Are you a gambling gardener? Late fall is the time to roll the dice to decide how to overwinter your dahlias, queen of the fall garden.
If we lived in zones 8-10, where winter temperatures rarely dip below 20 degrees Fahrenheit, we would cut the plants to the ground and call it good.
In zone 7, we may be able to leave them successfully if the winter is temperate and water doesn’t funnel into the tubers and cause rot.
Unlike the hardier bulbs, tubers are thin-skinned and will be destroyed if a freeze reaches a depth of 6 inches in your garden.
If you are willing to bet that your dahlias will tough it out in your well-drained soil, you can rejoice when they make it through or treat your dahlias as annuals and plant a fresh bunch in the spring. Many Pacific Northwest gardeners prefer to do this rather than to dig and store.
The following in-ground procedure will further safeguard your dahlias and requires just a few easy steps. After the first killing frost, which will blacken the stalks, or by mid-November (whichever comes first), cut the stalks to below ground level.
Inspect the area where the stalk connects with the tuber and cut off any hollow stalk that remains until there are no tube-like protrusions that could catch and funnel water down into the plant.
Cover the dahlia area with plastic to keep the winter rains at bay. Add a few inches of mulch, soil, leaves, straw or compost over the area to add protection from severe cold. In March, you can remove the extra layers of protection.
When sprouts appear in spring, you can leave the dahlias in the ground or choose to dig and divide the plants and get a jumpstart on the dahlia bloom season.
Once you have carefully divided them (see description below), let the cuts seal overnight before planting in containers. Don’t wait too long to replant or the tubers will become dehydrated.
Ensure each divided tuber contains an eye, the small white or pinkish swollen dot located around the base of the old stem. These eyes are the growing point for next year’s stem. If you find a tuber without an eye or one growing out of another tuber, discard it because it is unlikely to produce a healthy new plant.
Be sure to label your variety, as it is easy to forget when working with multiple plants.
Plant dahlias in potting soil or sand 2-3 inches deep about six weeks before you transplant them outside. Keep them in a warm area over 60 degrees Fahrenheit in soil that is slightly damp but not wet.
Transplant outside after all danger of frost is passed. If your start is over 12 inches tall, pinch it back before transplanting.
Dig a hole that will place the tubers 6 inches deep. Add a handful of bone meal in each hole, followed by the tuber. It is a good idea to place your stakes in at this time so you won’t pierce the tubers if you place them later in the season.
Place stakes about 2 inches from the tuber. Be sure to keep plants watered until the roots are established (approximately one week).
Why go to the bother of digging, dividing and storing dahlias in the winter, as required in zones 3-6? If you have a few treasured dahlias you absolutely do not want to replace, or if you want to share a specific variety with family and friends, this method ensures a better survival rate.
Also, if your plant is getting so entrenched that the tubers are growing too large, it may be getting “tuber-bound.” Separate them before they become a tangled mess, the stems weak and the flowers small.
For those willing to take the time and effort to dig and store dahlias, the process includes the following:
After a hard frost or in mid-November, cut stalks to 6 inches above the ground. Wait at least a week before digging to force the eye of the plant to develop and make dividing easier.
Dig your dahlias using a garden fork or shovel. Dig down on all four sides about a foot away and lift clumps carefully so their necks don’t break.
Wash off excess dirt using a garden hose and trim all feeder roots and long tapered roots. With a permanent marker, write the varietal name or code number on a tag or label — or directly on the tuber. Dry clumps overnight in a cool, dry location. Do not leave them in the sun.
Divide your clumps using a sharp paring or hooked-blade knife. Carefully cut the tuber away from the main stalk, being sure you get an eye with each tuber. Cut surfaces should be dried overnight before storing.
Various storing mediums can be used. Some gardeners use mesh net bags, others store in peat moss, sand or pet bedding, e.g., sawdust/shavings. Do not use potting soil.
Store in a cardboard box or line your containers with 10-12 sheets of newspaper. Store in a cool, dry area with temperatures between 40-60 degrees Fahrenheit. Tubers will shrivel if too warm or freeze/rot if too cold.
Check them once a month throughout the winter and remove any rotting tubers. Moisten lightly with a spray bottle if they appear to be dehydrating.
Plant in the spring as described above, whether in the greenhouse or directly into the garden later.
The more you read about overwintering dahlias, the more opinions you will find, especially regarding the preparation of the tuber for storage, the storage medium and the storage container.
Use the tried and true method above or experiment yourself and find what works best for you. There are also many pictorials in books and online videos that can help you visualize the process.