When to Plant Dahlias: Tuber Timing by Region, Depth and Winter Storage

Dahlias are tender tuberous plants from the mountains of Mexico (Iowa State) that flower from mid or late summer until frost (NC State). They range from 1-inch pompons to dinner-plate blooms over 10 inches across, and from 1-foot plants to 7-foot giants (Iowa State, WSU). They can't take frost: NC State says the tubers are injured below 25°F, so in most of the country they go in after the last frost and come out in fall.

When to plant dahlias by region

A soil thermometer at planting depth is the simplest check (GardenPal note). Our planting calendar shows your average last frost date, and dahlias go in around the same time as tomatoes and zinnias.

Starting dahlias early in pots

For earlier flowers, Iowa State plants tubers in 8-inch pots in early April, laid flat and covered with 1 to 2 inches of potting mix, then moves the pots into full sun once shoots appear. Oregon State and the RHS also start tubers in pots under cover. WSU plants potted dahlias out in mid-May or when they are 10 to 15 inches tall, which also keeps tender new shoots away from slugs and rabbits. Harden them off before planting out, as with any transplant (GardenPal note).

How deep and far apart to plant dahlias

Watering and feeding

• Wait to water: the tuber holds what it needs to sprout. Oregon State waits until the first two leaves appear, and WSU and UC Master Gardeners say to hold off until shoots show, because wet soil rots unsprouted tubers.
• Then water deeply: Iowa State gives about 1 inch per week once plants are 10 to 12 inches tall, and mulch helps hold moisture.
• Feed low nitrogen: Oregon State uses 5-10-10 starting about 30 days after planting and every 3 to 4 weeks, stopping in September. Too much nitrogen gives leaves instead of flowers (WSU, UC Master Gardeners).

Pinching, disbudding and cutting

Pinch out the growing tip for a bushier plant with more stems: Oregon State tops plants at 12 to 16 inches, WSU at about a foot or 3 to 5 sets of leaves, UC Master Gardeners above the third set of leaves, and Clemson about three weeks after planting. For bigger single blooms, remove the two side buds next to each main bud (WSU, Iowa State). Cut flowers in the cool of the morning when they are open or nearly open; Oregon State says they usually last 4 to 6 days in a vase. Deadheading keeps new flowers coming (WSU, Iowa State).

Winter: dig and store or leave in the ground

• Colder than zone 7: lift the tubers in fall (NC State). Iowa State cuts the stems after a killing frost and leaves the clumps in the ground 1 to 2 more weeks before digging; Oregon State digs after a hard frost.
• Storage: pack clumps in vermiculite, peat, coir or wood shavings somewhere cool and frost-free: 34 to 40°F (Oregon State), 35 to 45°F (NC State) or 40 to 55°F (Iowa State).
• Zones 7 to 10: tubers can usually stay in the ground under mulch (NC State); Clemson mulches 3 to 4 inches in the colder parts of South Carolina, and UC Master Gardeners leave them in the ground in mild California and divide every 2 to 3 years.

Slugs, mildew and viruses

Slugs and snails go for young shoots (UC Master Gardeners, RHS, WSU), and WSU also mentions rabbits. Iowa State lists thrips, aphids and powdery mildew late in the season, and says virus-infected plants (mottled, ring-spotted or distorted leaves and stunting) should be removed and kept out of the compost. Earwigs often hide in the flowers (GardenPal note).

Dahlias in the vegetable garden

Tall dahlias belong on the north side of a vegetable bed so they don't shade shorter crops (GardenPal suggestion), and a row of them doubles as a cutting garden. WSU notes that bees love dahlias, and open-centered forms such as singles and collarettes are the easiest for pollinators to use (GardenPal note from the flower forms Iowa State describes). See flowers that attract beneficial insects, when to plant cosmos and when to plant zinnias. Gladiolus are another summer cut flower planted around last frost; see when to plant gladiolus.

Frequently asked questions

When should I plant dahlia tubers?

After your last spring frost, once the soil is about 60°F (Oregon State, UC Master Gardeners): around March in mild California, mid-April in upstate South Carolina, and May in much of the North (UC Master Gardeners, Clemson, Iowa State).

How deep do you plant dahlia tubers?

In a hole 4 to 6 inches deep, laid flat with the eye up (Oregon State, WSU), covered with 2 to 3 inches of soil (UC Master Gardeners).

Should I water dahlias after planting?

Not until shoots appear, since wet soil can rot the tuber (Oregon State, WSU, UC Master Gardeners).

Can I start dahlias indoors?

Yes. Iowa State pots tubers in early April and plants them out after frost; WSU plants potted dahlias out in mid-May or at 10 to 15 inches tall.

Do I have to dig up dahlias in winter?

Below zone 7, yes: lift and store the tubers cool and frost-free. In zones 7 to 10 they can usually stay in the ground under mulch (NC State).

How far apart should dahlias be planted?

About 2 feet for most (Oregon State, RHS), closer for small types and 3 to 5 feet for the largest (UC Master Gardeners, Clemson).