When dahlias (Dahlia pinnata) are planted in a sunny location and well-draining soil they will reward you with large, colorful blooms from mid-summer until the first killing frost of fall/autumn.
These easy-care flowers perform their best during the hot, dry days of summer when most other flowers have withered under the summer sun. Dahlias are hardy in most growing zones and come in a wide variety of bloom colors and heights, making them a perfect choice for any home garden.
Use these tips to successfully grow and overwinter dahlias.
Dahlias only have two main requirements for thriving — plenty of sun and soil that does not remain soggy.
Select a location that receives at least 6-hours of direct sunlight each day. Incorporate 4-6 inches (10 – 15 cm) of compost into the soil to ensure good drainage. Amend the soil with compost even if the soil has good drainage–the compost will keep the plants fed and help prevent soil compaction throughout the growing season.
Dahlias are sold in the spring as tubers or bedding plants. The tubers are sometimes called ‘bulbs’ and look like a bunch of brown carrots with stems sprouting out of them.
You can start tubers indoors in early spring and transplant them outdoors to get a head start on the growing season. To start dahlias indoors, plant one tuber in a 6-inch (15 cm) container of potting soil. Place the tuber 3-inches (7.5 cm) below the soil surface, cover with potting soil, water thoroughly, and place container in a sunny window. Keep soil moist and turn container half a turn every 4-5 days to provide equal sun exposure to the plant.
After all danger of frost has passed in spring it’s time to plant dahlias outdoors. If planting tubers, plant them 3-inches (7.5cm) below the soil surface and cover with soil. Firm soil gently around the tubers.
When planting bedding plants, place them at the same level they were at in their containers and cover roots with soil. Water thoroughly.
Apply a 2-inch layer (5 cm) of organic mulch, like hay or shredded tree bark, at the base of plants to prevent weeds and help retain soil moisture. As the mulch slowly decomposes it will also help feed the soil and promote good drainage.
Dahlias need minimal after-care but do appreciate a little food and support. Apply a balanced organic plant food at spring planting time then again at the start of bloom time in mid-summer. Water only during times of prolonged drought and the plants begin to wilt.
Some varieties of dahlias can reach a mature height of 3-5 feet (90 – 150 cm) and will need to be supported with a trellis or stake. Dahlia blooms range in size from petite pom-pom blooms to open-faced, saucer-sized blooms and all make excellent cut flowers.
Cover plants with a sheet of newspaper to protect from early frost. Remove newspaper before the sun shines on the plants in the morning. This will allow the last remaining flower buds to open late in the season.
Dahlias are perennials but can be treated as annuals and replaced each year. If you want to save some garden money, dig up the tubers and plant them over and over again.
Before the soil freezes at the end of the growing season dig up the tubers. Cut the stems back to 6-inches (15 cm) and gently rinse the soil off each tuber. Lay tubers in a single layer on newspaper to dry for 24-hours in a location where they won’t freeze. Place clean, dry, tubers in a cardboard box and cover with peat moss or vermiculite and store in a cool, dark place until next spring.
Check on the tubers periodically through the winter. If storage conditions are too humid some tuber may become mushy. When this occurs, remove mushy tubers and discard, then increase ventilation to reduce the moisture level. If the tubers are wrinkled and dry, lightly mist them with water to help them re-hydrate.
Divide tubers in spring just before planting. Use a pair of sharp hand-held pruning shears and cut small tubers off of the large clump. Each tuber must have an ‘eye’, which is a small knob where it had been connected to the main stalk.
In climates that have cold, long winters, (growing zones 6 and below) it’s best to dig up the dahlias as described above and overwinter them indoors. Climates with mild winters (growing zones 7 and above) can overwinter dahlias in the ground.
In climates where winter temperatures rarely fall below 20° F (-6.66°C), leave your dahlia tubers right in the ground. After a killing frost in fall/autumn cut the plants back to 6-inches (15 cm) above soil level. The plants will start growing again when the weather warms up in spring.
In growing zones where the winter temperatures might dip below the 20-degree (-6.66°C) mark, dahlias will usually survive the winter outdoors if the tubers are insulated with a thick layer of mulch.
Cut plants back to 6-inches (15 cm) tall and cover them with 6-inches (15 cm) of lightweight organic mulch, like hay or fallen leaves. When the weather warms up in spring, gently pull the mulch off the plants so they can warm up and come out of the dormancy.
Another over-wintering method involves digging up the entire root ball and storing it overwinter. Remove the entire root ball and place it into a large container with damp potting soil or store them in cardboard boxes that are partially filled with damp growing mix, peat moss, or vermiculite.
Several clumps of dahlias can be placed together in a large black plastic trash bag for overwinter storage. Don’t seal the bag, just loosely gather the top so moisture stays in but air can still circulate.
At spring planting time, go through all of your saved dahlias tubers or clumps and discard any that are soft or completely dried out.