Dahlias are gorgeous flowers that bloom from midsummer through autumn. They bloom in a rainbow of colors and in sizes, ranging from petite 2-inch pompoms to giant 15-inch “dinner plates.” Most varieties reach 4 to 5 feet tall. The Dahlia plant is a member of the Asteraceae family, which also includes the sunflower, daisy, chrysanthemum, zinnia, and aster. The beautiful and colorful dahlia blooms are perfect for cut flower arrangements. These plants love moist, moderate climates and brighten up a sunny garden with a growing season that’s at least 120 days long.
Dahlias are actually fairly easy to grow. They are grown from tubers, not bulbs and can be planted in the spring once the danger of frost has passed. They will not grow in cold soil, so it’s best to wait until the soil temperature has reached 60 degrees. You can also start dahlias indoors in early spring, and then plant them outdoors once the soil has warmed. Grow dahlias in a location that will receive full sun (at least 6-8 hours a day). Use rich loamy soil that contains plenty of organic matter and drains well. If your soil is a denser clay type, add in sand, peat moss or manure to loosen the soil for better drainage. They thrive in a neutral soil pH of around 6.5. Plant tubers two to six inches deep, depending on the variety. The plants need good air circulation to thrive, so place smaller varieties about 12 inches apart and large cultivars up to three feet apart.
Do not water dahlia tubers until green growth shows above the surface. They will not need water before their root system develops. When new growth appears, water them regularly (once or twice a week) and fertilize them. Make sure to water deeply, as large tubers may be planted six inches deep. If summer days are especially hot and dry, you may need to water more frequently and never let the soil dry out. Use mulch to help keep the moisture level even. Dahlias are heavy feeders – the more food they receive, the larger the plant will grow, and the bigger (and more numerous) their flowers will be. Use a fertilizer with a high percentage of phosphorus (perhaps a 10-30-20 ratio) to promote blooming. For the amount to use, follow the product label instructions. Do not use one with a high percentage of nitrogen. Too much nitrogen creates lush foliage, but few blooms. If you plan to dig up and store your tubers for winter, stop fertilizing your plants at the end of August. You don’t want to encourage more growth late in the season as you prepare the tubers for dormancy.
Dahlias do best with a one-time task of pinching off the tops after the tubers have sprouted and formed branches. Wait until three sets of branches appear, and then top the stem just above the highest set of branches. This pruning tactic encourages the plant to grow more branches, thus creating more blooms. Within months, you’ll be treated to a garden full of stunning, colorful, and oversized blooms. Just be sure to deadhead faded blooms to keep the plant looking tidy and to encourage flowering. Dahlias, especially taller varieties, may bloom best when staked to keep the plant from flopping to the ground.
Common pests, like slugs, earwigs, caterpillars, and thrips, adore dahlias. Slugs are especially problematic when the foliage is young and tender. Once the plants mature, slugs are usually not a problem. Some gardeners have trouble with deer, while others claim the deer avoid their dahlias. This may depend on the other types of plants in your garden and what the deer prefer the most. To be safe, you might want to use repellent if you have deer visiting your yard. Dahlias are also prone to powdery mildew and other fungal diseases. Keep the foliage as dry as possible and space out your plants to provide good air circulation. If you notice an infection, treat it with neem oil or another natural solution. Stem rot can occur in dahlia beds that have been watered too profusely or in those that have poor drainage and heavy soil. To avoid this, always amend your soil before planting, and never allow for standing water in your garden beds. Dahlias also seem to topple over and wilt during the heat of a midsummer day. This is not necessarily a problem, as it’s the plant’s way of adapting to stress. If the bed is moist, your dahlias should perk back up once the sun goes down.
In our area, dahlias will not survive the winter. If you want to try and re-plant them the next year, the tubers will need to be dug up and stored during the cold months. To do so, first, select the healthiest plants from your garden, and then wait for the first hard frost. Then cut the plant back to 4 inches above the ground. Leave the tubers in the ground for one week before digging them up. Dig up each root ball starting at least one foot away from the stem. Carefully remove the tubers (taking care not to damage them) and clean off excessive dirt. Allow the root ball to air dry in a place that is sheltered from frost and out of direct light. Then, you can either bag and store the whole root ball, or carefully separate the tubers and store them individually. Store your tuber pots, boxes, or bags in a cool, dark, and humid place with temperatures between 40 and 50 F. A basement or root cellar works best. Do not let them freeze.
Hoffmann Hillermann Nursery & Florist 