My Dahlia Tuber Storage Experiment: 2025-26 Edition

Dahlia growers have no shortage of opinions about winter tuber storage. Use peat moss. Use vermiculite. Use wood shavings. Leave tubers exposed, wrap them individually, seal them in plastic, or give them plenty of air. Most of these methods probably work somewhere, for someone, under the right conditions, which is part of what makes the subject so confusing.

I have been growing dahlias for nearly ten years here in the Pacific Northwest, where our mild winters give us the option of leaving clumps in the ground. I usually do not, because I sell tubers in early spring and also pre-start many of them indoors for cuttings. My normal practice is to dig the clumps in November, wash and divide them, then store the individual tubers through the winter in my unheated basement.

Over the years I have tried several storage methods and eventually settled on sealed plastic bags containing clean pine shavings. That method has worked well for thousands of tubers, even though I regularly encounter growers who insist that tubers need air circulation during storage. Experience told me that my method worked, but it did not tell me how it compared with other methods under the same conditions. Last winter I decided to find out.

I began with 58 individual tubers, all divided from clumps of the same cultivar, a third-year seedling of mine. Using one cultivar removed at least one source of variation, since published dahlia research has shown that cultivars can differ in how well their tuberous roots retain weight during storage.

All tubers were thoroughly rinsed with fresh water, then allowed to air-dry at room temperature for 48 hours. I mixed all the tubers together, randomizing which plant’s tubers became part of which of the six test groups.

I divided the tubers into six groups with similar starting weights, ranging from 361 to 376 grams. One group had 8 tubers, three had 9 tubers, another 10 tubers, and the final group 13 tubers. I randomly assigned which batch of tubers would be stored using each method.

The three A groups were stored in closed zip-top plastic bags, while the three B groups were left open to the basement air. Within each set I tested three treatments: no packing medium, pine shavings, and peat moss.

On December 14, 2025, I counted and weighed the tubers in each group, arranged them for photographs, and placed them into storage. The basement was about 58°F (14°C) at the time, with relative humidity around 60 to 70 percent.

The six groups were:

   A-1:    13 tubers, 367 g, sealed plastic bags with no packing medium
   A-2:    8 tubers, 376 g, sealed plastic bags with pine shavings
   A-3:    9 tubers, 370 g, sealed plastic bags with peat moss
   B-1:    9 tubers, 361 g, open tray with no packing medium
   B-2:    10 tubers, 367 g, open Styrofoam box with pine shavings
   B-3:    9 tubers, 372 g, open Styrofoam box with peat moss

As winter progressed, the basement cooled gradually until it stabilized at about 52°F (11°C). With the arrival of our winter rains, relative humidity was commonly between 75 and 85 percent. I did not inspect the tubers, mist the storage media, or adjust the containers during the test, and the sealed bags remained sealed until May.

Humidity matters in tuber storage because a tuber is continually exchanging water with its immediate surroundings. Relative humidity describes the amount of water vapor in the air compared with the maximum the air could hold at that temperature, so changes in temperature change the relationship as well. Packing materials such as peat moss, pine shavings, and vermiculite can further modify the small environment immediately around a tuber by absorbing and releasing moisture. I was interested in seeing how much those differences would matter under the conditions in my own basement.

On May 3, 2026, about four and a half months after the experiment began, I removed all six groups from storage. I photographed them again, inspected the tubers, and recorded the combined weight of each group. Where tubers had begun to sprout, I initially included the new growth in the final weight.

The two largest losses were nearly identical. Tubers stored open with no medium lost 28.0 percent of their starting weight, while those stored open with pine shavings lost 28.6 percent. Peat moss behaved very differently. Even in an open box, those tubers lost only 3.5 percent. The sealed no-medium and pine-shavings groups also retained much more weight than their open counterparts, losing 5.4 and 8.2 percent respectively.

One result, however, needed a closer look.

The nine tubers stored in sealed bags with peat moss had gone into storage weighing 370 grams. When I weighed them in May, the scale read 380 grams. After more than four months in storage, the group appeared to have gained 10 grams.

Several of the tubers had sprouted, so I removed all of the new growth and weighed it separately. The shoots together weighed approximately 9 grams, leaving the tubers themselves at about 371 grams compared with 370 grams when the experiment began. Within the precision of the kitchen scale I used, the tubers had essentially maintained their original weight while also producing about 9 grams of new shoot growth.

The scale was measuring fresh weight, not moisture content. Water makes up much of a tuber's fresh weight, and the shriveled appearance of the groups that lost the most weight makes water loss an obvious part of the story. But a stored tuber remains alive, and the number on a scale also includes its living tissues and stored compounds.

Dahlia buds pass through seasonal dormancy, a physiological period when growth is suppressed even though the tissues remain alive. Research on dahlia crown-tubers found sprouting strongly suppressed during autumn and into early winter, followed by a later release from dormancy. Extended chilling accelerated that release, which helps explain why tubers that have spent months in cool storage may begin sprouting before they are planted.

Storage also brings biochemical changes within the tuber. Studies of stored dahlia tuberous roots have also detected changes in compounds associated with carbohydrate reserves. My experiment did not measure water content, respiration, or carbohydrate use separately, so the scale cannot tell us how much each process contributed to the final weight. Its value here is simpler: the six storage treatments produced very different amounts of fresh-weight loss.

The most relevant published research I found examined how well dahlia tuberous roots retained weight in different packing materials. In a 2015 experiment, researchers stored roots from five cactus-type cultivars for 131 days in sand, sand mixed with sawdust, or peat mixed with sawdust. Weight loss differed among cultivars as well as among storage treatments, but peat plus sawdust generally gave the best weight retention.

A related 2016 study reached a similar result. Peat plus sawdust again preserved tuber mass better than the other treatments, and the researchers found differences in biochemical composition associated with the storage conditions. Their experiment also included tubers stored without a substrate.

The conditions in those studies were quite different from mine. The tubers were stored at approximately 41 to 46°F (5 to 8°C), considerably cooler than my basement, and at only 30 to 40 percent relative humidity. The researchers also used mixtures of storage media rather than testing peat and sawdust separately. Still, peat was associated with better weight retention in their experiments, and it performed well in both the open and sealed treatments in mine.

Temperature has been studied directly as well. In 1930 and 1931, plant physiologists P. W. Zimmerman and A. E. Hitchcock stored dahlia root clumps under several temperature regimes, ranging from the mid-30s Fahrenheit to warm greenhouse conditions. Roots kept around 40 to 50°F dried much less and remained in better condition than those stored warm, and peat moss reduced drying compared with roots stored without packing material.

Their coldest treatment also showed why there is a lower limit. An equipment failure allowed the temperature to fall below freezing for several hours. The injured roots became soft and were readily attacked by fungi afterward. My basement began the winter warmer than the best storage range in that experiment but eventually settled at about 52°F, close to the temperatures at which they found relatively little drying.

The sealed-versus-open comparison interested me because I have often heard that dahlia tubers need air circulation during storage. I have not found an experimental dahlia study demonstrating a requirement for circulating air. The storage research I found concentrates instead on temperature, drying, packing material, freezing injury, and disease.

None of the 58 tubers in my experiment rotted, including those stored for more than four months inside sealed plastic bags. A few felt somewhat soft when I removed them from storage, but all appeared suitable for planting in my own garden. Sealed storage can certainly trap unwanted moisture if tubers go into the bag wet, condensation develops, or diseased tissue is already present. In those circumstances, ventilation may help dry the storage environment. My concern is with the broader claim that a sound tuber necessarily requires circulating air throughout storage, something neither my experience nor the dahlia research I found has demonstrated.

The weight results add another part to that picture. The open no-medium and open pine-shavings groups each lost about 28 percent of their starting weight, compared with losses of 5.4 and 8.2 percent in the corresponding sealed groups. Peat again behaved differently, limiting the loss to 3.5 percent even in an open container.

I began the experiment expecting the packing medium to be the main variable. By May, I was paying at least as much attention to whether the tubers had been sealed or exposed to the basement air.

Pine shavings make the comparison especially clear. In sealed bags, the tubers stored with pine shavings lost 8.2 percent of their starting weight. In an open box, they lost 28.6 percent, almost exactly the same as tubers stored completely exposed. Peat moss performed well in either arrangement: the open peat group lost only 3.5 percent, while the tubers in sealed peat finished storage at essentially the same weight at which they began.

There are limits to what I can draw from those numbers. I did not measure the starting moisture content of the peat or pine shavings, continuously log temperature and relative humidity, or determine the actual water content of the tubers. Those measurements would make it possible to distinguish some of the processes that this first test leaves together. For my purposes, however, the differences in fresh-weight retention are substantial enough to justify another, more carefully measured experiment.

They also reinforce the importance of the storage environment as a whole. A method that works in my cool, humid basement may behave differently in a heated house, an arid climate, or a colder storage room. “Store in pine shavings” or “store in peat moss” describes only part of a method unless we also know the temperature, humidity, and whether the storage container is open or closed.

One popular storage medium was missing from this experiment: vermiculite. I have never used it routinely and did not have any on hand when I set up the test, so this winter I plan a simpler comparison of pine shavings, peat moss, vermiculite, and a no-medium control. I will probably also include a sample of dahlia tubers tightly wrapped in plastic food film, the so-called Plastic Wrap Method popular with many growers.

I also plan to improve the measurements by recording the starting condition of the storage media and continuously logging temperature and relative humidity. If I have enough comparable tubers available, I will increase the number in each treatment. A planting trial would add another useful measure by allowing me to compare sprouting and subsequent growth rather than ending the experiment when the tubers come out of storage.

For now, the first test gave me a result I did not expect. Storage medium was only part of the story. Exposure to the surrounding air was associated with very large weight losses in two treatments, peat moss performed well whether open or sealed, and none of the 58 tubers rotted. That is enough to make the next winter's comparison worth doing.

Ciobanu, I., Buta, E., Husti, A., & Cantor, M. (2015). Research regarding the influence of cultivar and storage substrate at Dahlia variabilis Desf. cactus type. Agricultura – Știință și practică, 95(3–4), 197–203.

Ciobanu, I., Cantor, M., Stefan, R., Buta, E., Magyari, K., & Baia, M. (2016). The influence of storage conditions on the biochemical composition and morphology of dahlia tubers. Notulae Botanicae Horti Agrobotanici Cluj-Napoca, 44(2), 459–465.

Konishi, K., & Inaba, K. (1967). Studies on flowering control of dahlia. VII. On dormancy of crown-tuber. Journal of the Japanese Society for Horticultural Science, 36(1), 131–140.

Zimmerman, P. W., & Hitchcock, A. E. (1932). Dahlia root storage at different temperatures. Bulletin of the American Dahlia Society, 13(59), 44–45.