Dahlias: Digging, Dividing, and Storing

By Carol Moore, UC Master Gardener

Photo credit: Carol Moore UC Master Gardener		Photo credit: Kim Wilson UC Master Gardener

What is a Tuber?

A dahlia tuber is the tuberous root of a dahlia plant. It consists of three parts - the body, the neck, and the crown.

The Body: The tuber’s swollen body contains carbohydrates, water and nutrients. It provides nutrients until roots and shoots are established then the plant is fueled by photosynthesis.

Each year the mother tuber (the original tuber planted) will produce new tubers. The number of new tubers produced will vary. Tubers tend to be larger and more numerous when the growing conditions are ideal.

The Neck: Connects the tuber’s body to the crown. If the neck is broken, the tuber is not viable.

The Crown: On top on the neck sits the crown. Dahlia eyes sprout from the crown and are the source of the shoots. The crown must have a least one eye to produce a plant. Some tubers take a long time to “eye up.” Late blooming dahlias usually develop eyes late in the spring.

Photo credit: Carol Moore UC Master Gardener

Does tuber size matter? Tubers vary in size and shape.  To grow a plant with flowers, a tuber should be bigger than your pinky finger or a AAA battery. Some varieties have large tubers which produce a flowering plant but may not generate additional tubers by fall. To prompt additional tuber growth, cut off the back of the tuber before storing.

Why and When to Dig Up Tubers

Dahlia tubers are fleshy, thin skinned and may freeze or rot if left in the ground over winter. In colder climates, where the soil freezes at a depth of 4-6”, dahlias need to be dug up in the fall, by November 15^th or by the first killing freeze.

If left undivided, Dahlias form a massive clump of tubers that send up weak stalks and smaller blooms. Best to divide every 2-3 years at minimum. 

Fall Digging

Wait to dig up your tubers in the Fall until the foliage has turned yellow and brown. This allows the tuber to gather all the plant’s nutrients (stored in the leaves) for the following year.

Pros for fall digging:

• Easier harvest since the soil is drier (not soaked by winter rains).
• The tubers are softer and easier to divide.
• Frees up garden space to allow winter vegetable crops or cover crops.
• Protects tubers from predators, such as gophers.
• For florists or exhibitors, allows control over planting dates to manage timing of blossoms.

Cons for fall digging:

• Tuber skins may not fully harden, resulting in thin skin which is more likely to rot, shrivel and die. It is common to lose 20% of your fall tuber harvest to rot or dehydration during storage.
• Finding the perfect storage space (cold and dark) for the tubers and the cost of materials.
• Time and energy spent caring for the stored tubers over winter.

Spring Digging

Dahlia stems are hollow and collect water which will rot tubers. If you are digging your tubers in the spring, cut the plants down to the crown leaving no plant aboveground. Top each plant with 4” of mulch (compost, alfalfa, straw, pine needles). Do not water over winter months.

Pros for digging in spring:

• You can dig, divide and replant immediately.
• No storing or caring of tubers over winter.

Cons of digging in spring:

• Tubers become woodier, making it harder to divide.
• High risk of tuber rot due to winter moisture and hard freeze.
• Detection of spoiled tubers delayed until spring digging when replacement choices are less.

Digging and Washing Your Tubers

Equipment:

• Shovel and/or garden fork
• Garden gloves & boots & waterproof clothing (you are going to get wet)
• Sanitizing solution: 1 part bleach to 10 parts water (or) 5 tsp of Dawn dishwashing soap to 4 cups water
• Large buckets to dip your tubers in during the sanitation process.
• Containers to temporarily place your tuber clumps (plastic flats, baskets, cardboard boxes)
• Waterproof labels & marking pens.

Before digging:

• Identify any plants to be discarded. Plants that show signs of virus, have weak spindly growth or small blooms may be diseased and should not be composted but bagged and placed in the garbage.
• 1 to 2 weeks before fall digging, cut all the stems down, leaving five to six inches of the main stem. These stems will be used to as handles when digging and it promotes eye growth.

ALWAYS STERILIZE YOUR CUTTING TOOL AFTER EACH PLANT ENCOUNTER. 

Digging Tubers

• Start early – allows time for digging, washing, and air-drying tubers.
• Work clean to avoid spreading virus and disease amongst your tubers.
• Work slowly, using a garden fork, gently dig a foot away from the dahlia stem, encircling the plant. Gently rock to lift the dahlia clump of tubers.
• Once the tuber clump is out of the ground, carefully shake off the excess dirt. Handle carefully, tubers are fragile.
• Label your tubers to prevent loss of identification: Tie label tape to the tuber clumps and place your tuber clumps in their holding containers which should also be labeled.
• Beware of Crown or Leafy Gall : Any tubers that show signs of disease such as abnormal nodules on the crown need to be bagged and discarded. Do not compost. Be sure to sanitize your tools and replace gloves after handling.

Washing Tubers

• Tubers are washed to remove soil microorganisms, worms, and insects that may cause rot. Wash gently with a garden hose. Avoid forceful spraying which can injure tuber skin and increase risk of rot.
• Keep the label fastened to the tuber clump with an extra label attached to their holding tray.
• After washing tubers, dip tuber clumps into prepared sanitizing solution for a few seconds. Place back in their holding tray or lay on newspaper or cardboard to dry for the day.
• Do not dry tubers on concrete, which can draw the moisture from the tubers and make them shrivel.
• Dry outside for the day (if the weather is dry), and bring inside (garage, barn, shed) during the evening. They will need time to dry and cure for the next 24-48 hours.
• Drying time depends on the size of the dahlia tuber and current humidity. Too much drying time can cause tuber shriveling. Too little drying time can lead to tuber rot during storage.

Dividing Tubers

The process of dividing your tubers is the same whether you do it in the fall or in the spring.

Equipment needed:

• Sharp pair of garden scissors or clippers
• Garden knife
• Tree pruners
• Sanitizing solution: 1 part bleach to 10 parts water (or) 5 tsp of Dawn dishwashing soap to 4 cups water
• Labels & Markers

Dividing

Once the tubers have been dug up, washed, and allowed to dry, they are ready for division.

ALWAYS STERILIZE YOUR CUTTING TOOLS AFTER DIVIDING EACH CLUMP

1. With your garden clippers, remove: plant stalks attached to the tuber clump; fine fibrous roots; rotted or shriveled tubers; tubers with broken necks; piggyback tubers (small tubers growing off the larger tuber).
2. Mother tubers are usually darker and worn looking. Save if it looks healthy. Make sure there is a new crown attached to it, not just the same crown from the previous season.
3. When dividing a tuber clump joined to a large crown, make the first cut through the middle of the crown with tree pruners. You will likely need to sacrifice some of the tubers underneath.
4. Each tuber needs a piece of crown attached that is at least ¼ to ½ square inch in size. This size is likely to contain one or more eyes. Eyes may not be obvious especially in the fall.
5. Once you have made your division, cut away any extra stem tissue still attached to the crown.
6. Check the tubers for rot- cut a thin slice off the base of each tuber body. White flesh, with no sign of brown is healthy. If there are brown spots, slice until you reach healthy tissue. Dip the cut portion in antifungal powder to prevent rot.

Storing Tubers Location: Store tubers in a dry, cool, dark space. Garages, basements, attics, utility rooms, closets, and under the house are common places to store tubers. Temperature: Find a location that is consistently 40-50 degrees with 70% humidity. Storage: Store your tubers in bins/boxes filled with peat moss, vermiculite or wood shavings. Fill the bin with 1-2” of storage material and place tubers so they are not touching. Add another 2” of storage medium and another layer of tubers, then top off with 2” of storage medium. Label the outside of the boxes with the names of your tubers. Check on your tubers every few weeks. Remove and discard any rotted tubers. If the tuber looks shriveled and dry do not throw it out. Move it to a more humid location. If you see mold (bluish green powder) on your tubers it means the storage location is too humid. Brush off the mold and move the tubers to a dryer location.

Photo credit: Carol Moore, UC Master Gardener

Photo credit: Kim Wilson UC Master Gardener	Photo credit: Kit Long	Photo credit: Carol Moore UC Master Gardener

References

Crown Gall. https://ipm.ucanr.edu/PMG/GARDEN/FLOWERS/DISEASE/crowngall.html

Plakos Lori, All About Dahlias.  UC ANR. All About Dahlias - The Backyard Gardener - ANR Blogs

Plakos, Lori. Aall About Dahlias. Part 2. UC ANR.  https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=18896

Digging, Dividing, and Storing Tubers. https://www.dahlia.org/docsinfo/articles/digging-dividing-and-storing-tubers