How to Grow Dahlias: Planting Depth, Care and Storing Tubers

Plant dahlia tubers outdoors after the last frost, in full sun and fertile, well-drained soil, and keep the soil evenly moist all summer. NC State says dahlias need fertile, moist but well-drained soil and full sun to partial shade, and that they flower from late summer to the first frost. The tuberous roots can be injured below 25°F (-4°C), so winter is part of the plan: mulch over in zones 7 to 10 (NC State), or dig the tubers in fall and store them. Planting depth is the one place the sources differ, and the range is laid out below.

Sun: Full sun to partial shade
Origin: The Americas
Soil: Fertile, moist but well drained
Planting depth: 2–7 in by source and type (NC State, Illinois)
Spacing: 12–18 in apart (Illinois)
Bloom time: Late summer to first frost
Cold limit: Tubers injured below 25°F (-4°C)

What dahlias are

Dahlias grow from tuberous roots and come in a very wide range of flowers. NC State says the American Dahlia Society recognizes 29 groups based on flower size and shape. They tend to be tall (NC State), which matters for staking and for how deep you plant. Illinois Extension covers them in its piece on tender bulbs for summer color.

Sun, soil and water

Sun. NC State says full sun to partial shade. Illinois Extension says to plant in full sun for the best flowering.

Soil. NC State says fertile, moist but well-drained soil.

Water. NC State says to keep dahlias mulched and give them plenty of water. Illinois Extension says consistent watering is key for blooming.

How deep to plant dahlia tubers

The depth figures are not the same, and the difference is mostly about the height of the plant.

Source	Planting depth
NC State	3 inches (cover tubers to a depth of 3 inches)
Illinois Extension, shorter varieties	2 to 3 inches
Illinois Extension, taller varieties	6 to 7 inches

Across the two sources the range is about 2 to 7 inches. NC State’s 3 inches sits at the shallow end, where Illinois puts the shorter types. If you are growing a tall variety, Illinois Extension is the source that addresses it directly and calls for the deeper hole. Space plants about 12 to 18 inches apart (Illinois Extension).

How to plant dahlias

Wait for the last frost. Illinois Extension says to plant outside after the last frost date for optimal flowering. NC State says the tuberous roots can be injured below 25°F.
Pick a sunny site with good soil. Full sun to partial shade, in fertile, moist but well-drained soil (NC State).
Set the tuber at the right depth. About 3 inches (NC State), or 2 to 3 inches for shorter varieties and 6 to 7 inches for taller ones (Illinois Extension).
Space and support. 12 to 18 inches apart (Illinois Extension). NC State says dahlias tend to be tall and may require staking.
Mulch and water. NC State says to keep them mulched and provide plenty of water.

Dahlia care through the season

Water

Plenty of water and a mulch layer (NC State). Consistent watering is key for blooms (Illinois Extension).

Sun

Full sun to partial shade (NC State).

Support

Tall types may need staking (NC State).

Deadheading

NC State says to deadhead spent flowers.

Lifting and storing tubers

Do not wash them. NC State says to dry the tubers carefully and not wash them before storage.

NC State says to dig up the tubers in fall, dry them carefully, and store them in vermiculite or dry sand at 35 to 45°F (2 to 7°C). Iowa State says to allow the tubers to dry and then store them in a cool, dark and dry location. In zones 7 to 10 NC State gives the other option: mulch over in winter and leave them in the ground.

Dividing and propagating

NC State says dahlias are propagated by division or root cutting.

Dahlia flower groups

NC State lists these among the American Dahlia Society groups: single, anemone, collarette, waterlily, decorative, fall, pompon, cactus and semi-cactus. The Society recognizes 29 groups in all, based on flower size and shape.

Common dahlia problems

ColdTubers lost over winterThe roots can be injured below 25°F (NC State). Mulch in zones 7 to 10, or lift and store.BloomsFew flowersIllinois Extension says consistent watering is key for blooming, and Illinois says to plant in full sun.HeightTall plants flopDahlias tend to be tall and may require staking (NC State). Tall varieties are also planted deeper (Illinois Extension).

Grow these next

Dahlias belong with the tender summer bulbs. Gladiolus is planted from corms in spring and lifted in fall, and the lily guide covers the hardy summer bloomer that stays in the ground. For a bed that flowers from spring through frost, our bloom calendar shows how the months line up.

Frequently asked questions

When do you plant dahlia tubers?

Illinois Extension says to plant outside in full sun after the last frost date for the best flowering. NC State says dahlia tubers can be injured at temperatures below 25°F (-4°C).

How deep do you plant dahlia tubers?

Our sources give a range of about 2 to 7 inches. NC State says to cover tubers to a depth of 3 inches. Illinois Extension says taller varieties go 6 to 7 inches deep and shorter ones 2 to 3 inches.

Do dahlias come back every year?

In zones 7 to 10 NC State says to mulch over in winter. Where winters are colder, NC State and Iowa State describe digging the tubers in fall and storing them over winter.

How do you store dahlia tubers over winter?

NC State says to dig up the tubers in fall, dry them carefully without washing, and store them in vermiculite or dry sand at 35 to 45°F (2 to 7°C). Iowa State says to let them dry, then keep them in a cool, dark, dry place.

Do dahlias need staking?

Often. NC State says dahlias tend to be tall and may require staking.

Sources

Dahlia, NC State Extension Plant Toolbox
How to Overwinter Tender Perennials, Iowa State University Extension, Yard and Garden
Tender Bulbs Make Summer Delights, University of Illinois Extension

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