting display of blooms perfect for gardens and bouquets.
Understanding Dahlia Tubers
Dahlias grow from tuberous roots, often sold as clumps. Each viable tuber must have at least one "eye" (a small bud, usually near the old stalk) to sprout.
- Tuber Clump: How they are often dug and stored.
- Single Tuber: A clump can be divided into individual tubers before planting.
- Eye: The growth point for the new stem. No eye = no plant! Eyes can be difficult to see until they start to swell or sprout.
Planting Dahlias
Timing and technique are key for getting dahlias off to a good start.
When to Plant
- Outdoors: Plant tubers directly in the garden after all danger of frost has passed and the soil has warmed to at least 60°F (15°C). Planting in cold, wet soil invites rot.
- Indoors (Optional): Start tubers indoors in pots 4-6 weeks before the last frost date to get a head start on the season. Transplant carefully after frost danger.
Ideal Growing Conditions
- Light: Full sun is essential (6-8+ hours per day). Less sun results in leggy plants and fewer blooms.
- Soil: Fertile, well-draining soil. Amend generously with compost or aged manure. Avoid heavy clay that stays waterlogged. Slightly acidic to neutral pH (6.0-7.0) is ideal.
- Spacing: Space according to mature size: smaller border types 12-18 inches apart, larger varieties 2-3 feet apart. Good air circulation is important.
Do NOT water dahlia tubers immediately after planting unless the soil is bone dry. Wait until sprouts emerge above ground. Watering dormant tubers in cool soil is a leading cause of rot.
Planting Technique (Tubers)
Choose a sunny, well-drained location. Amend the soil with compost.
Dig a hole about 6-8 inches deep and wide enough to accommodate the tuber or clump. Mix in a handful of bone meal or low-nitrogen fertilizer at the bottom (optional), covering it with a thin layer of soil.
Important: If growing medium to tall dahlias (over 3 feet), insert a sturdy stake (5-6 feet tall) into the planting hole now, next to where the tuber will go. Installing it later risks damaging the tubers.
Lay the tuber horizontally in the hole with the eye(s) pointing upwards, about 2-4 inches away from the stake. If planting a clump, position it similarly.
Cover the tuber with 4-6 inches of soil. Do not water yet. Wait for sprouts to appear above ground (this can take several weeks). Once sprouts emerge, water thoroughly.
Caring for Dahlias During the Season
Watering
- Once growth is active, water deeply 1-2 times per week, providing about 1 inch of water total. Increase watering during hot, dry weather, especially for large plants in full bloom.
- Avoid letting plants wilt, but also avoid constantly waterlogged soil. Water at the base to keep foliage dry.
Fertilizing
- Dahlias benefit from regular feeding, but avoid high-nitrogen fertilizers, which promote lush leaves at the expense of flowers.
- Use a low-nitrogen, high-phosphorus and potassium fertilizer (e.g., 5-10-10 or similar "bloom booster" formulas) starting a month after planting.
- Apply fertilizer every 3-4 weeks according to package directions, stopping in early fall.
Pinching
- When plants reach about 12-18 inches tall and have 3-4 sets of true leaves, "pinch" out the central growing tip just above a leaf node.
- This encourages the plant to branch out, resulting in a bushier habit and more flower stems.
Staking
- Essential for varieties growing over 3 feet tall to prevent stems (often hollow) from snapping under the weight of blooms or in wind/rain.
- Tie stems loosely to the stake inserted at planting time as the plant grows, using soft ties or twine. Add ties every 12-18 inches. Cages can also be used for bushier types.
Deadheading
- Regularly remove spent flowers to encourage continuous blooming and keep plants looking tidy.
- Cut the stem back to just above a leaf node or main stem. This prompts the plant to produce new flowering side shoots.
Digging and Storing Tubers (Zones 7 and colder)
In areas where the ground freezes, dahlia tubers must be lifted and stored over winter.
When to Dig
- Wait until after the first hard frost has blackened the foliage. This signals the tubers to prepare for dormancy.
- Cut the stalks down to about 4-6 inches above the ground a few days before digging.
Digging Technique
Using a digging fork or spade, carefully loosen the soil in a wide circle around the base of the plant (at least 1 foot out) to avoid damaging the tubers.
Gently pry under the tuber clump and lift it from the ground. Shake off excess soil carefully. Avoid breaking the delicate necks connecting tubers to the main stalk.
Gently rinse off remaining soil (optional, some prefer dry brushing). Allow clumps to air dry/cure out of direct sun in a cool, frost-free place (garage, shed) for several days to a week until the skin feels dry.
Storing Tubers
- Storage Medium: Pack tubers in crates, boxes, or paper bags filled with slightly dampened peat moss, vermiculite, wood shavings, or sand. The goal is to keep them from drying out completely but prevent mold.
- Labeling: Label each clump or individual tuber clearly with the variety name!
- Temperature: Store in a cool, dark, humid but frost-free location (ideal temps 40-50°F / 4-10°C). An unheated basement, cellar, or insulated garage often works.
- Checking: Check tubers periodically through winter. If shriveling, lightly mist the storage medium. If mold appears, improve air circulation, trim affected parts if minor, or discard heavily molded tubers.
Dividing Tubers: When and How
You can divide clumps before storing in fall, or wait until spring before planting. Spring division is often easier as the eyes are usually more visible.
- Use a clean, sharp knife or pruners.
- Identify the eyes near the central stalk.
- Cut the clump apart so each division has at least one healthy tuber attached to a piece of the old stalk with at least one visible eye.
- Allow cut surfaces to dry/callous for a day before storing or planting.
Troubleshooting Common Problems
Pests (Spider Mites, Aphids, Thrips, Earwigs, Slugs)
Management
- Spider Mites/Aphids: Common in hot, dry weather. Use strong water spray or insecticidal soap.
- Thrips: Tiny insects causing distorted buds/flowers and streaked petals. Insecticidal soap can help; severe cases may need stronger insecticides. Remove affected blooms.
- Earwigs/Slugs: Can chew holes in petals and leaves. Traps (rolled newspaper for earwigs, beer traps for slugs) or baits can help.
Diseases (Powdery Mildew, Viral Diseases)
Management
- Powdery Mildew: White coating on leaves. Improve air circulation, water at base. Fungicides can prevent spread.
- Viral Diseases: Cause stunted growth, yellow mottling/streaks, deformed leaves/flowers. No cure. Remove and destroy infected plants (do not compost) to prevent spread via tools or insects. Buy tubers from reputable sources.
Conclusion
Dahlias offer an explosion of color and form for the summer and fall garden. While they require attention – particularly staking tall varieties and lifting tubers for winter storage in colder climates – the breathtaking blooms are well worth the effort. Provide full sun, fertile soil, consistent moisture, and proper support, and these garden stars will put on a dazzling show until the very end of the growing season.
