Here's how to overwinter dahlia tubers. Gardeners in zones 3-6 will need to dig up their dahlia tubers and store them inside overwinter. Gardeners in zone 7 can leave their tubers in the ground if the area is well-draining and a very thick layer of mulch is applied. However, indoor storage is still advisable in zone 7. And gardeners in zones 8-10 can leave their tubers in the ground year-round.
When the first frost arrives, dahlia foliage will turn a dark brown color and the blooms will become mushy. If possible, leave the tubers in the ground for 1-2 weeks after the first frost before you dig them. This will allow their skin to toughen up and cure for storage. Take care to watch the weather during these weeks and if a freeze is expected, dig them immediately, because a frozen tuber is a dead tuber.
While you wait for the tubers to cure label your dahlias so that varieties don’t get mixed up during the digging and storage process. Use a waterproof marker to write the variety name on flagging tape and tie the label securely around the base of each plant.
After 1-2 weeks of curing begin to dig your dahlias. Cut the stalks down to 4 inches, and use a pitchfork to gently loosen the soil around the tuber clump, being careful to start far away from the central stem so as not to damage the delicate tubers. Lift the tuber clump and gently brush off excess soil. Check the clumps for signs of disease and discard any that look questionable, and cut away any tubers that may have been broken or damaged in digging.
At this point you can choose to either wash your dahlia clumps, or you can leave the soil around them for storage. I find it beneficial to keep the soil on the clumps, and to store them just as they came out of the ground.
Next, turn the clumps upside down and place them into open paper bags or bulb crates. This will allow any excess water to drain out from the stocks before storage. Bring the tubers into a cool location out of direct sunlight to dry for a few days.
After drying, cut the stalks back to 1 inch, and place the clumps into large paper bags. Roll down the tops of the bags and place them into boxes. It’s a good idea to label your bags and boxes in case your initial tags are lost or damaged. Store the boxes in an unheated basement or any location that maintains a consistent temperature between 40-50 degrees F with 70% humidity. Make sure the storage area has no chance of dipping below freezing at any point.
Check the tubers once a month for signs of mold, rot, or shriveling and make any necessary adjustments. If the tubers are molding, rotting, or sprouting, the storage area and medium are likely too wet and warm. If the tubers are shriveling, the storage environment is likely too dry. A small amount of shriveling can be remedied by spritzing the tubers with a bit of water, but pay attention to what the tubers are doing and make any necessary adjustments to their storage medium and location.
Once the winter has passed and spring has sprung you can open your boxes and start the process of growing beautiful dahlias once again!