The American Dahlia Society (ADS) currently recognizes 42 species of dahlias. Although dahlias grow better in some regions of the United State than in others, the plants are beloved by fervent gardeners and dabblers alike due to their range of forms and colors and their long blooming season. Yet some aspects of dahlias will keep even the most ardent of green-thumbers from embracing them.
“The Dirt Diaries” author Lynn Hunt, who is also an accredited horticultural judge and consulting rosarian emeritus for the American Rose Society, decided to try her hand at dahlias six years ago when she moved from Maryland to Sapphire, North Carolina. “I discovered the head-turning beauty of dahlias,” Hunt said. “What a delicious addition to the landscape they are, with colors in every shade but blue, and blooms as small as a pincushion or as big as a dinner plate.”
Dahlias fill vases from late spring or early summer until fall, and are relatively “easy to grow,” Hunt noted. But when that first fall frost threatens, dahlia growers are faced with a dilemma: Should they dig up the tubers and store them, or take a chance and leave them in the ground?
The Allure of Dahlias
Drew English usually opts for digging and storing. Manager since 2018 of the almost century-old dahlia garden at the historic High Hampton Resort in Cashiers, North Carolina, English was first a fan of the flowers before he became their professional caretaker. “It was in the summer of 2010 that I first saw the High Hampton dahlia garden, and I was struck by the amazing variety of blooms,“ he said. ”I was in love! Having been born and raised in the low country of South Carolina, the dahlia was a plant that was foreign to me, but no longer. I read, studied, and investigated all there was to know about the species.”
English’s first personal dahlia purchase was a deep-purple dinner plate variety called “Thomas Edison.” “From that point on, my garden grew ... evolving from my humble little 12-foot by 6-foot fenced bed to a now terraced hillside garden of about 110 plants. Some of my favorite cultivars are ‘Hollyhill Spider Woman,’ a large purple and white variegated cactus bloom; ‘Show-n-Tell,’ a vibrant, red and orange dinner plate dahlia with huge, luscious petals; and ‘Maarn,’ with its perfectly delicious, orange-sherbet, spherical ball bloom.”
Since then, English has decided to make dahlias not just a source of enjoyment, but a profession as well. High Hampton’s dahlia garden has been moved around the resort property several times, but it’s currently front and center, near the entryway, and features at least 300 varieties of dahlia, including a few originally grown by the resort’s founder, Dr. William Halsted, who purchased varieties from Europe priced at upward of $1,000.
“The High Hampton garden has 14 ‘old stock’ tubers, also considered ‘legacy tubers,’ which gardeners have been passing down from year to year,” English said. “History and tradition have always been very important to me, and in some way, I feel a responsibility to continue the passion of the Halsteds and High Hampton by continuing to grow these marvelous plants.”
During the spring-to-fall growing season, dahlias are generous with their blooms. However, like all plants, they do have specific needs. According to ADS, the plants will grow in pots or beds, but they need an environment with adequate drainage and partial to full sun. Dahlias need to be planted in healthy soil full of organic matter, English said. “I plant 700 to 800 dahlias [at High Hampton], and add organic compost annually, and regularly test soil. A lot of pests and diseases can be eliminated just by having good soil.
“Dahlias love sun: six to eight hours a day. If they don’t get enough sun, they become stringy and leggy. But really hot sun in some Southern areas dries them out fast, and they wilt. That is why people don’t usually try to grow dahlias in the mid-to-lower regions of South Carolina, in Florida, etc. The mountainous areas and Pacific Northwest don’t have oppressive heat, and those regions have cooler summer nights, which allows dahlias to recuperate.”
Regarding watering, ADS warns that excessive watering of young plants can lead to rot. A rule of thumb for maturing and mature plants is to water them if the local rainfall is less than one inch in seven days. Pots, however, dry out more quickly and may require regular watering.
Back to Wintertime Prep
English said he’s still learning the best ways to preserve dahlias through the winter months. “Unfortunately, there is no hard and fast rule,” he said. “It’s trial and error, because every region’s climate and weather conditions are different.” Since dahlias grow from tubers, winter dormancy poses challenges. Deciding whether to dig them up and replant in the spring or just leave them in the ground depends on personal choice.“You’re always taking a chance if you leave them in the ground, because some winters are so harsh that they end up freezing—even if covered with plastic and plenty of mulch, such as straw,” English said. “But taking them out of the ground is tricky as well. I always pull the High Hampton tubers, because of the legacy of these dahlias. I can’t risk losing them to a cold winter.”
- After the first freeze, cut them back to the ground.
- Place cardboard or several sheets of newspaper down as a first layer, then put a thick layer of compost, and then use mulch (such as pine or hardwood) over the tubers to insulate.
The main risk when insulating dahlias for the winter, besides freezing, is rotting due to a particularly wet winter.
- Dig them up after they’ve gone dormant—meaning after the first freeze, when the plant stops growing.
- Trim off any large roots.
- Clean each tuber with a bucket of water and a hose, then let them dry off for a few days in the sun. “They still need to be somewhat moist, but not noticeably wet,” English said, “but they also don’t need to be too dried out, or they will shrivel.”
- Leave tubers as clumps, or divide.
- Store in a sturdy wooden box, vegetable box, or plastic container with holes cut in the lid for ventilation. Mediums for storage include wood shavings, peat moss, vermiculite, or nothing at all.
After the Thaw
Each spring, after the ground has warmed and there’s little threat of frost, stored dahlia tubers can go back into the ground. English plans High Hampton’s garden, driving stakes—which provide stability for the plants—and tagging each stake based on the type of tuber planted.ADS says: “Put the tuber in a hole several inches deep with the ‘eye’ on the tuber facing up. The eye is the point on the shoulder, or crown, of the tuber, from which the plant grows. If you are planting a number of dahlias in the same location, they should be separated by about two feet, to give each plant room to grow.”
After dahlias begin growing in late spring or early summer, English prunes them. While he says many gardeners gasp when he mentions this, a lack of pruning may result in weak stems. “Pruning, or pinching back, early on, when the plant is about a foot or two, creates hardy stems and a strong base that can hold those heavy blooms. Cut, or use fingers to pinch off, the plant right above the larger leaves, where tiny leaves are sprouting. It doesn’t take long for the plant to grow again, and within a few weeks it will bounce back and new buds will form—and gardeners get a lot more bang for their buck.”
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