Get It Right: When and How to Split Dahlia Tubers for Vibrant Gardens

Dahlias are versatile flowers, ranging from smaller ball dahlias to delicate daisy types to giant dinner plate blooms that can be up to eight inches in diameter. Colors and patterns range from neutral and romantic to bold and uniquely multi-colored, and to say flower enthusiasts are obsessed with them would be an understatement. They’re a joy to grow and fun to breed. To learn the differences between starting dahlias and planting tubers, check out our article, Growing Dahlias From Seed. Here, you’ll learn how to split tubers, cure them, and store them properly for Spring success.  

These elegant bloomers represent a considerable percentage of the flower-farming industry, and even though the tubers can cost a pretty penny, consumers are also willing to pay to obtain these unique beauties. Properly dividing the dahlia tubers you dig up in the fall allows you to expand your dahlia tuber stock and future dahlia plots, exponentially increasing your flower farming revenue. 

Related: 5 Signs Flower Farming is Right for You (and How to Start!)

Understanding Dahlia Tubers

Like some other plants, dahlias have a flowering stage that creates a seed, bringing the next generation of this species. However, dahlias also form underground tuberous roots that, when split, stored, and replanted, create a replica of the mother plant in future years. Dahlia seeds are not pure but rather unique hybrids made partly of the parents and partly of the plant that pollinated them. 

Dahlia tubers serve as a storage vessel, containing food, water, and nutrients needed to grow. They are stored similarly to potatoes, garlic, and other overwintered bulbs. These tubers each look slightly different, even within the same variety. Their size and shape change with growing conditions, variety, precipitation, and soil type, to name a few. Some are skinny, others plump and stout, while others might be slightly wrinkled. It’s what’s going on inside that matters. 

Dahlia Tuber Anatomy

Each dahlia tuber has a crown, a neck, and a body. 

• The crown was once attached to the mother plant and is where the new plant will emerge in the spring. Due to its connection to the mother plant, this is where the new plant's eye will form. We’ll discuss how best to divide tubers from the mother plant to ensure proper growth next season. 
• The neck is the slender area between the crown and the body, where nutrients and starches flow from the body to the new plant. The neck should be sturdy so there is no risk of a broken nutrient flow. 
• The body is the remaining part of the tuber where the starches, nutrients, and food are stored. Next season, your plants will receive their starting energy from the body of the tuber. 

Why Split Dahlia Tubers?

Splitting tubers allows growers to easily and inexpensively expand their dahlia supply. Depending on the age of the tuber and the variety, you may see 5 to 20 new tubers from just one purchased tuber, a considerable increase!

While some warm-weather growers can leave their tubers in the ground over winter without issues, digging them up and splitting them is advised. Storing tubers in a well-ventilated, climate-controlled area under your watchful eye is better than leaving them in the ground, risking disease, pest damage, and rotting. Plus, as they grow underground, you may need to realize how big tubers have become, leading to unintentional overcrowding that has adverse effects. 

Bacterial diseases that a dahlia plant contracts through the growing season transfer into the tuber and overwinter. A plant contracts disease through infected tools, human clothing, nearby infected plants, or a wound. If you don’t catch these quickly and before storage, your whole supply may become infected. 

Some dahlia tuber diseases to watch for: 

• Fungal diseases include powdery mildew, Erysiphe, botrytis blight, verticillium wilt, fusarium wilt 
• Viral diseases include dahlia mosaic virus and impatiens necrotic virus 

If you notice any white spotting, brown or black lesions, water-soaked spots, wilting, stunted growth, discoloration, or deformations, look at the plants further and identify the issue. Contact your local extension office for further assistance. 

Prevent diseases by:

• Practicing proper sanitation of tools, materials used in the garden, and your clothing. 
• Only purchasing tubers from trusted sources. 
• Avoiding overhead watering. 
• Immediately removing infected plants. 
• Growing disease-resistant varieties. 
• Providing good air circulation and ventilation during growth and storage. This includes proper plant spacing. 

When to Split Dahlia Tubers

Digging up all your dahlia tubers, especially if you grow them on a large scale, is loads of work to complete at the end of a long season, but splitting them now will allow for easier storage. Home gardeners, if you have the space, go ahead and label those massive root clumps and store them somewhere safe until late winter or early spring, when you have renewed spring energy. 

Regardless of when you split them, always allow the foliage to die back before digging them up. This usually doesn’t occur until a hard frost, which depends on your hardiness zone. Allow them to cure, and provide a well-ventilated, climate-controlled storage area. 

Related: The USDA's New Plant Hardiness Zone Map - Explained

Step-by-Step Guide to Splitting Tubers

Whether you’re splitting dahlia tubers in the fall or spring, start by gathering your materials:

• Clean, sharp shears or floral snips
• A clean, sharp knife 
• Newspaper or other packing materials like wood shavings, peat, or vermiculite 
• Optional plastic bags or liners for the bottom of your storage crates 
• Well-ventilated crates, mesh trays, or storage bins  
• Labels and a permanent marker
• Dahlia tubers
• Clear working space

Partners’ Garden doesn’t wash dahlias before storing them for winter, as the additional moisture invites mold and fungal diseases, but others do it without issue. They just gently shake or brush off excess dried soil. To wash them, use a gentle mist to release thick soil and allow them to dry overnight before splitting and storing them.

Lay your dahlia clumps out on your working space and inspect them. Discard any dried, desiccated, or fully molded pieces that will likely be unviable. A tiny bit of light fuzz on the end of a tuber can be brushed away. It shouldn't affect the viability as long as it hasn’t traveled into the tuber. Some varieties wrinkle more than others, and that’s just fine. With experience, you’ll learn to distinguish between normal wrinkles versus wrinkles caused by decay. 

A note about necks: Tubers without a neck and eye will not be viable, so don’t waste space storing them. It can be challenging to identify a broken neck. There may be a thinner band around the neck near the crown if it’s broken, and the skin may feel different. If you suspect it, look for strings that appear inside the neck. If these are visible in the broken section of the tuber, it will not survive because the nutrients from the body cannot flow into the eye. 

Good-quality tubers will:

• Be firm 
• Exhibit no sign of mold or rot
• Contain a healthy eye with at least 1 cm of crown attached 
• Have an intact neck. Hairline fractures are okay. 
• Not have any smell of rotting. 
• Have light brownish-tan-colored flesh without any spotting. 

Inspect your tubers before sitting down so you can focus on the splitting and storage process. Discard any that aren’t worth the storage space and effort. 

Splitting Dahlia Clumps 

Now, depending on the overall size of the clump and your personal preference, you may split them in half or quarters to create smaller areas to work on. If this was your first year with these tubers and they’re smaller, working on it as a whole might work just fine. Split large clumps by carefully cutting down from the center of the mother crown with your knife. If your goal is not to save the mother, it’s okay to cut into her. 

Pro tip: If you have a particularly expensive variety or one with few tubers to plant, you may save the mother. She’ll eventually produce more tubers, so it’s a way to retain this variety in your stock. 

Identify darker spots on the crown that indicate newly emerging eyes. You’ll want to keep these with their corresponding tuber and 1 cm of the crown. Slowly move around the crown, separating each tuber from the mother with its eye and intact neck. Discard as much of the woody, white stem as possible; it is vulnerable to pest damage and rot. 

Prepare yourself for some tuber loss. Sometimes, cutting into little tubers is necessary to break apart large clusters. It feels painful at first, but it’s part of the process that can’t always be avoided. Avoid damage by using sharp, clean tools and practicing patience. 

Does Size Matter? 

In the case of dahlia tubers, yes, to an extent. Tubers smaller than the size of a AAA battery won’t create a productive plant because they likely haven’t completed their growth stages properly, meaning they don’t contain all the food, energy, and nutrients for the next generation of dahlias. 

Pro tip: If the bodies of two small conjoined tubers are equal to the size of a AAA battery combined, you may keep them together and plant them as one unit. You may separate one large mass into three to four smaller chunks and plant them. This works well for people too nervous to cut into the crown and risk losing the tubers. 

On the other hand, tubers can also be too large. When food and energy reserves are over-abundant, the plant has no reason to expand in search of anything else. The same goes for planting the clump you dug up in the fall. While you might think this is a good idea, it may backfire on you. You might see some foliage, but the plant will not be productive. 

Storing Your Split Dahlia Tubers

Keep as much soil in the garden as possible, gently brushing it off your tubers before pulling them out of your fields. Allow your dahlia tubers to dry and cure for several weeks to prevent rot and disease during storage. Provide good air circulation and space for them to breathe during the curing process and keep them out of direct sunlight. 

Pro tip: Avoid setting tubers on your driveway or directly onto concrete, which will dry them out. 

Many growers love the simple black crates purchased from their local bulb retailer after they receive deliveries from Holland for tuber and bulb storage on the farm. The crates come in seven-inch and nine-inch heights, and their sturdiness ensures they last for many years. Other options include mesh trays, cardboard boxes, well-ventilated baskets, and sturdy paper bags. Ideally, your containers can be neatly stacked. 

When the tubers are ready for storage, arrange them in crates according to size and variety and add labels. Labels can be as simple as thick cardboard with a variety written on it in bold, large letters and placed inside the crate.  For extra security, add two labels per container in case one slips out during transport or storage. Label options include, but are not limited to: 

• Colored plant labels
• Flagging tape attached to the base of the tuber 
• Write directly on the tuber with a marker 
• Cardboard 
• Tape adhered to the outside of the crate 
• Metal tags

Use a labeling system that can transfer from the field into storage. Once each crate has labels you like, stack them and arrange them in your storage space. It's recommended that they be set on a platform or palette so they’re not directly on the floor to keep moisture from seeping in and hungry mice from snacking on them. 

Properly store dahlias at temperatures between 38 and 55°F (3 and 12°C) and relative humidity levels of around 80%, which keeps them from drying out. Never freeze dahlias, and keep moisture down. Run a dehumidifier in the room if necessary to prevent mold. Use an uninsulated area of your home, like a basement, attic, closet, or root cellar. Set traps if critters become an issue. Packing material between tubers and inside each container, like vermiculite, peat, wood shavings, or newspaper, will wick away excess moisture while keeping the environment moist and preventing the tubers from drying out. 

Troubleshooting Storage Issues

Monitoring dahlias while they’re in storage is wise, especially if it’s your first time. For instance, go into your cellar after one or two weeks to ensure everything seems good. Perhaps you need to adjust the dehumidifier or the direction of a fan blowing. Maybe the temperature is fluctuating too much. Improving the storage area sooner than later will save you from tuber loss and disappointment. Once you learn what you’re looking for, once-a-month check-ins should suffice. 

On your trips into the storage area, record notes on how things look, feel, and smell. Squeeze a few tubers to ensure no softening, which may signify decay. Inspect any with brown or black spots. Find the source of any sour or rotting smells and remove it immediately. Like other bulbs and ripening fruits and vegetables, dahlia tubers release chemicals while rotting that can quickly spread to others. 

If you suspect tuber rot, cut it in half to investigate further. Without seeing the inside of the tuber after inspecting the outside, you won’t learn how to identify rot correctly. On the other hand, small, dried-up tubers are more resilient than you think. Pot them in 5” Heavy Duty Seed Starting Pots with fresh soil. If they sprout, you’ve just grown your supply; if they don’t, you haven’t wasted much soil or space in your dahlia garden. 

Related: How to Successfully Start Dahlia Tubers Indoors

Pro tip: Pot small tubers indoors this winter. Snip the green growth and transfer it to a 3.3” Heavy Duty Seed Starting Pot with fresh soil when they've sprouted and grown to about three inches. Allow them to grow. Do this as often as possible, as space allows, which may be five to seven cuttings per plant over the winter. You’ve created dahlia clones and multiplied this variety’s stock without much investment or energy! 

Common Mistakes and How to Avoid Them

Splitting and storing dahlia tubers properly takes practice, and mistakes are common but avoidable. Here are a few to avoid: 

Keep records of your monitoring, mistakes made, and how you remedied faulty conditions like high humidity. Like everything else in gardening and farming, we learn from our mistakes! Keep photos and written journals so you can look back and review them. 

If you want to read more about ways to avoid mold and rot when splitting and storing tubers this article, Digging, Dividing, and Storing Tubers from the American Dahila Society is helpful. 

FAQs

When is the optimal season to split dahlia tubers?

Typically, experts will advise splitting tubers in the fall before storage. Splitting them into smaller chunks, cleaning off excess soil and moisture, and identifying any signs of disease make storage safer and more efficient. However, home gardeners growing on a small scale may place large root clumps into a bag or cardboard box and split them in the spring. 

What are the signs of a healthy dahlia tuber for splitting?

Ensure each tuber has an intact neck with no strings exposed and at least 1 cm of the crown attached. Coloring and smell should be healthy, with no signs of disease. After storage, the tuber should have white, fleshy innards remaining; when squeezed, they should be firm, not mushy, and not have visible signs of mold or brown or black spots with no odor. 

How can I ensure my split tubers don’t dry out in storage?

If you have some small tubers whose viability is questionable going into the storage period, keep them separate or make note of which varieties have them. Pay closer attention to those, and bring a spray bottle when you check in. Spritz tubers lightly if they seem dry, and visit them more often than the rest of your supply to ensure they don’t dry out. 

Balance moisture with air circulation using simple measuring sensors. Relative humidity should be between 75 and 85%. Packing material like vermiculite, peat, or wood shavings will help keep the storage containers moist while wicking away excess moisture. 

What tools are necessary for splitting dahlia tubers?

To pack and store tubers efficiently, clean, sharp shears or snips, a knife, and a working surface are needed. Labels, newspapers, crates, packing material, tape, and markers are also used. 

How often should I check on stored dahlia tubers?

If you’re new to the process, we advise checking in after a few weeks, then monthly, if the environment seems conducive to proper storage. Remember, take swift action and remove any sign of infection immediately. 

Written by Jenna Rich of Partners’ Gardens.