How to Grow Dahlias - Everyone's Favorite Cut Flower

When I think of a flower that everyone loves, I think of dahlias. They are the epitome of cottage garden beauty and excellent for cutting and arranging in bouquets.

Dahlias are native to Central and South America, specifically the mountainous regions of Mexico and Guatemala. They were first discovered by Spanish explorers in the late 18th century and were named in honor of Swedish botanist Anders Dahl.

The first recorded cultivation of dahlias was in the Botanical Gardens of Madrid, Spain. In the early 19th century, dahlias became very popular among European gardeners, who began hybridizing and cultivating them for their beautiful blooms and planting them in cottage gardens. 

This led to the development of the wide variety of dahlia cultivars we have today, ranging in size from small pompon blooms to large dinnerplate blooms. And they continue to increase in popularity. You’ve probably seen them in flower gardens, potager gardens, cut flower bouquets, and landscaping. 

When to Plant Dahlia Seeds & Dahlia Tubers

Dahlias can be planted from seed or from tubers. If you are starting dahlia seeds, I recommend starting them indoors in February (8 to 12 weeks before your estimated last frost date). They need a lot of time to grow to ensure they bloom in their first summer. If you give dahlias a long enough growing season, they will form tubers you can save to grow next year! 

Start them in a high-quality seed starting mix that drains well, using grow lights to ensure they receive ample light. If you’re new to seed starting, check out this tutorial on how to start seeds indoors. You will likely need to pot up your dahlia seedlings at least once before it’s time to transplant them outside.

Once the danger of frost has passed, harden off your seedlings, then transplant them into the garden. Dahlias are tropical and will not tolerate any frost, so ensure the weather is warm enough! 

Dahlia tubers can be planted as early as one week before your average last frost date. The tubers in the ground will be protected from a frost (not a hard freeze) because they are in the ground.

The Right Soil Type

Dahlia tubers do not like to sit in a lot of water, so they need well-draining soil. Never plant dahlias in a location where water settles or floods.

Before planting my dahlias, I amended my raised bed with a good-quality potting mix containing perlite, vermiculite, and even a little forest humus. These ingredients help the soil hold onto the right amount of water while still draining well.

• Vermiculite - Vermiculite is an expanded volcanic rock that is very porous, allowing it to hold onto water and nutrients so they don’t drain through the soil.

• Perlite - Perlite is an expanded volcanic glass used in high-quality seed-starting mixes to prevent compaction and improve drainage and soil aeration. There are different grades of perlite. I prefer using fine to medium perlite. Extra-fine will work, too, but you will need to add more.

• Humus - Humus is dark, organic matter in compost and soil that improves water retention and soil structure.

When the roots are healthy due to great soil, they will form more tubers. If you have good tuber development, at the end of the season, you can dig up your plant and divide the tubers to have more dahlia plants for future years.

How to Plant Dahlia Tubers

Dahlias can be grown in USDA zones 2 -11, but they are perennials only in growing zones 8-11.

1. Location - You can plant dahlias in-ground, in raised garden beds, or in large pots, but they need plenty of soil for the roots. Pots should be at least 7 gallons, and raised beds should be at least 8 to 10 inches deep! Some varieties of dahlias can grow 5 to 6 feet tall, so choose a location without overhead obstructions.

2. Sunlight - Dahlias thrive in full sun and will not produce blossoms if they get inadequate sunlight. Plant dahlias where they can receive at least six hours of sunlight per day.

3. Amend the Soil - Ensure the soil drains well, as dahlias do not tolerate waterlogging. Amend with quality potting mix and some compost to improve drainage if you have heavy clay soil.
   

4. Fertilize the Soil - Apply a balanced fertilizer at planting and again when flowers start to set to promote healthy growth and vibrant blooms. We use Trifecta+ with amazing success, but use whichever balanced fertilizer you trust.

5. Soil pH - Dahlias really like a pH right around 5.5 to 6.0, which is a little more acidic than your average garden crop. Test your soil pH with a soil pH meter, then add some powdered sulfur to temporarily acidify the soil if needed. If your soil pH is above 6.0, you will need to amend with sulfur each year! Pro Tip: Having a little powdered sulfur in the soil can help preserve tuber quality and prevent things like tuber rot and other dahlia diseases.

6. Planting Depth - Dig a hole that is 4 to 6 inches deep. Splay out the tubers so they are somewhat flat, and plant them with the eyes (buds) facing up. Cover them with soil up to the original soil level; don’t make a hill over the tubers.

   

7. Plant Spacing - Space them around 18 inches apart. If you are planting multiple rows, space the rows 3 feet apart.

8. Water - Forming tubers requires substantial energy and water. If your soil drains well, you can water dahlia plants deeply once a week and more frequently during hot, dry periods. If your soil is more heavily compacted, dial back the watering so your plant can dry out in between.

9. Support - Stake your dahlias with bamboo stakes, use tomato cages, or another type of support early in the growing season. As dahlias grow, they become top-heavy and require support to prevent bending or breaking. 

How to Grow Dahlias

Dahlias produce tons of blooms, but these blooms do not last forever. Deadheading your dahlias regularly promotes growth and prevents the plant from diverting energy into producing seeds. 

Simply snip off the spent blooms with a pair of snips or scissors. As you cut dahlias, they will continue to grow and produce all summer long!

Pest Control

The main pests you will likely encounter when growing dahlias are aphids, as well as ants, mid to late season when the blossoms are opening up. These organic pesticides are reliable at controlling them:

• Neem Oil - Neem oil repels a wide variety of pests and is especially useful in fighting aphids, whiteflies, fungus gnats, and Japanese beetles.

• Pyrethrin - Pyrethrin is fast-acting and can be used on vegetables, fruits, flowers, and ornamentals up to the day of harvest to control aphids, scales, spider mites, caterpillars, and more. 

• Insecticidal Soap - Another reliable option for controlling ants and aphids is insecticidal soap. Follow the manufacturer's instructions and apply the soap solution to your plants, focusing on the affected areas.

Overwintering Dahlias

In cooler climates (growing zones 2-7), dahlia tubers should be dug up each fall after the first frost and replanted in spring. This is because dahlias are not cold-hardy and need protection during the winter months. 

After the first frost, cut back the foliage to the ground. Dig the plant up like a potato and gently brush off any soil. Clip tubers from the root system, being very careful not to damage the "eyes" which grow very close to the main stem.

Store the tubers in peat moss, sand or vermiculite and keep them lightly damp at all times, but not soaking. Place them where the temperature stays around 40-50° Fahrenheit.

Take care not to break the neck below the eye of the dahlia tuber, it will kill it. Just like potatoes, if you have multiple eyes on your tubers, you can divide them to plant multiple plants in the spring.

If you don’t know where the eyes are, it is best to just plant your dahlia as is. You will have multiple plants growing, but there is no ill effect to doing that, and you’re not risking killing your dahlia.