How Light, Hormones, and Propagation Methods Affect Tuber Formation
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 2: What Makes a Dahlia Root Become a Tuber?, we took a deep dive into the specific environmental cues and internal changes that tell a dahlia root it's time to become a tuber. In this article, we'll explore how human intervention, particularly in propagation, can sometimes accidentally disrupt this natural process.
One of the many things I enjoy about growing dahlias is propagating them from cuttings, allowing me to multiply my favorite varieties quickly. I have many dahlia friends who are just as enthusiastic about taking cuttings as I am.
For us, and other growers, a common frustration is finding that some beautiful, healthy-looking dahlias grown from cuttings produce only one or two viable tubers by the end of the season. Sometimes, they make none at all.
Why does this happen? Often, the very conditions we create to encourage rapid rooting and top growth in cuttings can inadvertently work against tuber formation.
A dahlia cutting at 75 days old. Adventitious roots are already swelling to form tubers.
The "Greenhouse Effect": Light and Warmth as Tuber Suppressors
When we take dahlia cuttings, our goal is usually fast root development and vigorous shoot growth. We provide warmth, consistent moisture, and often plenty of light. While ideal for rooting, these conditions can send the wrong signals to a young dahlia plant when it comes time to make tubers.
Long Daylength, High Light Intensity
Growers often start cuttings early in the season, when natural daylengths are still on the up-swing. Where I live, safe planting outdoors is around mid-May. In order to have well-established dahlia cuttings that are ready for the garden, I usually start my cuttings in February.
Although the days are a bit over 15 hours long in the middle of May (planting time), three months earlier when I take my first cuttings, there is just 10 hours of natural daylight. This means that I have to use artificial lights to trick the plants into thinking the days are longer than they are.
While essential for photosynthesis and general plant growth, continuously long periods of light actively suppress tuber initiation in dahlias. This relationship between photoperiod and tuber suppression in dahlias is well documented in horticultural research, including controlled trials showing significantly reduced tuber formation under prolonged long-day conditions.
Remember from Part 2 that dahlias interpret shortening days as a signal to form tubers. Maintaining "long day" conditions effectively tells the plant to keep putting energy into leafy growth, not underground storage.
When a cutting I started on February 15 and grew for three months under supplemental light is finally planted in the garden, where the days are still more than 15 hours long, the signal to begin forming tubers can become lost or delayed.
This dahlia, grown from a cutting, shows a healthy root system on planting day.
Warm Temperatures
I have a heated greenhouse, but before that I started seeds and rooted cuttings in my garage. Experience (and science) tells me that dahlias root best at temperatures ranging between about 65–75°F.
Like me, other growers typically root cuttings in warm environments to encourage rapid cell division and root emergence. While beneficial for rooting, consistently warm soil and air temperatures can also inhibit the formation of tubers.
The gradual cooling of late summer and early fall is a natural cue for tuber development. Prolonged warmth can override this signal. In some regions, unusually warm conditions now persist well into autumn, further delaying tuber formation.
In essence, the environment we provide for rooting cuttings mimics summer conditions. The plant receives the message: “Keep growing! Winter is a long way off!” This delays or prevents the critical hormonal shifts needed for tubers to form.
Hormonal Interference and Physiological Age
Beyond environmental cues, the internal physiology of the cutting itself and any treatments we apply as growers can play a role. These insights about hormone levels and physiological age come not only from grower experience, but from formal studies exploring how rooting conditions affect tuberization in dahlia cultivars.
Rooting Hormones (Auxins)
Many of us use rooting hormones (synthetic auxins) to encourage faster and stronger root development on cuttings. I use rooting powder, and a friend uses Clonex gel. Experienced growers from my local American Dahlia Society (ADS) chapter insist that no chemical enhancement is necessary; they root their cuttings in sand, perlite, vermiculite, or some other medium.
While generally helpful for rooting, high levels of certain auxins may disrupt the hormonal balance that drives tuber formation. The plant’s internal chemistry is finely tuned, and adding external hormones can steer it away from making tubers.
Physiological Age of the Cutting
The age of the cutting material itself can also make a difference. My usual approach is to wake up my tubers, often intact clumps or pot tubers, in early February. With artificial light and heat, I coax them into growth in my garage or greenhouse until they produce tender new shoots I can harvest as cuttings.
What I only recently learned is that cuttings taken from very young, vigorously growing mother plants tend to stay focused on vegetative growth. Without realizing it, the early-spring cuttings I take by the hundreds may be carrying an inherited tendency to delay or skip tuber formation altogether.
The sprouts on this tuber clump are ready for vegetative propagation as cuttings.
As a tuber, clump, or actively growing plant begins to receive tuber-inducing signals such as shorter days, things shift. Cuttings taken later in the season often carry some of that “tuberization readiness.” This helps explain why some growers see better tuber production from cuttings taken in mid-summer instead of early spring.
It’s actually a pretty good word, but in this series (as with my other articles) my goal is to try my best to understand the science, then make it accessible to fellow growers who, like me, may not have formal scientific training. I don’t shy away from using scientific terms, but I take care to explain these often specialized and unfamiliar expressions.
In this series, my workaround has been to use the more everyday terms that gardeners say, such as “tuber formation” or “tuber-making” whenever possible. Instead of “adventitious” (also common in the science journals), I might say “roots that may swell into tubers.”
But in a few places, I’ve kept the terminology that scientists prefer. When you see them, please know that I recognize them as words you’ll probably never hear said at a local dahlia society meeting!
At 75 days, this dahlia cutting has developed feeder roots and adventitious roots that will form tubers.
Practical Takeaways for Tuber-Producing Cuttings
Starting cuttings early under ideal rooting conditions is effective, but consider these strategies if you want to maximize tuber production from them:
• Acclimatize and Plant Out Early
Get rooted cuttings into their final garden beds as soon as safely possible. This exposes them to natural daylength cycles and soil temperature fluctuations.
• Reduce Artificial Light
Once rooted, transition cuttings away from intense, extended artificial light if your goal is tuber production.
• Time Your Cuttings
If you can, take a second flush of cuttings in mid-summer. These might have a stronger tendency to make tubers, influenced by the mother plant’s physiological state and the naturally shortening days after summer solstice.
• Focus on the Mother Plant
Remember, the most reliable way to get tubers is often from the original mother plant, which experiences a full season of natural environmental cues.
This dahlia grew for two seasons. The darker tubers formed during the first year and the lighter-colored ones during the second.
Heads-up: Not all propagation problems are hormonal or environmental.
In some cases, bacterial infections like leafy gall can cause malformed growth at the tuber crown. This may look like unusual sprouting or dense leafy clusters at the base. If you spot these signs on tubers from cuttings, discard them—gall diseases can spread silently through tools or cuttings.
I explore these diseases, how they spread, and what to do if you encounter them in my article Dahlia Gall Disease: Unmasking Crown Gall and Leafy Gall .
Understanding these factors helps explain why some cuttings might produce beautiful flowers but make tubers without eyes, or no tubers at all. This isn’t a failure on your part. It’s a consequence of the many biological signals that govern tuber formation, which our propagation practices as growers sometimes override.
In Part 4: Disrupted Tuber Formation: What’s Really Going On? , we'll take a look at how human intervention, environmental stress, and other factors can interrupt the tuber-forming process.
For Further Reading: Scientific Claims and Documentation
In "Part 3: Why Some Dahlia Cuttings Don’t Make Tubers," we've demystified a common grower frustration. 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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High light intensity, particularly long daylength, and warmth during propagation of cuttings can suppress tuber formation. (See Claim 3.1 )
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Hormonal treatments, such as high levels of auxins used for rooting, can sometimes interfere with subsequent tuberization. (See Claim 3.2 )
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The physiological age of the cutting material (juvenility) can also influence its capacity for tuber formation. (See Claim 3.3 )
Practices like taking cuttings too late in the season, when mother plants are already signaling tuberization, can improve tuber yield from cuttings. (See Claim 3.4 )