When to Plant Dahlia Tubers in New Hampshire?

Spring Planting

• In New Hampshire, dahlias tubers should be planted in late May or early June, once the soil has warmed and all danger of freezing temperatures has passed. Two to three weeks before planting, loosen the soil for your dahlia bed to a depth of 12 inches and work in a 2- to 4-inch layer of compost or other organic matter to improve fertility and drainage. Resist the urge to plant your dahlias early in the spring. Wet soil and cold temperatures can rot the bulbs.

Fall Planting Problems

• As with planting your dahlias too early in the spring, planting in the fall allows your dahlias to sit in cold, wet soil that can lead to rot. In New Hampshire, where winters often bring long freezes, dahlia tubers will be destroyed if planted in the fall or left in the ground over the winter. These plants are native to Mexico and Guatemala, and the plants are unable to withstand a temperature lower than 20 degrees Fahrenheit.

Planting Dahlias

• Dahlias should be planted in a sunny area of the garden that receives at least five to six hours of sun each day. Prior to planting, inspect your dahlia tubers and discard any that feel mushy, look withered or appear rotten. Plant each tuber in a hole that is approximately three times as deep as the tuber is round. Leave 1 to 3 feet of space between each tuber, depending on the mature height of the plant; taller varieties will need more space, while shorter varieties can be planted closer together.

  For taller varieties of dahlias, insert a stake or tomato cage near them to help support the plants as they grow. Once you've finished planting, water the area well. If you like, add a 2-inch layer of mulch to the bed to help retain moisture and suppress weeds.

Overwintering Dahlias

• Although dahlias grow in all U.S. Department of Agriculture hardiness zones, the tubers can only be left in the ground in areas with mild winters in U.S. Department of Agriculture plant hardiness zones 7 through 10. In New Hampshire, you'll need to dig up and store your dahlias in October if you want to save them for next year's garden.

  Once the first frost destroys the stems and foliage, cut the stems to a height of 4 to 6 inches. Three to five days later, dig up the tubers and allow them to cure in a dry place for 24 hours. Use a soft brush to remove any soil from the roots and then place them in sand, peat moss, sawdust or perlite. Try to arrange the roots so they aren't touching one another. Store the tubers in a dry, dark place that remains 40 to 50 F degrees until you're ready to plant them in the spring.