How To Grow Tree Dahlias

Tree dahlias (Dahlia imperialis), also known as giant dahlias, are beautiful tall perennial plants that really stand out in the garden.

They’re easy to care for and great for providing color in the garden when most other flowers have finished blooming.

Here’s how to grow stunning tree dahlias in your flower garden.

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Tree dahlias are the largest dahlia variety, growing to a height of 6 to 20 feet (2 to 6 metres) tall.

They’re native to Colombia, Mexico and Central America but they can be grown in many different areas, including cool climates.

Flowers

Tree dahlias bloom from fall until winter and the stems produce multiple flower heads, each holding up to twenty flowers.

The flowers can grow 4 to 6 inches (10 to 15 cm) in diameter and the flowers can range in color from mauve to pale pink or white.

Climate

Tree dahlias grow well in most areas, including subtropical and cool climates, but the flowers aren’t frost hardy.

The tubers can survive cold winter temperatures with a thick layer of mulch on top of the soil to protect the underground tubers.

If you live in an area that has long freezing winters with temperatures regularly dropping below 23°F (-5°C), you can lift the tubers in fall and keep them indoors over winter.

Soil

Dahlias like rich, easily draining soil and they’ll grow well in acidic, sandy or loamy soil.

Dig in some compost or broken down manure before planting and loosen the soil several inches deeper than required to help with drainage. 

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Planting tree dahlias

Choose a site that’s sunny, protected from the frost and wind and dig a trench about 4 inches (10 cm) deep.

Lay a cane horizontally along the trench. They shoot from the nodes and grow fast so you won’t have to wait long to have a beautiful clump of dahlias.

Tubers should be planted 6 to 8 inches (15 to 20 cm) deep with the eye facing upwards.

Add a layer of mulch around the base of the plant to protect the roots and keep the soil moist.

It’s also a good idea to set up a few stakes after planting to protect the plants from strong winds and rain.

Spacing

Tree dahlias need plenty of room to spread out in the garden so space the tubers out at least 2 feet (60 cm) apart to allow the foliage and flowers to grow.

Sunlight

Tree dahlias will grow well in full sun or partial shade but they need at least six hours of sunlight each day to flower well.

Too much shade means that they’ll produce less flowers.

Water

Tree dahlias can be watered once to three times a week, depending on how warm the weather is.

Always check that the soil has dried out slightly before watering because overwatering can cause the tubers to rot.

Also, take care to avoid wetting the foliage to reduce the likelihood of fungal diseases.

Pruning

Tree dahlias grown in areas with mild winter temperatures keep growing and blooming until Christmas and sometimes longer.

This means that the plants become very tall and high winds can easily break the stems if they’re not staked.

It’s a good idea to regularly pinch back the tips to keep the plants to a manageable height and cut them back to ground level once they’ve finished flowering.

Fertilizer

Tree dahlias can be fertilized when they start blooming to promote beautiful big flowers.

Use a good quality fertilizer that is specifically for flowers like Bloom Booster Flower Food.

Pests and diseases

Dahlia imperialis can be affected by a number of different fungal diseases including powdery mildew and gray mold.

Avoid overhead watering and check the leaves regularly for signs of fungal growth.

Remove any affected leaves and spray the entire plant with a fungicide.

The new growth can also be affected by aphids but they can be eliminated with a natural insecticidal spray.

Root and stem rot is another common problem with dahlias so it’s important to choose a spot in the garden with well draining soil and take care not to overwater the plants.

Can tree dahlias be grown in containers?

Tree dahlias can be successfully grown in large containers situated in a sunny spot in the garden.

You can prune the plants to keep them to a smaller size and they look beautiful in containers.

Propagation

Tree dahlias can be propagated from seeds, stem cuttings or tubers.

You can collect the seeds by leaving the flowers on the plant to mature and dry out.

Once the flowers are dried, gently crush them or break them apart to collect the seeds.

Dahlias can also be propagated by cutting the stems into pieces with at least two nodes and planting them in the garden.

The tubers can also be dug up and divided to propagate more plants.

Overwintering tree dahlias

In mild climates, tree dahlia tubers can be overwintered in the garden.

Once the stems have wilted, cut them back to about 4 inches (10 cm) above the ground.

Cover the soil with a thick layer of mulch to help protect the tubers during winter.

If you live in an area where the temperature frequently drops below 23°F (-5°C), you’ll need to dig up the tubers before the ground freezes. 

Place the tubers in the sun for 2 to 3 days to dry out and then store them in a cool, dry place over the winter.

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So there are my tree dahlia plant care tips.

With the right care you’ll be able to enjoy the attractive foliage and spectacular flowers of these tall perennial plants for many years to come.

Have you tried growing dahlia imperialis flowers in your garden? Let me know in the comments below.

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