May Is Dahlia Planting Time

Mother’s Day signals the time to plant dahlias. Like chrysanthemums, many people associate dahlias with the fall but dahlias can bloom from the beginning of summer to frost. Dahlia flowers are available in a wide range of sizes, colors, and shapes. Each bloom can be 2 inches across to more than 10 inches, in all colors except blue. Plants can reach one to 6 feet tall. Some plants have beautiful dark foliage instead of green leaves. Although there are 40 plus species there are thousands of cultivars. In addition, there are numerous forms such as the single, peony, anemone, collerette, star or single orchid, double orchid, cactus, waterlily, ball, and pompom.

Planting Tubers or Seed

To grow dahlias, you can either purchase tubers or start them from seed. If you purchase a tuber that is a named variety you will know exactly what the plant will look like. Plus, if you purchase cultivars that have been trialed and proven to do well in this area, you will have a good chance of success. With dahlias, it is important to purchase one that is known to do well in your area.  It is best to ask for recommendations from local dahlia growers. Seed is cheaper than tubers but there is a lot of variability with plant vigor and flower color. Although the seed will grow and produce a plant with pretty flowers for the garden, the flowers may not be exhibition quality.

Tubers can be planted outside in the ground after the average last frost date (Mother’s Day in the Washington DC metro area). They prefer warm soil with temperatures between 60 and 65 degrees minimum. Tubers also can be started indoors in April in containers under fluorescent lights or by the window to initiate growth. Seed should be started indoors under lights because planting seed in cold soil may retard the germination rate. Starting seed outdoors in May will only delay the time to reach blooming stage.

Caring for Dahlias

“Put the tuber in a four-inch hole and cover so that it is just peeking through. This way you can see the growth. When it grows, add more soil,” advises John Spangenberg, member of the National Capital Dahlia Society and owner of Crazy 4 Dahlias. John is a long time dahlia enthusiast who also sells tubers from his website.

Growing a dahlia plant is similar to growing a tomato plant: full sun and plenty of water and food. A dahlia can grow in less than 6 hours of sun but would not produce as many flowers. After planting the tubers, insert stakes such as tomato cages, peony hoops, or posts. In the beginning, dahlias will require plenty of water, generally one inch of water per week. Dahlias are heavy feeders and will need fertilizer throughout the summer. Seaweed-based liquid fertilizers or slow release fertilizers work well. Dahlias appreciate a leaf or straw mulch to keep the tubers cool and to prevent weeds.

Encouraging More Flowers

In the beginning of the growing season, John recommends topping the plants to encourage bushier, sturdier plants with more flowers. The center bud (not flower bud but central growth) should be pinched back. “When you see three to four sets (or pairs) of leaves, break the center top off,” explained John.

Later in the season when flower buds appear, disbud or cut off smaller, lateral flower buds to encourage the top bud to form a single, larger flower. When a dahlia flowers, there are three stems with three buds in a v-shape. When the outer two smaller buds are the size of peas they should be cut leaving the center flower bud.

“The more you cut your flowers, the more flowers you get,” said John. If you don’t cut a flower for a vase, make sure you at least deadhead them. Deadheading is cutting off and disposing flowers that are past their prime to encourage the plant to produce more flowers. For the vase, it is best to cut flowers in the morning when the weather is cool and the plant is most turgid.

Saving Tubers in the Fall

Dahlias are native to Mexico. Here in Washington DC they are treated as tender perennials and may or may not come back the following year. In order to ensure that the plants can be grown again next year, most gardeners lift and store the tubers in October.

“In the fall when get the first frost, cut the plant a couple of inches above the ground and let sit for a week or dig them up,” explained John. “You want to have the eyes develop and swell to be able to see them well. It helps to see the eyes when dividing the tubers. You can divide in the spring or fall but it is easier to divide in the fall.”

Dahlia tubers are swollen roots. Each tuber has to have an “eye,” which is a growing point in order to grow. From that eye the stem will emerge. In May, a single tuber with an eye is planted for a single plant. In the fall, when the plant is lifted out of the ground, there will be more new tubers joined together in an area called the crown. The “eyes,” or viable growing points, are in the crown. This can be stored as is or divided to create more plants.

John uses vermiculite in a box to store his tubers but there are many methods to store tubers. He finds vermiculite works best because it absorbs and releases moisture. Tubers should be in the coolest place in the house where there is constant temperature such as a crawl space or basement or a closet next to the outer wall of the house.

Because they are native to Mexico, one would think that dahlias would be easy to grow here with our sunny, warm summers. In fact, dahlias are native to a mountainous region in Mexico with more wind, less humidity, and cooler temperatures. Thus dahlias grow very well in the Pacific Northwest but have some difficulty in the mid-Atlantic. They need quite a bit of water, yet as heavy feeders, the rain can leach the nutrients. Plus the humidity can encourage disease. “In this area, we have issues with slugs, earwigs, rabbits, groundhogs, and deer,” said John. “Plus we have noticed that Japanese beetles prefer white and yellow flowered dahlias.”

Selecting Dahlias for Washington DC Metro Area

To choose a dahlia that performs well here, look to the National Capital Dahlia Society for recommendations. A branch of the American Dahlia Society (ADS), the National Capital Dahlia Society is comprised of dahlia enthusiasts and breeders who meet on a regular basis. Every year they manage a trial garden at the Agricultural History Park in Derwood, MD. “The display garden is a trial garden to test new introduction from across the country to see how they do here. We look at bloom quality and plant vigor and report this to the American Dahlia Society,” said John.

If you are really interested in learning more about dahlias, the American Dahlia Society has a national dahlia show every year and this year it will be in our area at the Sheraton Reston Hotel, Reston, Virginia, from September 22-25, 2022. This will be hosted by the National Capital Dahlia Society and will be open to the public, free of charge. More information is on the National Capital Dahlia Society’s website.