When to Plant Dahlias

The rule: warm soil around the last frost

Dahlias are tender. Frost kills the tops, and the tubers rot if they sit in cold, wet soil. UF/IFAS puts it simply: don’t plant until the soil has warmed. Most extension services tie that to the average last frost. WSU, Iowa State, Colorado State and an Illinois Extension guide to tender bulbs all plant after it. Another Illinois Extension article allows planting one to two weeks before it.

The dates for your zone above combine those views: from about two weeks before your average last frost to about a week after. In a cold, wet spring, use the later end. WSU also suggests planting when the soil is a bit dry rather than wet.

Why later is not a problem

Dahlias bloom from midsummer until the first hard frost, and planting a few weeks late costs little. UGA notes that many dahlia growers are in no hurry: in Georgia, the most beautiful flowers come from late plantings that peak in September and October, and June plantings often produce the most perfect fall flowers.

Where the growing season is short, starting tubers indoors is the way to get flowers sooner.

Starting dahlias indoors

To get flowers sooner, start tubers in pots under cover:

• Iowa State plants each tuber horizontally in an 8-inch pot in early April, half filled with potting mix and topped with another 1 to 2 inches. Once growth appears, the pots go into a sunny window or under lights.
• WSU’s Thurston County Master Gardeners start them in pots in a greenhouse or other protected spot in April and plant out in mid-May or when the plants are 10 to 15 inches tall.
• UGA suggests starting tubers in pots or gallon cans when you’re short on time to prepare the bed.

Harden off potted dahlias for several days before planting out, starting in shade and gradually adding sun (Iowa State). Plant after frost danger has passed.

Timing by region

Pacific Northwest. In western Washington, WSU plants any time after the last frost, which there falls between about April 15 and June 1. Mother’s Day is typically safe.

Upper Midwest and Rockies. Iowa State and Colorado State plant after the last spring frost. Iowa State’s indoor start in early April gives earlier bloom in short seasons.

Southeast. UGA plants in April in south Georgia and May in the northern half of the state. The Coastal Plain’s summer heat makes dahlias harder to grow there, though flowers improve during cool, cloudy spells. Later plantings bloom into fall.

Florida and warm-winter areas. UF/IFAS planting months are May for zones 7 and 8; March, October and November for zone 9; and September through December for zones 10 and 11. In zones 9b to 11, dahlias are generally planted in fall for winter and spring color, because they dislike high summer heat. Choose heat-tolerant varieties; UF/IFAS Gardening Solutions suggests ‘Elsie Huston’ for Florida, along with light afternoon shade and a good layer of mulch.

What happens if you plant too early or too late

Too early. Tubers in cold, wet soil can rot before they sprout, and shoots that come up before the last frost can be killed.

Too late. In the South and the Pacific Northwest, late plantings simply bloom later. In short-season northern gardens, a late start leaves fewer weeks of bloom before the first fall frost ends the season.

The end of the season

Dahlias keep blooming until a hard frost. After frost blackens the plants, cut them back, wait a week or two, then dig and store the tubers at 35 to 50°F until it’s time to plant again. In zones 9 and 10, UF/IFAS says tubers can stay in the ground. For planting depth, staking and care, see how to grow dahlias.