Part 4: Disrupted Tuber Formation: What’s Really Going On?

Copyright © 2025 by Steve K. Lloyd
All Rights Reserved

In Part 3: Why Some Dahlia Cuttings Don’t Make Tubers, we explored how human intervention, particularly during dahlia propagation, can accidentally disrupt this natural process. In this article, we’ll look into other factors that can interfere with tuber formation, including commercial growing practices and the stubborn problem of plants that fail to make tubers for no obvious reason.

Even when we follow all the rules—providing ideal light, temperature, and moisture, and managing cuttings carefully—dahlias can still fail to produce viable tubers. This is one of the most frustrating aspects of dahlia cultivation, especially when nothing appears to be wrong.

This article explores some of the hidden reasons tuberization fails to occur, including commercial production methods and certain unexplained behaviors.

To fully understand why dahlia tuber formation can be disrupted, we must consider practices in commercial dahlia production. While home growers rely on environmental cues and good cultural practices, large-scale commercial operations sometimes employ a different set of tools: chemical growth regulators (CGRs).

These powerful substances aim to manipulate plant growth in specific ways, often to achieve a desired retail appearance or to meet production schedules.

A friend who operates a retail nursery once told me, “Flowers sell.” What she meant is that customers are drawn to potted plants that are already in bloom, choosing them over more robust plants that have grown naturally and are poised to show their colors after planting.

These blooming nursery center dahlias, planted in gallon pots, have almost certainly been manipulated with chemicals

CGRs are synthetic compounds that mimic or interfere with natural plant hormones. Commercial dahlia growers might use them to:

Delay or accelerate bloom development to hit specific market windows.

While effective for these commercial goals, the primary aim of these chemical applications is not robust tuber formation. Their use can have downstream effects on what happens once you plant that blooming dahlia you picked up at the grocery store or garden center.

Commercial growers use several types of CGRs. Each works in a different way and can affect a dahlia's overall growth, including its ability to form tubers. These include ethephon (an ethylene releaser), daminozide and SADH (gibberellin inhibitors), and maleic hydrazide, all of which interact with hormonal pathways relevant to tuberization (see Part 7).

Ethephon (Ethylene Releaser): This compound releases ethylene, a natural plant hormone. Commercial growers might use it to reduce plant height or encourage branching. While it can influence flowering, this chemical also interacts with the plant's natural signals for tuber formation. This means it can subtly shift the plant’s energy away from developing tubers.

Daminozide (Alar) and SADH: These are gibberellin inhibitors. Gibberellins (GAs) promote vegetative growth like stem elongation, and high levels can inhibit tuber formation. By blocking GAs, these chemicals lead to more compact plants. In some cases, this hormonal shift might even indirectly encourage tuber formation. However, commercial growers typically use them for height control, not necessarily to boost tuber production.

Maleic Hydrazide (MH): MH is another growth retardant that can reduce plant height and increase branching. While MH helps create a desirable bushy plant for sale, it changes the plant’s internal chemistry. This can affect how the plant uses its energy, potentially impacting the size or quality of tubers.

For years, I searched for a dahlia called ‘Electro Pink’, hybridized in New Zealand by Dr. Keith Hammett, the creator of the dark-foliage ‘Mystic’ series. This cultivar has circulated in Australia and the U.K. for years, but is hard to find in the U.S.

To my surprise, I found two potted ‘Electro Pink’ dahlias at an independent nursery in a small town about 50 miles from home. Both plants looked healthy, they were in bloom, and their leaves gleamed with the dark tinge characteristic of Dr. Hammett’s unique cultivars.

I happily bought them both and, within a few hours, had carefully planted them in my garden. As I watered and fussed over them for the next few weeks, I started to wonder: How did they arrive at this nursery? How were they propagated? And would they produce viable tubers?

When we buy a dahlia plant from a grocery store or garden center—especially one that looks unusually compact or perfectly shaped—it might carry residues of growth regulators. That raised a practical question: What does this mean for my hoped-for tuber harvest?

Growing just 18” tall in small pots, these blooming dahlias are likely to be poor tuber producers, at least during their first growing season.

The plant’s first-season growth habit might appear shorter or bushier than its true genetic potential.

Depending on which CGRs were used and when they were applied, these chemicals can interfere with the hormonal signals that trigger tuber formation. This can lead to smaller tubers, fewer of them, or none at all in the first season.

CGRs are commercial tools, not typically used by home growers. Your best strategy remains optimizing light, temperature, soil moisture, and balanced nutrition to encourage natural tuber development.

Understanding commercial treatments helps clarify why store-bought dahlias might behave differently than those grown from tubers you’ve overwintered or purchased from a specialist.

These retail plants are grown for shelf appeal. Compactness and first-season bloom are the goals. Growers may use CGRs to enhance those traits, but the result may be poorer tuber production. In contrast, dahlias from a small farm or specialty grower are usually propagated with tuber quality in mind, without the chemical shortcuts.

The use of chemical growth regulators does not necessarily doom a store-bought potted dahlia to a single season. Given the right care, most will produce at least a few tubers, although they often remain small and congested. You can store these clumps intact, rather than dividing them into separate tubers. The following season, plant the entire “pot tuber clump” whole.

While this does not always work, when I’ve used this method I have been rewarded with larger, dividable tubers at the end of the second growing season. I grow a number of dahlias that first came to my garden in pots, and they make beautiful tubers.

Beyond commercial interventions, there are frustrating instances where dahlias simply fail to produce tubers despite seemingly ideal conditions.

For years I grew one plant of ‘Lilac Bull’, a formal decorative dahlia that bloomed in a shade of purple I didn’t have anywhere else. All season, the plant grew well, made nice flowers, and seemed perfectly healthy. But every fall, when I dug it up, I found the original mother tuber with maybe one or two golf-ball-sized tubers at best. What was going on?

These kinds of results baffle even experienced growers. While not every failure has a clear cause, possible explanations include:

Mild, hard-to-detect stresses such as waterlogging, compacted soil, or temperature fluctuations can interfere with tuber signals.

As we’ll explore in Part 6: The Science of Better Tuber Harvests, phosphorus and potassium must be carefully balanced. A quiet deficiency or excess can inhibit tuberization.

Some varieties have a weaker natural tendency to make tubers or need more precise environmental conditions. This is especially true in cultivars where flower traits have been prioritized over tuber strength.

Sometimes, a tuber forms without a viable eye: the small growth point needed to sprout the next season. These tubers cannot be propagated and are often mistaken for healthy ones. Scientists know little about how or why this occurs, but it may involve disrupted hormonal signals at the crown.

Unseen pests like aphids, nematodes, or fungal pathogens can compromise the root system’s ability to form tubers properly.

This generous tuber clump grew in a single season from one tuber-grown dahlia.

Not all failed tubers are the result of environment or genetics. In some cases, bacterial diseases like crown gall or leafy gall may be to blame.

Crown gall (Agrobacterium tumefaciens) causes woody, tumor-like swellings at the crown or on tubers.

Leafy gall (Rhodococcus fascians) creates distorted, leafy shoots at the crown or near developing eyes.

These diseases are often incurable, spread easily, and mimic problems such as blind tubers or malformed sprouts. If you see abnormal swelling or strange leafy growth during division, discard the entire clump and sanitize your tools. Do not propagate.

For more, see my article Dahlia Gall Disease: Unmasking Crown Gall and Leafy Gall, available on this site.

While not every failure can be fully explained, recognizing the most common disruptions—whether hormonal, environmental, genetic, or human-made—helps demystify how dahlia tubers form (or don’t). This is especially true when we consider the effects of commercial growth regulators and the environmental cues discussed earlier in Part 2: What Makes a Dahlia Root Become a Tuber?

This giant clump is from Dahlia ‘Teddy’, a variety which consistently produces good tubers in the author’s garden.

For Further Reading: Scientific Claims and Documentation

In "Part 4: Disrupted Tuber Formation: What’s Really Going On?", we've uncovered the less obvious reasons for tuber struggles. 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.

• Commercial growth regulators (CGRs) are used to manipulate plant architecture (e.g., height, branching) and flowering time for market purposes, which can have downstream effects on tuber formation. (See Claim 4.1 )

  
  

• Specific CGRs like Ethephon, Daminozide (Alar), SADH, and Maleic Hydrazide (MH) impact plant growth by altering hormonal balances, potentially affecting resource allocation for tuber development. (See Claim 4.2 )

  
  

• Dahlias purchased as potted plants from garden centers may have been treated with CGRs, which can alter their initial growth habit and potentially delay or reduce tuberization compared to tubers grown for propagation. (See Claim 4.3 )