Part 2: What Makes a Dahlia Root Become a Tuber?

Copyright © 2025 by Steve K. Lloyd
All Rights Reserved

In Part 1: What Makes a Tuber a Tuber?, we established that a dahlia tuber isn't just any root. It is a specially modified type of root serving as an energy store and the plant’s regrowth engine.

What growers need to understand is that healthy dahlia tubers don’t “just happen”. Despite the repeated mentions on social media of some magic 120-day growing period, even that measure does not tell the whole story. By understanding the hidden cues of dahlia tuber formation and growth, we gain invaluable insight into how to coax our dahlias into producing those healthy, robust tubers we need for next season.

But what tells a dahlia it’s time to stop producing long, fibrous roots and start thickening them into tubers? Environmental cues and internal hormones work together in a delicate balance to set that process in motion.

Dahlias, like many plants that form corms, bulbs, and tubers, are highly responsive to their surroundings. They interpret changes in light, temperature, and moisture as cues to prepare for dormancy, shifting their energy from active growth to underground storage.

Dahlias belong to a group of plants called geophytes (plants with underground storage organs such as tubers, bulbs, or rhizomes). I first encountered the term in an absorbing  University of California article , which explained it means “earth plant.” That translation instantly reminded me of geocaching, and now, whenever I plant a tuber, I picture “geocaching” a dahlia that will surprise me with flowers a few months later.

Photoperiod means the number of daylight hours a plant receives in a 24‑hour period. For dahlias, this is the most important environmental signal for tuber initiation.

Here in the Pacific Northwest, we enjoy more than 16 hours of daylight in June but scarcely eight hours in late December. It’s no surprise that I’m up early and strolling the dahlia garden, coffee in hand, on bright summer mornings, but far less eager to rise on dark winter days. Plants respond the same way to changing daylength.

Dahlias are “short-day” plants when it comes to tuber formation, meaning they begin initiating tubers as the days naturally diminish in length after the summer solstice. Extended daylight hours in summer encourage vegetative growth, such as stems and leaves.

However, as the daylight hours recede in late summer and early fall, the plant senses this seasonal shift and starts directing its energy underground. This process often begins before any visible changes appear in the plant's top growth.

Stem growth, leaf production, and flowering all thrive in warm soil. But as soil cools in late summer and early fall, tuber formation is encouraged. Warm soil supports fibrous root growth, but a gradual cooling signals the dahlia that its active growing season is winding down and it’s time to divert resources below ground. Prolonged soil temperatures above 75–80°F can inhibit tuber formation, even if other conditions are favorable.

Adequate moisture is essential for plant health, but overwatering or waterlogged soil reduces oxygen in the root zone, which can suppress tuber development. As dahlias prepare for dormancy, they still require water, but consistently soggy conditions can harm tuber formation. Good drainage is critical throughout the season and becomes even more important as the plant transitions to storing food in its tubers.

Environmental cues don’t directly cause roots to swell — they trigger hormonal changes that direct how the plant grows and stores energy.

I first learned about the role of hormones in plant roots from the article “The Physiology of Adventitious Roots ”, hosted by the U.S. National Institutes of Health’s National Library of Medicine. Although the illustrations featured other plants, the underlying processes matched what I later read in dahlia‑specific studies: the same primary hormones are involved across many species.

Gibberellins (GAs) are plant hormones that generally promote stem elongation and vegetative growth. High levels of GAs inhibit tuber formation. As days shorten and temperatures cool, GA levels naturally decline, removing a block to tuber initiation.

Abscisic acid (ABA) is a hormone linked to dormancy and stress responses. In dahlias, tuber formation often follows an increase in ABA, triggered by shorter days and cooler temperatures.

Ethylene and auxins (auxins are hormones that regulate growth and development in plant tissues) also play roles in root development and communication between plant parts. Their exact functions in dahlia tuber formation are still being studied.

When growth‑promoting hormones fade and dormancy‑promoting hormones rise, the plant shifts resources into tuber formation, with other hormones guiding the process. These hormonal shifts stimulate internal changes in the root itself, including increased cambial activity and the development of new storage tissue. As a result, the fibrous root begins its transformation into a tuber.

A few years into my dahlia‑growing obsession, I began hybridizing my own varieties from seed. The first time I tipped a baby plant out of its 3.5‑inch starter pot and saw tiny tubers already forming, I was thrilled and also a bit confused.

I had always heard that most tuber growth happens late in the season, yet here my season had barely begun and my little dahlia already had baby tubers. Later research showed that the most critical time for tuber initiation is typically 4–8 weeks after planting. Depending on your climate and planting date, that often means mid to late summer.

During this window, shortening days and cooling soil send a strong signal to the plant to begin converting specialized roots into storage organs. Until I understood this, I thought my early‑season work of planting, fertilizing, watering, and pest management only benefited the visible parts of the plant. Now I know the first half of the season is just as important for the roots and tubers I will dig in November.

By recognizing these environmental and hormonal cues, gardeners can better align their practices to what is happening underground. While we cannot directly control a dahlia’s hormone levels, we can create the external conditions that encourage the shift from leafy growth to robust tuber formation.

For Further Reading: Scientific Claims and Documentation

In "Part 2: What Makes a Dahlia Root Become a Tuber?", we've unraveled the environmental signals and internal hormonal shifts that prompt a dahlia root to swell into a plump tuber. 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.

• Tuber formation in dahlias is largely influenced by photoperiod (daylength), typically favored by shorter days. (See Claim 2.1 )

  
  

• Cooler soil temperatures also contribute to robust tuber development. (See Claim 2.2 )

  
  

• Soil moisture and drainage are critical; waterlogging can inhibit tuber formation by reducing oxygen to the roots. (See Claim 2.3 )

  
  

• The critical window for tuber initiation typically occurs 4 to 8 weeks after planting, influenced by these environmental cues. (See Claim 2.4 )

  
  

• Hormonal balance, particularly the interaction between gibberellins (GAs) and abscisic acid (ABA), is key to tuberization. High GAs inhibit, while ABA promotes. (See Claim 2.5 )