Mastering Dahlia Tuber Storage: A Practical Guide Rooted in Science

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The dahlia community is filled with advice and opinions about every aspect of dahlia growing and care. Much of it is excellent, some of it is incomplete, and a small percentage is just plain wrong. The complicated subject of dahlia tuber storage is no exception. Nobody wants to fail, yet many do. Were they misled by bad advice, confused by good advice, or simply unlucky?

This article explains the science behind successful dahlia tuber storage. If you want a step-by-step “Do This” plan, you won’t find it here. But if you’d like to learn about the factors that determine how well your dahlia tubers store through the off-season, and what you can do to increase your chances of success, read on.

During the growing season, dahlia roots swell and begin storing carbohydrates and other nutrients. These enlarged root segments are tubers, and properly stored, they will grow to create a genetic copy of the parent plant. Every fall, growers in cold-climate zones begin digging and lifting dahlia tuber clumps for winter storage.

Tubers can be rinsed to remove soil before storage, or soil can be lightly brushed off, allowing the tubers to enter storage with a protective coating of dirt. Growers may divide the clump right after lifting or wait until spring, just before planting.

If dividing, rinsing the tuber clump helps remove soil and exposes the eyes from which the plant will grow. Some growers prefer compressed air to blast away dried soil instead of using water.

This article focuses on storage, so I won’t go into detail about dividing tubers. There is ample information on dividing available through Facebook groups, YouTube, and the websites of experienced growers.

Dahlia tubers are biologically programmed to grow when conditions are right, and temperature plays the most significant role in triggering growth. If the tuber temperature exceeds 55-60°F (13-16°C) and the tuber has viable eyes, a small shoot will appear and begin to elongate.

If planted in soil, the shoot becomes the plant’s stalk, growing toward the surface in search of light. If it finds light, it forms leaves and begins photosynthesis. If not, it will continue growing until it either finds light or exhausts the stored nutrients, killing it.

Stored below 55°F (13°C), tubers remain dormant, “waiting” for better conditions. However, dahlia tubers cannot survive freezing. At 32°F (0°C), water inside the tuber crystallizes, irreparably damaging cells.

The ideal storage temperature range is 38-50°F (3-10°C), with a target of 40-48°F (4-9°C).

Proper storage isn’t just about temperature—humidity also plays a critical role.

Relative Humidity (RH) measures how much water vapor is in the air compared to the air’s maximum capacity at a given temperature. Warmer air holds more moisture, so RH changes as temperatures fluctuate. For instance, 50% RH means the air contains half the moisture it could hold at that temperature.

Dahlia tubers are fleshy, water-storing organs. While it’s okay for them to lose some moisture during dormancy, excessive dehydration will shrivel the tubers, depleting the nutrients needed to grow in spring.

Temperature and relative humidity form the two key pillars of successful dahlia tuber storage. Like Goldilocks, neither temperature nor humidity can be “too low” or “too high”; both must be “just right.”

When I teach tuber storage in my in-person classes, I ask students to list potential storage locations. If someone heats their home to 75°F (24°C) all winter, it’s too warm to store tubers. An unheated shed may expose tubers to freezing temperatures, which can be just as damaging.

In such cases, creative solutions are necessary. Many growers turn to refrigerators to maintain the optimal range of 38-50°F (3-10°C). However, kitchen refrigerators often dehumidify the air, drying out tubers. A wine cooler, chest cooler, or manual defrost fridge without a dehumidifier offers a better solution.

Once you find the right temperature, the next step is ensuring proper humidity. Too much moisture can lead to rot, while too little will desiccate the tubers.

Growers in damp regions, like Western Washington where I live, benefit from high winter RH (70-95%), while those in dry areas, such as Colorado or Eastern Washington, struggle with low humidity. The choice of storage containers and mediums depends on local RH levels.

When the air is damp, the container and storage medium should buffer humidity swings to prevent rot. In dry environments, these materials should help retain moisture, often requiring the grower to lightly moisten the storage medium.

Some growers caution against storing tubers in sealed containers, believing they need to “breathe.” But consider this: How do tubers breathe when buried under six inches of soil? Yes, there are air spaces within the soil particles, but the reality is that dahlia tubers absorb and release moisture in only three ways:

• They take up moisture from the soil through their roots, which we see as the long “tails” attached to tubers when we dig them up.
  
  

• They expel moisture by sending it up the stem of the growing plant and into the leaves and flowers, where it is exchanged with the atmosphere.
  
  

• They exchange moisture directly through the tuber surface, either taking up moisture from the soil or giving it back, based on the amount of water in the soil AND the temperature (remember the “relative” part of “relative humidity”).

Since stored dahlia tubers don’t have functional roots, and they lack stems, leaves and flowers, the first two ways of regulating moisture are not available to them. Only the third method is possible: Exchanging moisture through the tuber's skin, either giving it away if the storage environment has a lower RH, or absorbing it if the external RH is higher.

Every single hack for storing dahlia tubers (once the temperature issue is solved) has to do with maintaining the correct relative humidity in either the air or the material that is in direct contact with the dormant tuber. Simple as that.

Peat Moss

Peat moss is a natural material derived from partially decomposed organic matter, primarily sphagnum moss, accumulated over thousands of years in peat bogs. This slow decomposition occurs under waterlogged, low-oxygen conditions, which preserve the organic matter and prevent complete decay.

• pH Level: Peat moss is naturally acidic, with a pH range of 3.5 to 4.5.
  
  

• Moisture Retention: Peat moss can absorb and retain 10 to 14 times its dry weight in water. In terms of volume, it holds approximately 60-72% water, making it an excellent material for moisture regulation.
  
  

• Sterility: Peat moss is sterile when harvested, meaning it is free of weeds, pests, and pathogens.
  
  

• Environmental Impact: The environmental sustainability of peat moss harvesting is a topic of debate. Although Canada claims to harvest peat sustainably, some argue that peat bogs grow too slowly to be considered a renewable resource. Monitoring sustainable practices and peatland restoration efforts is crucial to balancing environmental needs with horticultural applications.

Vermiculite

Vermiculite is a naturally occurring mineral that undergoes a process called exfoliation when exposed to intense heat, expanding into a lightweight, porous material.

• pH Level: Vermiculite is neutral to slightly alkaline. It does not affect soil acidity, making it a versatile choice for many growing conditions.
  
  

• Moisture Retention: Vermiculite has an exceptional ability to retain moisture, holding between 220% and 325% of its dry weight. Depending on the grade, it can retain 20-50% water by volume.
  
  

• Sterility: As a heat-processed material, vermiculite is sterile and free from pathogens, making it ideal for storing tubers without the risk of disease.
  
  

• Other Uses: Vermiculite is often used in horticulture for moisture retention and aeration, providing a favorable environment for root development.

Pine Shavings

Pine shavings, commonly used for animal bedding, are a by-product of the lumber industry. They are derived from softwood species, often white pine, and undergo processing to remove moisture and harmful pathogens.

• pH Level: Pine shavings have a slightly acidic pH, typically ranging from 4.5 to 5.5.
  
  

• Moisture Retention: These shavings absorb moisture effectively, holding up to 100% of their dry weight in water.
  
  

• Sterility: Kiln-dried pine shavings are considered biosecure due to the heat treatment, which kills bacteria, fungi, and pests. They are also screened to remove dust, reducing respiratory risks for animals and gardeners.
  
  

• Storage Application: Pine shavings can help buffer humidity swings in moist environments, preventing tubers from becoming waterlogged while retaining some moisture to avoid desiccation.

Compost

Compost is made from decomposed organic matter such as wood, yard waste, and other plant matter. It is biologically active, supporting nutrient cycling through microorganisms.

• pH Level: Bagged compost typically ranges from neutral to slightly alkaline, with a pH between 6.0 and 8.0, depending on the materials used.
  
  

• Moisture Retention: Finished compost holds 40-50% moisture—similar to a damp sponge. If too dry, microbial activity decreases; if too wet, it risks becoming anaerobic, producing foul odors.
  
  

• Sterility: While not sterile, well-processed compost achieves pathogen control by maintaining temperatures above 131°F (55°C) during decomposition. This kills weed seeds and harmful organisms.
  
  

Storage Application: Compost can help maintain a stable moisture environment for tubers. However, since it is biologically active, it is important to monitor for any signs of decomposition or mold growth during storage.

The best storage material is the one that works for you. Educate yourself about these options, experiment with different methods, and talk to experienced growers to learn what works in your region. Beginners might find success by dividing tubers across multiple storage methods. That way, if one method fails, other tubers might still emerge healthy and ready for spring.