There are as many ways of storing dahlias as there are people who grow them. In certain regions, some gardeners choose to leave their dahlia tubers in the ground over the winter. We cannot do this in the northeast, because when the ground freezes the tubers will rot. Your best option is to dig the tubers and store them in a cool, dry place for the winter.
After the dahlias experience a killing frost, the stalks turn black and the flowers die. At this point, all the dahlias should be cut down to 4” to 6” in height from the ground. Leave them in the ground for two weeks so the dahlias can begin to cure. If you do not get a killing frost by Thanksgiving, start the process of cutting and lifting.
Dahlia tubers grow to all different sizes—small and large, long and short, fat and skinny. No matter what the size, the tubers will be easiest to divide if they are harvested after this two-week period; however, you can store the clump(s) and do your dividing in the spring. If you do, you need to know that the stems do become very hard which makes it more difficult to cut the tubers apart. Either method is fine and ultimately when you divide your tubers comes down to personal preference.
I prefer to divide in the fall because cutting the clumps is easier at this time. If you have a very large number of dahlias and not much area for storage, you may also wish to divide in the fall.
Recommended Method of Digging Tubers
Use a garden fork. Dig a circle about 12 inches around the plant and lift the clump carefully out of the ground. (Be careful not to damage the tender new tubers.) If the neck of a dahlia tuber is broken, it will NOT grow. However if the bottom part of the tuber gets cut off during the lifting process, don’t worry. The cut will heal and the tuber will be just fine.
Also, dig only as many clumps as you can process in one or two days. The eyes are prominent only for 48 hours after being dug; after that, they recede and are not visible until the following spring. If the tubers stay in the ground and the ground isn’t frozen, you can lift clumps over the course of a week or two and spare your back!
Use can also use a shovel; if you do, push the blade into the ground about 8-10” from the stalk of the dahlia plant. Most growers don’t recommend using a shovel because you are more likely to cut some of the tubers off with it. This does not cause any harm because a dahlia tuber does not require the entire tuber in order to grow. However, if you cut the tuber(s), you must allow the cuts to callous over (heal). If you don’t, the cut provides a perfect surface for mold or fungus to grow.
Preparing Tubers for Storage
Method 1: Lift the tuber and gently shake off excess soil. Use a gentle spray from your hose to clean and remove the remaining soil from the clump. Allow the clump to dry for a day or two in a cool dry place. You are now ready to divide the clump.
Method 2: Lift the tuber and gently wipe off the dirt on the clump. Allow the clump to dry for a day or two in a cool dry place. Use a soft, clean brush (e.g., an inexpensive brush that comes with a dustpan) and remove as much soil as possible. Take care not to brush too hard because you will break the stems at the “neck”.
After cleaning (by either method), cut off all the spidery feeder roots and cut the main stalk back to the crown of the clump.
Use a sharp paring knife, X-Acto knife, lobster scissors or garden clippers to divide the tubers. Discard any damaged tubers and any that don’t contain an eye.
Preparation for Fall Division: If dividing in the fall, use a fungicide (such as Daconil) to kill fungus on the tubers. After you divide the tubers, place them in a five-gallon bucket with ¼ cup of Daconil and soak for a minimum of 20 minutes.
Next, remove the tubers from the bucket (wearing rubber gloves) and put them on ‘raised’ nursery trays to keep the tubers out of water. Keep the tubers in the trays overnight; you may wish to run a fan to help them dry. The next day, if dry, place the tubers in micro-perforated bags or Ziploc bags filled with horticultural vermiculite, dampened peat moss, sawdust, or pet shavings, then place them in a wax-coated or heavy cardboard box, or a plastic container with holes for ventilation, and store them in a cool dark place that does not freeze for the winter. If you put them in Ziploc bags, leave the bags open at the top. If you close them the tubers will rot!
Alternatively, mix 1/8 cup garden sulphur in a 2-gallon heavy plastic bag containing horticultural vermiculite. Place cut tubers in the bag and shake to coat. The sulphur will adhere to the cuts and act as a fungicide. Use gloves when handling the sulphur, and take care not to breathe in the powder; it can cause health problems in some individuals.
The ideal storage temperature is 45 degrees F, but anywhere between 40 and 50 degrees is fine. Don’t forget to label your tubers before storage. Use an ink pencil if the tuber is wet or a Sharpie marker (fine point) if the tuber is fully dry. You can write on the tuber itself.
Preparation for Spring Division: After you have brushed off the dirt, trimmed the feeder roots, treated with fungicide, and the tuber has dried and cured (a skin has formed on the tubers), cut the main stem as close to the collar as possible. Lay out two to three sheets of newspaper and place the clump of dahlias stem side down. (Use black and white newspaper.) Insert a label and wrap the clump in the newspaper. Place the wrapped clump in a plastic grocery bag and write the name of the dahlia on the bag (yes, again!) and tie the bag once. Don’t make a knot – you will want to check the bags during the winter to make sure there is no rot.
Place the bags in a large plastic container in which you have drilled holes for aeration, or in a covered wax-coated or heavy corrugated cardboard box. To absorb any excess moisture, place newspaper on the bottom and top of the storage container.
Store the tubers in a dark dry area where the temperature will remain at about 40-45 degrees F.
Check your tubers periodically during the winter for signs of rot or desiccation. Excess moisture will make tubers rot. Not enough moisture will desiccate tubers and make them shrivel and die. Unwrap the newspaper to check the clump of tubers. If moisture is excessive and rot has begun, cut out any rot and re-wrap in dry newspaper. Otherwise, just re-wrap in the same paper. If the tubers appear to be shriveling, mist with warm water – especially if storing in peat moss, vermiculite or pet shavings.
An excellent video on dividing dahlia tubers is also available at this website: http://www.youtube.com/watch?v=gROv1nhrRQE
NOTE: No matter which method of preparing the tubers you choose, the secret of successful storage is to allow a skin to form on the tubers – much like the skin on a potato.
To produce a new plant, each tuber must have an eye (the new growth bud). This eye appears at the point where the tuber connects to the main stalk (called the collar). Each tuber on the clump will not necessarily have an eye.
Our November meeting will focus on dividing and storing tubers. Various methods will be demonstrated and discussed.