Dahlia Plant Care Guide
Dahlia is a genus of bushy, tuberous, herbaceous perennial plants native mainly in Mexico, but also Central America, and Colombia. A member of the Asteraceae or Compositae, dicotyledonous plants, related species include the sunflower, daisy, chrysanthemum and zinnia. There are at least 36 species of dahlia, with hybrids commonly grown as garden plants. Flower forms are variable, with one head per stem; these can be as small as 2 in (5.1 cm) diameter or up to 1 ft (30 cm) (“dinner plate”). This great variety results from dahlias being octoploids—that is, they have eight sets of homologous chromosomes, whereas most plants have only two. In addition, dahlias also contain many transposons—genetic pieces that move from place to place upon an allele—which contributes to their manifesting such great diversity.
The stems are leafy, ranging in height from as low as 12 in (30 cm) to more than 6–8 ft (1.8–2.4 m). The majority of species do not produce scented flowers or cultivars. Like most plants that do not attract pollinating insects through scent, they are brightly colored, displaying most hues, with the exception of blue.
The dahlia was declared the national flower of Mexico in 1963.
Plant type: Flower
Sun exposure: Full Sun
Bloom time: Summer
In cold climates of North America, dahlias are known as tuberous-rooted tender perennials, grown from small brown biennial tubers planted in the spring.
These colorful spiky flowers generally bloom from midsummer to first frost, when many other plants are past their best. They range in color and even size, from the giant 10-inch “dinnerplate” blooms to the 2-inch lollipop-style pompons. Most varieties grow 4 to 5 feet tall.
Though not well suited to extremely hot and humid climates, such as much of Texas and Florida, dahlias brighten up any sunny garden with a growing season that’s at least 120 days long. Dahlias thrive in the cool, moist climates of the Pacific Coast, where blooms may be an inch larger and deeper.
- Dahlias thrive in humus-rich, well-drained soil in full sun. Choose a location with a bit of protection from the wind.
- Large dahlias and those grown solely for cut flowers are best grown in a dedicated plot in rows on their own, free from competition from other plants. Dahlias of medium to low height mix well with other summer flowers.
- If you only have a vegetable garden, it’s the perfect place to put a row of dahlias for cutting (and something to look at while you’re weeding!).
- Dahlias start blooming about 8 weeks after planting, starting in mid-July.
- Don’t be in a hurry to plant; dahlias will struggle in cold soil. Plant them a little after the tomato plants go in; in Zone 6, that’s in early June.
- Some gardeners start tubers indoors a month ahead to get a jump on the season.
- Avoid dahlia tubers that appear wrinkled or rotten. A little bit of green growth is a good sign.
- Don’t break or cut individual dahlia tubers as you would potatoes. Plant them whole, with the growing points, or “eyes,” facing up, about 6 to 8 inches deep.
- There’s no need to water the soil until the dahlia plants appear; in fact, overwatering can cause tubers to rot.
- Dahlias are heavy feeders. In general, they benefit from a high-nitrogen liquid fertilizing every week in early summer and then a high-potassium liquid fertilizing every week from midsummer to early autumn.
- Like many large-flower hybrid plants, the big dahlias may need extra attention before or after rain, when open blooms tend to fill up with water or take a beating from the wind.
- Bedding dahlias need no staking or disbudding; simply pinch out the growing point to encourage bushiness, and deadhead as the flowers fade.
- For the taller dahlias, insert stakes at planting time. Moderately pinch, disbranch, and disbud, and deadhead to produce a showy display for 3 months or more.
- Dahlia foliage blackens with the first frost, but tubers may be covered with a deep, dry mulch and left in the grounds in Zone 8 and warmer. Elsewhere, they must be lifted for the winter.
Taking Up the Tubers
Dahlias don’t handle frost well. In cold regions, if you wish to save your plants, you have to dig up the tubers in early fall and store them over the winter.
- Before the first frost nips the foliage, cut back stems to 6 inches and lift out the plants, using a spading fork.
- Gently shake the soil off the tubers.
- Cut rotten tubers off the clump and leave upside down to dry naturally.
- Pack in a loose, fluffy material (vermiculite, dry sand, Styrofoam peanuts).
- Store in a well-ventilated, frost-free place—40 to 45 degrees F is ideal, 35 to 50 degrees F is acceptable.
- Take out the tubers in the spring, separate them from the parent clump, and begin again.
- 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.
Picking a favorite dahlia is like going through a button box. There is a great spectrum of color, size, and shape. Here are some popular choices:
- ‘Bishop of Llandaff’: small, scarlet, intense flowers with handsome, dark-burgundy foliage
- ‘Miss Rose Fletcher’: an elegant, spiky, pink cactus plant with 6-inch globes of long, quilled, shell-pink petals
- ‘Bonne Esperance’, aka ‘Good Hope’: a foot-tall dwarf that bears 1-1/2-inch, rosy-pink daisies all summer that are reminiscent of Victoria bedding dahlias (though it debuted in 1948)
- ‘Kidd’s Climax’: the ultimate in irrational beauty with 10-inch “dinnerplate” flowers with hundreds of pink pentals suffused with gold
- ‘Jersey’s Beauty’: a 7-foot tall pink plant with hand-size flowers that brings great energy to the fall garden.
Wit & Wisdom
The dahlia was named for Anders Dahl (botanist), born on March 17, 1751.
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)