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Dahlias: How to Plant, Grow, and Care for Dahlia Flowers | The Old Farmer's Almanac

Dahlias

Botanical Name
Dahlia spp.
Plant Type
Sun Exposure
Bloom Time
Hardiness Zone
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How to Plant, Grow, and Care for Dahlias

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Dahlias are gorgeous flowers that bloom from midsummer through autumn. Available in a rainbow of colors and a number of sizes, dahlias are a lovely flower to grow in any garden. Here’s how to care for them in yours! 

About Dahlias

Dahlia is a member of the Asteraceae family, which also includes the sunflower, daisy, chrysanthemumzinnia, and, of course, aster.

Dahlias inspire awe and a good cheer. Growing vegetables? Put a row of dahlias on the border, where they will not shade your edibles. They make for lovely cut flowers.

Dahlias love moist, moderate climates. Though not well suited to extremely hot climates (such as southern Florida or Texas), dahlias brighten up any sunny garden with a growing season that’s at least 120 days long. 

The tubers are planted in the ground in late spring. They are considered a tender perennial in colder regions of North America. They are reliably winter hardy in hardiness zones 8 to 11, although gardeners in zones 6 and 7 may have luck keeping them in the ground as well. In colder zones, dahlias can either be treated as annuals or dug up after the first frost and stored indoors for winter. (See what hardiness zone you’re in!)

Picking a favorite dahlia is like going through a button box. In addition to coming in a rainbow of colors, dahlia flowers can range in size from petite 2-inch lollipop-style pompoms to giant 15-inch “dinner plate” blooms. Most varieties grow 4 to 5 feet tall.

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Planting

Dahlias thrive in 6 to 8 hours of direct sun, especially morning sunlight. They benefit from protection from wind, as strong winds can blow over tall dahlias if they are not supported. Consider their size at maturity when planting.

Dahlias will do best in rich, well-draining soil with a pH level of 6.0 to 7.5. Amend heavy clay soil with aged manure or compost to lighten and loosen the soil texture for better drainage.

When to Plant Dahlias

  • Dahlias will not tolerate cold soil. Plant when the soil reaches 60ºF (15°C) and any danger of frost has passed.
  • Planting dahlias a few days after tomatoes are planted in the ground is a good rule of thumb.
  • Some gardeners start tubers indoors in containers a month ahead to get a jump on the season. Medium to dwarf-size dahlias will do well in containers.

How to Plant Dahlias

  • Avoid planting dahlia tubers that appear wrinkled or rotten. Pink “eyes” (buds) or a little bit of green growth are good signs. 
  • Plant large dahlias and those grown solely as cut flowers in a dedicated plot where they will be free from competition from other plants. Set tubers in rows spaced 3 feet apart. If you plant dahlias about 1 foot apart, they make a nice flowering hedge and will support each other.
  • Plant medium- to low-height dahlias, usually in the 3-foot tall range, among other summer flowers. Set them 2 feet apart.
  • Plant the smallest bedding dahlias, grown from seed, 9 to 12 inches apart.
  • To plant the tubers, start by digging a 6- to 8-inch deep hole.
  • Set a tuber into the hole with the growing points, or “eyes,” facing up. 
  • Do not break or cut individual dahlia tubers (as you would with potatoes).
  • Cover the tuber with 2 to 3 inches of soil. (Some say 1 inch is adequate.)
  • As the stem sprouts, fill in with soil until it is at ground level.
  • Do not water the tubers right after planting. This encourages rot. Wait until the sprouts have appeared above the soil, then water.
  • The planting hole should be slightly larger than the root ball of the plant and incorporate some compost or sphagnum peat moss into the soil. It also helps to mix a handful of bonemeal into the planting hole. Otherwise, do not fertilize at planting.

Growing Dahlias in Containers

Medium- to dwarf-size dahlias do well in containers that have drainage and are big enough to support the plant at maturity. Generally, a 12x12 inch container will suffice.

  • Use a soilless mix and co-polymer moisture-retaining crystals, per the package’s guidance.
  • Follow the depth requirements.
  • Cover the tuber with a few inches of soil-crystal mix.
  • Spray water on the tuber, if necessary, until growth starts.
  • Do not water if the soil is damp 1 inch below the surface.
  • Fertilize through summer as directed.
  • Add soil if the roots become exposed.

Check out our video to learn more about growing dahlias in your garden:

Growing

When dahlias are established, water 2 or 3 times a week and more in hot, dry climates. Be prepared to tend to plants before or after rain. Large, open blooms tend to fill up with water or take a beating from the wind. 

  • Dahlias start blooming about 8 weeks after planting, usually by mid-July.
  • Dahlias benefit from the occasional application of a low-nitrogen, liquid fertilizer, such as 5-10-10 or 10-20-20. Fertilize after sprouting and then every 3 to 4 weeks from midsummer until early autumn.
    • Note: Do not over fertilize, especially with nitrogen, or you risk small or no blooms, weak tubers, or rot.
  • When plants are about 1 foot tall, pinch out 3 to 4 inches of the center branch to encourage bushier plants and increase stem count and stem length.
  • For large flowers, try disbudding. Remove the two smaller buds next to the central one in a flower cluster. The plant will put all of its energy into fewer but considerably larger flowers.
  • Bedding dahlias need no staking or disbudding. Simply pinch out the center shoot just above the third set of leaves to encourage bushiness.
  • For more blooms, deadhead as flowers fade.
  • Do not bother mulching the plants. The mulch harbors slugs, and dahlias like the sun on their roots.

White dahlias

  • For taller dahlias, insert stakes at planting time. Moderately pinch, disbranch, disbud, and deadhead to produce a showy display for 3 months or more.
  • Dahlia foliage dies back with the first light frost in fall. Dahlias are reliably hardy in USDA Hardiness Zone 8 and warmer and can simply be cut back and left in the ground to overwinter. Cover with a deep, dry mulch. 
  • Farther north, the tuberous roots should be lifted and stored during the winter. (Some readers find, however, that dahlias will survive in Zone 7 or even Zone 6 if the winter isn’t too severe.) See Harvesting (below) for more information.

pink dahlias

Harvesting

Dahlias as Cut Flowers

The more you cut dahlias, the more they’ll bloom—and dahlias are beautiful in a vase. For a bouquet, cut stems in the morning before the heat of the day and put them into a bucket of cool water. Remove stems’ bottom leaves and place the flowers into a vase of water. Place the vase in a cool spot and out of direct sun. Check the water daily. The bouquet should last about a week.

Digging and Storing Dahlias for Winter

Unless you live in a warmer region (USDA Hardiness Zone 8 or warmer), you’ll have to dig up dahlias in late fall. Native to Mexico, Dahlias won’t survive freezing temperatures. Digging and storing dahlias is extremely easy and simple, and will save you the money that would otherwise go into buying new ones each year.

  • Gardeners north of Zone 8 are advised to lift and store the tuberous roots during winter.
  • Some Zone 7 growers claim that dahlias will survive winter in the ground if the weather isn’t too severe, but this is risky. They will not survive freezing temperatures. In Zone 6 or colder, plan to dig them up.

→ Find your USDA Hardiness Zone here.

  • Wait to dig up dahlia tubers until the top growth dies back or is killed by the first hard frost. See your fall frost dates for an idea of when frost usually arrives in your area.

  • Dahlia foliage blackens with the first frost. Take it as a warning to begin digging up (lifting) dahlias. Complete the task before a hard frost.

  • Delay cutting dahlia stems until right before digging, because the stems are hollow and can collect water, which in turn promotes crown rot and tuber decay.

  • Cut off blackened foliage, leaving 2 to 4 inches of top growth.
  • Carefully dig around tubers with a pitchfork, garden fork, or shovel. Avoid damaging them. The “neck” on dahlia tubers is delicate and can be easily damaged while digging.
  • Lift the clump and gently shake off the soil. Or, swish them around in a tub of water or use a garden hose to wash away any clumps of soil. Soil contains microorganisms that can cause decay in storage, so it’s best to remove as much as possible.
  • Cut off rotten tubers. 

How to Store Dahlias in Winter

  • After being dug up, the dahlia tubers then need to be allowed to dry and cure. Leave the clumps outside in the sun upside down to dry naturally for a few days. Or, place the dahlias in a well-ventilated area with a constant temperature between 60°F and 70°F and out of direct sunlight. 
  • When dry, pack them in loose, fluffy material, such as vermiculite, dry sand Styrofoam peanuts, or wood shavings, all of which work. Place tubers inside plastic bags or cardboard boxes filled with material that maintains moisture around the tubers but allows air flow. Cover them with more storage medium before placing them in a cool storage spot.
  • Store in a well-ventilated, frost-free space: 40º to 45ºF is ideal, 35º to 50ºF is acceptable.

Tuber clumps can either be left intact for the winter and divided in the spring, or they can be divided in the fall. Some gardeners find that it is easier to divide in the fall, and divisions are more convenient to store. 

Check on the dahlia tubers occasionally over the winter. Remove any tubers that have started to rot before the decay spreads to healthy tubers.

Readying for Summer

In spring, separate healthy tubers from the parent clump and discard wrinkled or rotten ones. Plan to plant the survivors.

Each tuber must have at least one “eye” or piece of the crown attached or it will not develop into a blooming plant. The eyes are little pink bumps at the base of the stem.

→ See our gardener’s article on how to unpack and divide your stored dahlia tubers!

If this all seems like too much bother or you do not have the right storage place, skip digging and storing, and just start over by buying new tubers in the spring.

Wit and Wisdom
  • The dahlia was named for Anders Dahl (Swedish botanist), born on March 17, 1751.
  • In the 16th century, dahlias grew wild on the hillsides in parts of Mexico. There, they were “discovered” by the Spanish, who remarked on the plant’s beauty.
  • Both dahlia flowers and tubers are edible. The tubers taste like a cross between a potato and a radish.

The Dahlia you brought to our isle
Your praises forever shall speak

‘Mid gardens as sweet as your smile
And colour as bright as your cheek
.
–Lord Holland (1773–1840)

Pests/Diseases
  • Slugs and snails: Bait 2 weeks after planting and continue to bait throughout the season.
  • Spider mites: To avoid spider mites, spray beginning in late July and continue to spray through September. Speak to your garden center about recommended sprays for your area.
  • Earwigs and Cucumber Beetle: They can eat the petals though they do not hurt the plant itself.
  • Aphids
  • Deer: Find a list of deer-resistant plants to grow around your dahlias.
  • Powdery Mildew: This commonly shows up in the fall. You can preventatively spray before this issue arises from late July to August. 

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