How to Grow Dahlias for Summer Color

Updated on February 9, 2018
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Caren White is a Master Gardener and instructor at Home Gardeners School. She has been a volunteer at Rutgers Gardens for over a decade.

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Growing up, my mother grew flowers and my father grew vegetables. The only exception to this division of labor was my father’s dahlias. He loved them, especially the large dinner plate dahlias. There was even an informal neighborhood competition to see who could grow dinner plate dahlias with the biggest flowers.

What Are Dahlias?

Dahlias are flowering plants that grow from underground tubers. They are related to sunflowers, chrysanthemums and zinnias. Dahlias are originally from Mexico so they are not cold hardy. North of zone 8, they are grown as annuals or the tubers are lifted from the garden and stored indoors until the following spring.

A modern single dahlia which resembles its ancestors
A modern single dahlia which resembles its ancestors | Source

Dahlias were cultivated by the Aztecs for their tubers which were eaten rather than for their flowers which were small and daisy-like. The plants made their way to Europe in the 16th century after the Spanish colonization of the New World. Starting in the late 17th century, European hybridizers began developing the different flower forms with which we are familiar today. They are the single flowered dahlias, which resemble their ancestors, the pompom dahlias, the cactus dahlias with their spikey petals, decorative dahlias which resemble chrysanthemums and the enormous dinner plate dahlias whose flowers can be as large as 12 inches across. Within those flower forms, there are 14 recognized groups of flower types.

A decorative dahlia.  It looks very much like its cousin the chrysanthemum.
A decorative dahlia. It looks very much like its cousin the chrysanthemum. | Source

How to Grow Dahlias

Unless you buy plants, what you will receive when you purchase dahlias are brown tubers that resemble carrots. The tubers should be planted in the spring when the soil reaches 60⁰F. You can also start your tubers indoors 4 to 6 weeks before your last frost. Plant them in individual containers 3 inches below the top of the soil. You should insert a stake in the container to support the plant as it grows. Depending on the variety, the plants can grow 3 to 6 feet tall.

In the garden, select a site that gets full sun and has protection from the wind. Plant the tubers 6 to 8 inches deep. Smaller dahlias should be planted 2 feet apart while the larger ones should be planted 3 feet apart. Plant the tubers like carrots with the pointy end downwards and the fatter end which has the growth tip pointing upwards and level with the top of the soil. You should also install support stakes at planting time so that you don’t damage the tubers later in the season trying to push the stakes into the ground. The stakes should be 5 to 6 feet tall. Tie the stems to the stakes as they grow.

Do not water the tubers after planting. This will cause them to rot. Wait until you see the plants emerging from the soil and then give them a good soaking. Thereafter you can water 2 or 3 times a week to keep the soil moist.

Pompom dahlias.  They remind me of golf balls.
Pompom dahlias. They remind me of golf balls. | Source

How to Grow the Biggest Flowers

Dahlias will start flowering 8 weeks after planting, usually in July. They should be deadheaded regularly to ensure continuous blooms until the plants are killed by frost in the fall. To encourage dinner plate dahlias to produce their large flowers, they should be disbudded. All dahlias send up a stalk with a terminal bud on the end and other buds along the sides of the stalks. All of these buds will open into flowers. If you want large flowers on your dinner plate dahlias, you should remove all of the side buds so that the plants will concentrate all of their energy into the single terminal bud. This is known as “disbudding”. Unfortunately it means that you will only have one flower per plant if you want the largest flowers.

Cactus Dahlias.  Cactus flowered zinnias are nearly identical.  Not surprising because zinnias are in the same family as dahlias.
Cactus Dahlias. Cactus flowered zinnias are nearly identical. Not surprising because zinnias are in the same family as dahlias. | Source

Storing Your Tubers For Next Year

In the fall, the frost will kill the plants but don’t dig the tubers up immediately. Wait for a week after the foliage dies. This allows the tubers to achieve full dormancy for the winter. Gently dig up the tubers then cut away the foliage. Brush the soil from them but don’t wash them. Wetting them at this stage will cause them to rot over the winter. Allow the tubers to air dry for 24 hours.

Line a cardboard box with newspaper and put the tubers in the box in a single layer. Cover them with sand, sawdust or peat moss that is slightly moist. Store the box in a cool dark place that remains 40⁰F to 50⁰F all winter. Check your tubers regularly. If they start to shrivel, mist the packing material very lightly.

And don’t forget your labels! If you are growing more than one type of dahlia, label each variety. No matter which variety you are growing, the tubers all look alike.

In the spring when the soil has warmed, remove your tubers from their winter quarters. This is a good time to divide them if they had grown into large clumps last summer. Using a sharp knife, cut the clumps of tubers into smaller groups leaving at least 3 “eyes” in each clump. The plants grow from those eyes just like potatoes do. Then plant them in your garden after the soil has warmed.

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