Breeding Insights from Nature and the Greenhouse
Copyright © 2025 by Steve K. Lloyd
All Rights Reserved
All scientific claims and conclusions presented in this article are thoroughly documented with direct references in Part 7: For Curious Readers & Researchers .
In Part 4: Disrupted Tuber Formation: What’s Really Going On?, we explored how environmental cues and horticultural practices influence dahlia tuber formation. But anyone whose dahlia garden contains many different varieties knows that some just seem to be better tuber producers than others, regardless of how they're grown.
This observation raises the question: Are traits like tuber size, shape, clump formation, and storability inherited? The answer is a resounding yes. Genetics play a significant role in determining a dahlia's tuber-producing potential.
The Blueprint in the Genes: Polygenic Inheritance
A single “tuber gene” doesn’t control tuber formation in dahlias. Instead, the combined effect of multiple genes working together determines it. Scientists call this polygenic, and it explains the wide range of tuber habits across dahlia varieties. These genes interact in various ways with environmental factors to determine the final outcome. This kind of multi-gene influence has been confirmed in scientific breeding literature.
For several years, I grew the open-centered single dahlia ‘Junkyard Dog’. To many, this is not an attractive cultivar, but I find it to be healthy, with robust stems and foliage. Best of all, it is a prolific bloomer and a favorite destination for insect pollinators.

The author with a clump of 'Junkyard Dog' dahlia tubers
Its other virtue? ‘Junkyard Dog’ is a champ at producing tubers. But the strange thing is, this dahlia does not produce a circular clump of rounded tubers. Instead, they form in the shape of fat carrots or long English cucumbers, radiating from the stem like spokes on a wagon wheel. I measured one tuber at 18 inches long and barely 2 inches in diameter.
‘Junkyard Dog’ tubers store well and make good plants the following year, but their length makes them tricky to divide, difficult to store, and a challenge to ship to customers since most of its tubers are too long to fit in a standard postal box.
What Genetics Influence
Tuber Size and Shape: Some varieties naturally produce large, round tubers, while others produce slender, elongated ones. These inherent shapes and sizes depend largely on genetics. Environmental conditions affect how large a tuber grows in a season, but the plant’s DNA programs its size and shape potential.
Clump Density and Formation: You might notice some dahlias form tight, compact clumps of tubers, while others spread out with more individual, finger-like tubers. Genetics also influence this “clump architecture,” affecting how easily you can divide a clump for winter storage.
Number of Tubers: While cultural practices can encourage more tubers, some varieties are simply more prolific, producing a higher number of viable tubers per plant than others under similar conditions.
Eye Formation: The tendency for tubers to reliably form viable eyes (the growth points for next season’s plant) also depends partly on genetics. Some varieties are notoriously shy about producing obvious eyes, making division more challenging.
Storability and Dormancy: The genetic makeup also influences a tuber’s natural dormancy period and its ability to store well over winter, including resistance to rot and dehydration. This is why some varieties seem to keep better than others, even in the same storage conditions.
If you’re interested in learning more about successfully storing tubers over the winter, check out Mastering Dahlia Tuber Storage: A Practical Guide Rooted in Science.

Freshly-divided tubers of Dahlia ‘Junkyard Dog’ from the clump pictured above.
Breeding for Better Tubers
Dahlia breeders, whether commercial or hobbyist, understand these genetic tendencies well. When crossing dahlias and evaluating seedlings, they aren’t just looking for beautiful flowers. They are also assessing other essential traits, including tuber quality.
Dr. Keith Hammett is a horticulture scientist and renowned dahlia breeder from New Zealand. While a guest on the Quince Flowers podcast , he discussed how a cultivar’s tuber-making characteristics can help guide its fate in the market for dahlias.
“The plants that the flower farmers are going crazy over are coming from Holland by the container load,” he said. “Most of those are old cultivars, and they're there for two reasons: because they're largely virus-tolerant (but they're full of the virus), and also, the product is a tuber.”
In Dutch factory farming, dahlia tubers are planted and harvested by machine. “So the tuber shape has determined whether they stayed in commerce or not,” Dr. Hammett continued. “If they've been difficult to harvest with the potato harvester, those cultivars haven't been kept in the system.”

Dahlia tubers can be pre-sprouted indoors, which helps promote earlier growth once planted in the garden.
Selection for Tuber Traits
Breeders can enhance desirable tuber characteristics over generations of selection. If a seedling consistently produces a strong, healthy clump of well-formed, easily dividable tubers with clear eyes, breeders are more likely to select it for further propagation and introduction. This process of selection—sometimes formal, sometimes intuitive—has been documented in scientific studies of dahlia improvement.
Conversely, breeders might discard a seedling with stunning blooms but poor tuber production, or flag it as needing special care to propagate. That was the case for ‘Lilac Bull’, the stingy tuber producer in my garden that I talked about earlier. I finally got it to produce three good tubers, allowing me to retire the hard-working mother tuber that had struggled over multiple seasons.
Trade-offs in Breeding
Sometimes breeders make trade-offs, prioritizing a unique flower color or form even when tuber production isn’t exceptional. This is often how poor tuber producers enter the market. Their floral beauty outweighs their propagation challenges for some growers.
Breeders often propagate and sell these varieties as cuttings rather than growing them from tubers. In my garden, ‘Czarny Charakter’ falls into this category. Judging from what I read on the dahlia groups on social media, there are quite a few others.

These tubers formed over two growing seasons from a single dahlia planted from seed.
Implications for the Home Grower
Understanding that genetics pre-program much of a dahlia's tuber potential gives growers valuable context. It empowers us to make informed variety choices, knowing that sometimes a plant’s genetic blueprint determines its underground habits more than our growing methods can.
Variety Selection Matters: When choosing dahlias, especially if reliable tuber production and overwintering are high priorities for you, do some research beyond just flower aesthetics. Look for varieties known to be good tuber producers in reviews from other growers or dahlia societies.
Not All Failures Are Your Fault: If you've diligently followed best practices but a particular variety consistently disappoints with its tuber production, it might be due to its genetic predisposition rather than your gardening skill.
Learn from Experience: Keep notes. Over time, you'll identify which varieties consistently perform well (both above and below ground) in your specific climate and soil, helping you refine your collection based on both beauty and reliability.
In Part 6: The Science of Better Tuber Harvests , we'll bring together all this knowledge to provide actionable advice on what growers can do to encourage bigger, healthier tuber clumps.
For Further Reading: Scientific Claims and Documentation
In "Part 5: Are Tuber Traits Genetic?", we've discovered the significant role genetics play in a dahlia’s tuber-producing potential. Every substantive claim we've made is backed by peer-reviewed research and authoritative texts.
For a complete list of these claims and their supporting scientific documentation, please visit Part 7: For Curious Readers & Researchers.
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Tuber formation in dahlias is a polygenic trait, meaning it is influenced by multiple interacting genes rather than a single gene. (See Claim 5.1 )
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Different dahlia cultivars exhibit varying tendencies for tuber size, shape, eye formation, and storability, even under ideal conditions, due to genetic differences. (See Claim 5.2 )
Breeders can select for tuber-forming traits through seedling selection over generations. (See Claim 5.3 )