Dahlias are bold performers in summer garden

  • Some dahlias sport ball-shaped blooms.

    Some dahlias sport ball-shaped blooms. Courtesy of Diana Stoll

  • Dark foliage shows off the flowers of dahlia Mystic Haze.

    Dark foliage shows off the flowers of dahlia Mystic Haze.

By Diana Stoll
Updated 6/22/2018 6:23 AM

No doubt about it, dahlias are flamboyant. Not content to share attention in the summer garden, they try to steal the show with flashy blooms in just about every color except blue. Dinner-plate varieties with flowers the size of, well, dinner plates are often the first to come to mind when considering dahlias, but there are types with flowers as small as 2 inches across and others in between.

Their flowers offer a wide range of shapes, too. Dahlias are categorized by flower type -- single, double, anemone-flowered, ball, cactus, semi-cactus, decorative, mignon, peony and waterlily. Flowers of all types are held on sturdy stems.


Dahlias begin flowering in early to midsummer and continue until frost, long after many other plants have called it quits for the season.

The stature of plants varies so there is a size for every garden. Some reach a foot tall, perfect for the front of the garden or containers like Figaro Mix. Although these plants grow just 12 to 14 inches tall, they offer an abundance of impressive double and semidouble flowers in a bright mix of yellow, orange, pink and red. Others are garden goliaths up to 6 feet tall, ideal for the back of a perennial border.

The foliage of dahlias is attractive from the time it emerges in spring until it is blackened by frost. Some varieties boast burgundy or dark-colored leaves that contribute additional color to the garden and containers while providing a lovely backdrop for flowers. Bishop of Llandaff is a colorful, miniature-flowered peony type, growing 2 to 4 feet tall and boasting brilliant scarlet flowers and attractive bronze-red foliage.

In northern Illinois gardens, dahlias are most often grown as annuals and discarded after frost. Their tubers can, however, be dug from the garden and stored in a dormant state in a dark, frost-free space for the winter. Cut the foliage to about 6 inches after frost. Dig tubers and allow them to dry before storing them in vermiculite.

Tubers can be started indoors in moist potting mix several weeks before the average last frost date. Plant them several inches deep in moist potting mix, place pots under lights or in a sunny window, and water sparingly until new growth emerges. Begin feeding with a fertilizer higher in phosphorus and potash than nitrogen, like 5-10-10 or 10-16-14, when the foliage is a few inches tall. Plants started indoors should be gradually introduced to conditions outdoors to harden them off before planting in the garden.

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Dahlia tubers can also be planted directly in a sunny spot in the garden after the last frost. Dig deep holes and amend the soil with organic matter before planting tubers, with their points facing down, 6 inches deep. Their spacing will depend on the type of dahlia. Large dinner-plate varieties need considerably more room than smaller types. Water thoroughly after planting and then wait until new growth appears before watering again. Begin fertilizing when plants are a few inches tall. Do not overfeed -- it will result in lush, but weak growth.

Sizable transplants are also available for purchase at local garden centers. Dig a wide hole and add organic matter before planting these at the same depth they are growing in their pots.

Large-flowered dahlias, like Purple Gem with its intense purple, cactus-type flowers, or Kelvin Floodlight with its bright yellow, dinner-plate blooms, need staking to hold up their ample blooms. Insert a sturdy stake when planting to reduce the chance of injuring tubers later.

Deadheading regularly will encourage more flowers and keep plants looking their best. Cut some for bouquets, too. Their blooms are not only flamboyant in the garden, they are flashy in a vase for as long as a week.

• Diana Stoll is a horticulturist, garden writer and speaker. She blogs at gardenwithdiana.com.

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