North American Dahlia Society

Digging, Dividing, and Storing Tubers

by Alan Fisher

Good, mature roots from healthy plants generally keep well, and any of several methods of storing tubers should yield 90 to 95 percent success. However, tubers from some dahlia varieties tend not to keep well, and some cultivars tend to give very sparse tubers. For these varieties, one should consider planting extras and growing pot roots.

Care of tubers starts with proper treatment during the growing season. While most dahlias start producing tubers in the spring, the roots do not mature until late in the growing season. See Sec. III for cautions on avoiding excessive fertilizer late in the season.

Before digging, go through the garden and throw away any weak stock. Dig up and throw into the trash any plants that were weak during the growing season or that produced inferior blooms. If any plants had signs of virus that could affect plant or bloom performance send them to the local landfill or incinerator.

The longer the tubers are in the ground curing, the more fully developed the tubers and the better the likelihood of their keeping over the winter. While one can start digging before frost or before the rainy season along the Pacific Coast (and may need to do so if his garden runs to thousands of plants), small growers should seriously consider letting their dahlia roots continue to grow and mature as long as practical. Most areas have a light frost that ruins partially opened blooms and top foliage followed by a hard freeze that may be a few weeks later. Dahlias continue to grow and the roots continue to mature after light frosts, and they may continue to grow after the first killing freeze (depending on how deep the freeze affects the soil.)

Cutting the Stalks

If one cuts a few days before digging, the eyes tend to come out, so the clumps are easier to divide accurately. However, if water gets into the stem, it can promote crown rot and ruin the tubers. Moreover, if one uses the same tool to cut all the stalks and leaves the plants in the ground, some growers fear that the tool could spread virus from one plant to another. If one digs and divides the clump right after cutting the stalk, there is less time for any virus to travel to the tubers. (The latest virus research findings indicate that cutting instruments seem rarely if ever to transmit virus among dahlia plants. However, one may wish to be careful until additional evidence becomes available.) Growers who cut the stalks a few days early should cover the open stalk with aluminum foil to minimize any water going down to the crown. (After the first frost, the stems always seem to have plenty of water in them, so some water there is unavoidable.) Leave enough stem (at least a few inches) to facilitate handling the clumps easily.

As soon as one cuts off the tops, an opportunity arises for the variety name to become separated from the clump. Carefully keep the tag with the proper clump at all times until finishing marking the individual tubers. Unmarked tubers (variety unknown) are worth a lot less than marked ones. In addition to the variety name, carefully indicate the best plants -- propagate from the best, not the worst stock.

Dig and handle the clumps with care. A dahlia tuber's neck is fragile, especially right after digging. To remove the clumps, dig on all four sides of the plant, about a foot away from the main stalk. When all four sides are loose from longer feeder roots, push the shovel or tined fork under the clump and lift carefully. Carefully remove any large clumps of dirt and turn the clump upside down to drain out any water in the stem. If one digs in the morning and leaves the clumps out for a couple of hours, the tubers will be much less fragile. After a couple of hours, one can remove the dirt with less opportunity of breaking fragile tubers.

When ready to clean the clump, use a garden hose to wash away as much dirt as possible. (Dirt contains microorganisms, so one wants to remove the dirt before storing the divisions.) At this time, the clump is ready for cutting.

Cutting clumps presents another tradeoff. It is much easier to divide roots in the fall (some varieties become so hard over the winter that one would need a power saw to separate them in the spring), but it is correspondingly harder to find the eyes before they start to sprout. Many growers divide their roots in the fall. Gardeners generally have more time in the fall than spring, and it is easier to remove dirt from and apply fungicide to divisions than to clumps. Growers uncertain about finding the eyes can cut off the tops several days before planning to dig so the eyes will have time to become more visible. Alternatively, just cut -- some divisions will have eyes.

If a variety's tubers are hard to keep or it makes tubers with especially thin necks and small crowns or especially small, thin tubers, then those lacking extensive experience may wish to divide the clump into sections and leave a few tubers together. One can keep the tubers from each division together with 3/4 inch masking tape.

Another suggestion for varieties whose roots are difficult to keep is to grow the plants in pots. Keep a rooted cutting or tuber in a 4 inch pot, plant the pot so the top is about an inch below the ground, and then treat the plant the same as any other dahlia in the garden. Because of the inch of soil above the pot, the feeder roots will come above the pot. However, the roots form and keep better if confined this way -- even if one treats the plant like a show dahlia and not like a pot root.
Roots grown in pots are often smaller than roots grown in the open, especially since growers generally select varieties that are poor root makers to grow in pots. To harvest tubers in pots, cut off all but the top inch or two of stalk and dig up the pot. Cut away any hair-like roots that extend from the pots. One may shorten thick tubers that extend from the pot. Place the name tag in the pot and wrap the entire pot in half a dozen sheets of newspaper. Place the wrapped pots in brown grocery bags and store them with other tubers (see below). In the spring, remove the pot and start watering. After a few weeks, one may take cuttings (see sec. IV) or (after the eyes develop shoots) unpot the roots, separate them, repot, and start the sections as individual plants.

Dividing the Clumps

In dividing clumps, each division must have a piece of the crown with an eye. See Figure 3 to illustrate where to cut. Remove all of the stem, because any remaining tends to promote crown rot and ruin the tuber. ALWAYS STERILIZE CUTTING TOOLS AFTER DIVIDING EACH CLUMP. Some experienced growers believe that when a clump has virus, the cutting tool may spread the virus to other tubers and ruin more of the stock. Researchers, however, believe that cutting tools are highly unlikely to spread virus. The only effective way to sterilize a cutting tool is with fire: heat the blade over a flame and let the blade cool before reusing it. One can use hook-shaped carpet knives (inexpensive from a hardware store) and rotate several knives to economize on time spent waiting for individual knives to cool after sterilizing them.

While cutting the clumps, carefully inspect the divisions. Almost everyone throws away the "mother root." Sprouts from the mother root tend to rely on it for nutrients and develop fewer feeder roots and thus poorer plants and tubers than new divisions. One prominent grower saves the mother root, uses it for cuttings, and then throws it away.

Tubers need only be large enough to keep well through the winter without shriveling. Indeed, many experienced growers prefer small to large roots unless they plan to use the roots only to take cuttings (and then throw them away). Some varieties seem not to push to develop feeder roots if their tubers are large. For these varieties, smaller tubers (or large tubers with all but the top inch or two cut off and thrown away) will produce stronger plants and better blooms than larger tubers. However, smaller tubers are preferable only if they are fully mature. Immature roots from late, partially formed laterals seem more likely to rot than larger, more mature roots. A good compromise is to rely on larger, more mature roots but to cut off all but the top few inches and throw away the excess. Keep enough of the tuber to mark its variety name clearly (see below).
Remove all feeder roots and any stalk (both promote rot). If the inside of the crown has any brown or rusty colored areas, cut them away. The discoloration probably indicates crown rot, and the tuber is unlikely to keep. After cutting divisions, use a hose or indoor laundry tub (in cold weather) to wash the tubers again and remove any dirt missed when first washing the clump.
After rewashing, cut the end of the tuber. Any brown or rusty colored area in the middle or part of the way out indicates rot. If any of these signs are present, cut away toward the crown to see whether the tuber has a clear section that includes the crown. If so, the tuber should be viable. If not, throw it away.

Some tubers have insect holes part way down from the crown. Insect holes are only a problem if they make room for an organism that will promote rot. Cut above the insect holes and try to find a portion of the tuber without any brown or rusty area (as in the previous paragraph).

Treating for Fungus and Marking the Tubers

After cutting the divisions, treat the cut ends with a fungicide, such as Cleary's 3336 (systemic and low in human toxicity), Captan, or sulphur. One may use the Captan or sulphur dry or follow directions to mix any of the chemicals in water and dip the roots. No one expresses any concern about spreading virus by using the dip for successive clumps. When using a liquid dip, place the tubers from one clump in the solution for about 15 minutes. Remove the tubers and transfer them to an empty shoe box or other open container (one per clump) to start drying. Never dry tubers on concrete, because it tends to draw the moisture from tubers and make them shrivel. Keep the tag with the cultivar name with the clump at all times. With a few containers, one can keep each clump in a separate dip and soak each clump approximately 15 minutes.

Some growers add a systemic insecticide to the dip. Growers who use a systemic should wear rubber gloves whenever handling wet tubers! Various authors recommend anywhere from 15 to 30 minutes for the dip. Some experienced growers see no noticeable difference in the success rate using or not using a fungicide. Perhaps a fungicide is more necessary in high humidity climates than in areas with low humidity. Discard any tubers that float in a dip, because they will not keep.
After removing the tubers from the dip, mark each division with enough information to identify the variety name and (if there are multiples of each variety) which of the plants produced the tuber. The "Bottle of Ink in a Pencil" brand indelible pencil works well if the tuber is a little wet. Have a small container of water nearby to wet the pencil. Some commercial growers sell "Bottle of Ink in a Pencil." One indelible pencil should mark tubers from more than 100 clumps. Since indelible pencils are toxic, never put the tip into the mouth.

The easiest method to mark tubers is to write the name of the variety. For long names, one can use a code -- but DO NOT LOSE THE CODE! Some commercial growers write or stamp a number for each variety. In addition, indicate some way (such as with *) any especially good tuber (such as one to use to take cuttings next season).

After cutting, dipping, and marking the tubers, let them dry. Expect to let the tubers dry for about 24 hours for small roots and 24 to 36 hours for medium to large roots. Drying time varies depending on temperature and humidity. Do not dry the divisions on cement, because cement tends to draw out water and promote shriveling.

Anyone unsuccessful in working with an indelible pencil can still use a thin indelible marker to name the tubers once they have dried. The problem is that these markers do not always write well on tubers. Be certain to have several fresh markers and rotate them. Some nursery catalogs sell nursery markers, and office supply or drug stores sell less expensive (and sometimes less reliable) markers. Indelible pens work only on completely dry tubers. When a tuber is wet, the ink spreads and whatever one writes becomes unreadable.

Some growers attach labels with wires to mark each tuber. One grower cuts strips from empty plastic detergent containers, writes the cultivar name with an indelible marker, and attaches the strips to tubers with plastic wire.

Storing Tubers over Winter

There are numerous methods to store tubers over the winter. Various methods seem to work about equally well, as long as the procedure keeps the tubers cool (above freezing but ideally below 50 degrees) and allows an exchange of moisture between the tubers and the storage medium. The containers, however, must retain the moisture in the storage medium. If the moisture escapes, the tubers tend to shrivel.

Most growers seem to use vermiculite in plastic bags to store tubers that have dried for one to two days. Coarse vermiculite works better than the fine horticultural vermiculite. One opinion is that fine vermiculite tends to keep moisture too close to the tubers and make them sprout and develop too quickly. Also, vermiculite dust is hard on the lungs. The finer the vermiculite, the worse that problem.

A prominent grower in the Northwest uses slightly moistened sand in five gallon containers. The tubers keep very well but develop sprouts and feeder roots earlier than those in coarse vermiculite. Also, sand is much, much heavier. Perlite is not a good medium, because it does not absorb excess moisture, and its dust is unhealthy to breathe.

Experienced growers warn against peat moss. Dry peat moss tends to make tubers shrivel while moist peat moss tends to promote rot. Numerous growers use wood chips to store tubers, but some warn that wood chips leach moisture from roots. An inexpensive source of wood chips is pet bedding (available from pet supply outlets). Specify the higher quality wood chips that are supposed to be dust free.

Dipping tubers in paraffin wax was fairly popular in the Northwest several years ago. Tubers dipped in wax tend to be very slow to develop eyes. For cultivars that develop eyes very late, storing the tubers in wax would seem unwise. For varieties that develop sprouts and feeder roots in December, the wax method would seem more sensible. However, the wax treatment has not become popular despite several articles promoting it. The failure of this method to become popular probably indicates that the extra effort is not worth the trouble.

Numerous containers work well for storing tubers. While the discussion assumes coarse vermiculite, one can substitute wood chips without any additional changes. Add some vermiculite, put in some tubers, then add more vermiculite. Some growers use a separate bag for each clump. If a clump generates many divisions, one may need two bags. Each bag, however, must contain at least as much vermiculite (in volume) as tubers.

After filling the vegetable bags, stack them in doubled brown grocery bags and keep them in the coldest part of the basement or in another cool area that should stay around 40 to 45 degrees. An insulated garage with a space heater for the coldest part of winter works well for some growers. Some growers contend that the bags must be air tight while others only twist them so a little air can escape. Some growers in areas with very humid winters pierce holes in their plastic bags. In general, the smaller and thinner the roots, and the lower the average humidity during the winter, the more air tight one would want the bags.

One can also place the tubers in Styrofoam, wood, or cardboard containers with vermiculite separating layers. If one uses a wood or cardboard box, moisture could escape from the vermiculite. To retain the moisture, use newspaper at least eight sheets thick to line all sides of the container and keep the top of the container closed.

One should store the tubers at a nearly constant, cool temperature. Most growers seem to recommend a range of 40 to 45 degrees. Freezing temperatures ruin tubers, and higher temperatures encourage microorganisms and fungus to destroy them. Also, warm temperatures prompt tubers to develop sprouts and feeder roots too soon. One prominent grower uses a discarded refrigerator, not plugged in, with a plastic gallon container of ice changed every few weeks to provide cooling and humidity.

One should inspect tubers monthly during the winter. Throw away any tubers that show signs of rotting. By checking frequently, one can throw away rotting tubers before the rot spreads to otherwise healthy tubers. Some growers say that if one stores tubers so that they do not touch (as one could in a wood container but not in plastic bags), then rot will not spread. A rotting tuber releases a gas that hastens the developing of eyes and sprouts. A rotted tuber can therefore be useful for tubers that are very late to develop. Anyone who uses a rotted tuber for this purpose should check the bag frequently to ensure that the rot does not spread to the healthy, late tubers.

In early spring, move the tubers to a warm location (dark but room temperature) to encourage eyes to develop. One can add a teaspoon of water per quart of bag space 15 days before removing the tubers. After adding the water, retie the bag and put it into a warm location.