Part 6: The Science of Better Tuber Harvests

Copyright © 2025 by Steve K. Lloyd
All Rights Reserved

In Part 5: Are Tuber Traits Genetic?, we examined the genetic foundations of tuberization. What traits are passed down, what determines the natural predisposition of some varieties for strong tuber production, and how breeders can select for better tubers without sacrificing flower quality.

After a season of stunning blooms, every dahlia grower dreams of digging up a treasure trove of healthy, plump tubers. This final part reveals the science-backed strategies to make that dream a reality, ensuring your prized varieties return year after year.

My own passion for dahlias, like that of most growers, centers on the beauty and endless variety of their late-summer blooms. Collecting seeds, coaxing them into germination, and watching delightful surprises bloom in the seedling patch is just part of it. To sustain that joy and expand my collection through breeding, the tuber itself becomes paramount.

In the first parts of this series, we've explored the intriguing biology of dahlia tubers, the environmental signals that trigger their formation, and the factors, including propagation methods, commercial practices, and genetics, that can disrupt them.

Now, let's bring it all together into practical advice. How can we, as home gardeners and small-scale dahlia farmers, encourage our dahlias to produce big, healthy tuber clumps that overwinter beautifully and bring us joy year after year?

A dahlia farming friend of the author’s shows off a nice-sized tuber clump.

The most powerful tools at our disposal are the environmental factors we discussed in Part 2: What Makes a Dahlia Root Become a Tuber? By providing the right conditions, we can help guide our dahlias to prioritize underground storage at critical stages during the growing season.

Plant your dahlias when the soil has consistently warmed, typically after all danger of frost. However, don't rush it too early. Planting too early into cold soil can stress the plant and delay active growth.

If your growing zone often sees heavy rains in the spring, the combination of wet and cold often promotes rot before tubers have a chance to grow.

Planting at just the right time allows the plant to develop good top growth during the long days of summer, then naturally transition to forming tubers as days shorten and soil cools. Studies consistently confirm that shortening daylength and cooler temperatures are primary environmental cues for tuber formation.

Allow your dahlias to experience the natural progression of seasons. As days shorten in late summer and early fall, this is the primary cue for tuber initiation and bulking.

If you’re growing in a greenhouse, avoid providing excessive artificial light to mature plants late in the season, as this can confuse their natural photo-period signals.

While you can't control the weather, ensuring good soil health helps. Well-draining soil that you don't mulch excessively (which can keep it too warm) will allow for a natural cooling as fall approaches, promoting tuber development.

In very hot climates, any steps you can take to cool the soil slightly, such as afternoon shade for the root zone or light mulching that doesn't trap excessive heat, might help, although natural daylength remains the more critical factor.

Ensure consistent moisture during active growth and flowering. However, as the season nears its end and plants naturally begin to die back after a frost, you can gradually reduce watering.

Waterlogging at any stage deprives roots of oxygen and inhibits tuber growth. Good drainage is non-negotiable for healthy tubers.

These dahlia tubers show damage from growing in too-wet garden soil.

In her book DAHLIAS: Seed to Bloom: The Dahlia Grower's Companion, expert grower and hybridizer Kristine Albrecht writes that she makes it her practice to begin weaning her dahlias off water in the 30-day period before lifting tubers.

She cuts irrigation back by one-third the first week, and again the second week. After that, she stops watering altogether until digging day. She does make exceptions for any plants that are still developing seed heads.

Understanding how environmental cues and genetic factors shape dahlia tuber formation is vitally important, but what about the nutrients our plants absorb?

Just like us, dahlias thrive on a balanced diet, and smart nutrient management can significantly boost our tuber harvest. It's not just about applying fertilizer; it's about understanding how different nutrients (and even the hidden world of soil microbes) work together to cultivate plump, viable tubers.

While all plant growth relies on essential macronutrients like nitrogen (N), phosphorus (P), and potassium (K), their ratios and availability can dramatically influence how our dahlia allocates its energy — whether it prioritizes leafy growth or underground storage.

Dahlias' nutritional needs shift throughout their life cycle; early vegetative growth requires different support than later tuber bulking.

Nitrogen (N): The Growth Driver (Use with Care) 

Nitrogen is essential for lush, green foliage and strong stems. However, too much nitrogen, especially later in the growing season, can push your dahlia to produce more top growth at the expense of tuber development. Research also shows that excessive early nitrogen can delay the plant’s transition to tuber production.

Think of it as telling the plant to keep building its house rather than stocking its pantry. Some studies suggest that even a slightly reduced recommended dose of nitrogen, when combined with organic sources, can lead to excellent tuber yield.

This means you might be able to scale back on nitrogen-rich amendments once your plants are established, encouraging them to shift focus underground.

Phosphorus (P): The Root Builder

Often overlooked, phosphorus is foundational for strong root development and, by extension, tuber formation. Studies confirm that phosphorus is vital for energy transfer and early root development, while potassium directly supports sugar transport and storage during tuber bulking. These roles are well established in tuber-forming crops across horticulture.

Ensuring adequate phosphorus in your soil from the start can set your dahlias up for success below ground. Because both phosphorus and potassium tend to accumulate in garden soils, the only way to know for sure whether yours could use more is to obtain a soil test from a qualified lab or agricultural extension service. I talk more about that in the following section.

Potassium (K): The Storage Master

Potassium plays a vital role in moving carbohydrates (sugars) from the leaves, where they're made, down to the tubers for storage. It's the nutrient that helps those tubers bulk up and become dense energy reserves for overwintering.

Sufficient potassium is key as your dahlias mature and start actively forming and swelling their tubers. But don’t add more unless your soil test indicates it’s needed. For the same reason, be careful with so-called balanced fertilizers, such as a 5-10-10 blend commonly sold as “flower and vegetable food.” Those last two numbers (the “10”) mean you are adding twice as much phosphorus and potassium with each application as you are adding nitrogen.

These dahlia seedlings are all about the same age. The plants in back received fertilizer. They are much taller, and bloomed sooner.

Before adding any amendments, it's critical for gardeners to understand their existing soil chemistry. In many regions, especially the Pacific Northwest, phosphorus and potassium can accumulate in soils, sometimes to unhealthy levels, while nitrogen often depletes.

A soil test is the most accurate way to determine your specific soil's needs, preventing over-application and ensuring your dahlias receive precisely what they require without encouraging nutrient imbalances. Avoid generalized fertilizing advice and base your approach on data from your own garden.

Because my county here in northern Washington State is largely agricultural, science testing is easier to come by than in some metropolitan areas. The company I use and recommend is called Simply Soil Testing USA. Their lab is about a half-hour away from my garden, but they offer a very convenient mail-in sample option for customers all over the country, with test results delivered by email.

Science is increasingly showing that it's not just about the bag of fertilizer; it's about the living soil.

Many growers who propagate dahlia cuttings have adopted the advice of Kristine Albrecht and other experts, and apply a dose of mycorrhizal inoculant when they pot up their cuttings. One commercial product claims its product “helps improve root growth, nutrient uptake, and overall plant health.”

However, what some growers don’t realize is that a robust mycorrhizal network often already exists in healthy garden soil, and just like fertilizer, more doesn’t always mean better.

For a deeper dive into this specific topic, I highly recommend A Gardener’s Primer to Mycorrhizae: Understanding How They Work and Learning How To Protect Them by Linda Chalker-Scott, a horticultural professor and self-described “Gardening Myth-Buster.”

If Linda’s science-backed gardening advice resonates with you (as it has with me), she also has an informative YouTube series worth watching, and you can sample her fabulous plant and gardening knowledge in the 3-part Joe Gardener podcast “Gardening Myths BUSTED” (Listen or read the transcript summary here ).

This gardener stands atop a large mound of aged compost

Beyond mycorrhizae, the concept of Integrated Nutrient Management (INM) offers a sophisticated approach that combines organic, inorganic, and biological components. Multiple trials have confirmed that combining organic manures, microbial inoculants, and reduced synthetic fertilizer yields stronger plants and more abundant tubers.

Studies consistently show remarkable results when organic additions and other beneficial microbes complement traditional fertilizers:

• Organic Manures : Farm Yard Manure (FYM) and vermicompost aren't just food; they improve soil structure, water retention, and nutrient availability. Researchers found that combining these with chemical fertilizers led to dahlias with more vigorous growth, more tubers, and higher overall tuber weight.

• Bio-inoculants : Microbes like Azotobacter species are nature's tiny helpers. These beneficial bacteria, which often occur naturally in soil or are available as commercial products, can fix atmospheric nitrogen and solubilize phosphorus in the soil, making these vital nutrients more accessible to your plants. They can even secrete growth-promoting substances. The research suggests that using these biological partners can supercharge your dahlia's ability to produce quality tubers.

By embracing an integrated strategy that balances NPK inputs with organic matter and fosters beneficial soil biology, you’re doing more than just feeding your dahlias. You’re cultivating an underground ecosystem designed for success.

Even after the spectacular display in our dahlia garden slows down in early autumn, the tuber-making process continues beneath the soil. As long as the foliage remains green and unfrosted, the plants continue to photosynthesize and transfer energy to their tubers.

Resist digging up your dahlias right after the first light frost. A light frost might blacken the foliage, but the plant's system can still be moving resources below ground for a week or two. One Japanese study showed that from mid-September to mid-November, tubers continued to gain dry matter and carbohydrates, supporting the practice of delaying harvest when conditions allow.

If you live in a climate zone where it frosts, consider waiting until a hard frost kills the foliage back completely, or at least a week after a lighter frost, before digging.

Many gardens never experience cold frosts, and others don’t see freezing temperatures until well after the winter rains have started, making digging a chilly, mud-splattered mess.

It’s perfectly okay to lift your tubers while the weather is still dry and manageable, so long as the plants have had a chance to run through most of their natural cycle.

On the other hand, if your safe planting date didn’t happen until June or even July, allowing them to grow as long as possible, right up until shortly after a killing frost, may allow the underground system to mature fully.

You can read more about the basics of digging tubers in  Learn the Basics of Lifting Your Dahlia Tubers  on this website. There are lots of good video tutorials to watch online, if you’re a visual learner.

By applying these science-backed strategies, you significantly increase your chances of harvesting plump, healthy dahlia tubers, setting yourself up for another successful season of blooms.

Claims Made in This Article

To support the advice offered in "Part 6: The Science of Better Tuber Harvests," here are the claims made, all thoroughly documented with direct references in Part 7:

• Cool, moist soil and shortening daylength are key triggers for robust tuber formation. (See Claim 6.1 )

  
  

• Excess artificial light or high nitrogen fertilizer early on can delay or suppress tuber formation. (See Claim 6.2 )

  
  

• Integrated nutrient management, combining inorganic fertilizers with organic manures and bio-inoculants, significantly improves plant growth and tuber yield in dahlias. (See Claim 6.3 )

  
  

• Phosphorus is fundamental for root development and energy transfer, foundational for tuber formation, while Potassium is vital for carbohydrate transport and storage, directly impacting tuber bulking. (See Claim 6.4 )

  
  

• Allowing plants to grow for a week or two after blooming slows, as long as frost hasn't killed foliage, can promote continued tuber swelling and eye formation. (See Claim 6.5 )