Dahlia / Garden Dahlia
Dahlia pinnata
description
Dahlia is a tuberous garden favourite with an exceptional range of flower shapes and colours.
benefits
- Flowering
- Suitable for containers
warnings
- Toxic to cats
- Toxic to dogs
How to care for Dahlia
sunlight
Native to sunny, open parts of Mexico and Central America, dahlias flower best with plenty of direct light. Give them the brightest outdoor spot you have.
Full sun
Aim for about 6-8 hours of direct sun daily for sturdy stems and abundant blooms. In very hot climates, light afternoon shade can help flowers last longer, but too much shade means fewer blooms and lanky growth.
watering
Keep dahlias evenly moist once they are growing, but never leave their tubers in wet, airless soil. Water slowly at the base so the root zone is thoroughly moistened, then let the surface begin to dry.
After planting
Water newly planted tubers only if the soil is dry, then check about weekly. Until shoots appear, they use little water and are prone to rot in soggy soil.
Active growth and flowering
In summer, water deeply about once or twice a week in the ground when the top 2-5 cm of soil feels dry. Pots may need water every 1-3 days in hot, sunny weather. Check more often during heat, wind, and flowering, and less often in cool weather or heavy soil.
After flowering and dormancy
As foliage yellows after the first frost, reduce watering. Once growth has died back, keep lifted or overwintering tubers almost dry until spring.
humidity
Preferred humidity
Dahlias are comfortable in ordinary outdoor air. Their Mexican upland origins suit them better to moderate humidity and good airflow than constantly damp conditions.
They usually tolerate about 30-60% humidity if the soil is watered well. Very dry air can cause crisp leaf edges, limp growth despite moist soil, or encourage spider mites. Prolonged humidity above 70%, especially with still air, can promote powdery mildew.
- Space plants well and remove crowded lower leaves.
- Water the soil in the morning so foliage dries quickly.
- For potted dahlias in hot, dry weather, group pots nearby and water consistently rather than using a humidifier.
temperature
Growing season
Stored tubers
Native to Mexico's mild highlands, dahlias grow best in warm weather but are not frost hardy. Plant tubers after the last frost, and keep growing plants sheltered from cold, exposed spots. Hot spells above about 29°C (85°F) can shorten flowering, especially if the soil dries out.
nutrition
May - August
Once new shoots are growing, feed every 3-4 weeks with a flowering fertilizer that is lower in nitrogen and richer in potassium. Use a liquid feed at about half the label strength, especially in pots. Stop feeding by late August so the tubers can mature, and do not feed dormant stored tubers.
Nitrogen
Nitrogen supports leafy growth, but too much can make dahlias tall and soft with fewer flowers. A low-nitrogen flowering feed provides enough without encouraging excess foliage.
Potassium
Potassium supports sturdy stems and flowering. Regular light feeding during bud formation is more useful than one heavy dose.
soil
Rich, loose, well-draining soil
Dahlia tubers need moisture and nutrients for vigorous flowering, but they rot in heavy, soggy ground. Mix garden soil or potting compost with compost and a little coarse grit or perlite if drainage is slow.
pH 6,0 - 7,0
This is slightly acidic to neutral, which suits dahlias well. Most good-quality garden soil and multipurpose compost fall in this range, so avoid strongly acidic mixes and add garden lime only if a soil test shows the soil is very acidic.