The Dahlia Underground: Hidden Life of Cuttings and Tubers

 Part Two in the “The Dahlia Clock” Series

Copyright © 2025 by Steve K. Lloyd – All Rights Reserved

The story of a dahlia’s life beneath the soil is just as dramatic as the story above ground. While we tend to focus on flowers, the balance of energy between blooms and roots—and how that balance shifts across the season—is at the heart of understanding the Dahlia Clock.

The key lies in recognizing that photoperiodic signals—the plant’s interpretation of daylength—drive not only flowering but also the timing and quality of tuberization.

Long days promote vegetative growth and stem production; short days encourage tuber initiation. But neither phase is static—both are part of a dynamic, responsive cycle.

The duration of light and dark periods they experience is important to dahlias (Author’s photo)

Short days are the primary environmental cue that encourages adventitious roots to transform into tubers (2.1). This conversion is often reinforced by cooler temperatures. Together, these signals reprogram certain roots to act as carbohydrate sinks, storing energy for the next growing season.

Tuberization is not merely the swelling of existing roots; it represents a photoperiod-driven developmental reprogramming of selected adventitious roots into storage organs. This shift depends on sustained carbohydrate flow from the shoots to support the growth and maturation of fully formed tubers (2.2).

Prolonged warm nights or delayed short-day exposure can disrupt this reprogramming. The result is often disappointing—thin, stringy roots rather than the well-formed storage organs that gardeners prize.

Only specialized adventitious roots will swell and form tubers. At 75 days, tuber development is already starting (Author’s photo)

Growers have a powerful but often underappreciated ability to manipulate the tuberization process by managing daylength, temperature, and energy availability.

Early in the season, providing long days and warm temperatures encourages vigorous top growth. This vegetative vigor later supports better tuberization—if the transition to shorter days is well-timed.

However, if long days are extended too far into late summer—either by artificial lighting or by planting too late—the plant may continue to invest in vegetative growth and fail to trigger timely tuberization.

Conversely, if short-day conditions begin too early or are reinforced by cool weather, the plant may prioritize storage over stems and blooms, cutting short the floral season.

The Dahlia Clock’s influence is particularly evident in the tug-of-war between flowers and tubers. Both act as carbohydrate sinks, drawing energy from leaves and stems (2.3).

Late in the season, as nights lengthen, the plant shifts priority toward tuber formation. This is why blooms initiated in late August often fail to fully develop or have shorter vase life: the plant is diverting resources underground.

Understanding this competition helps growers make informed decisions about disbudding, feeding, and even harvest timing. It’s not just about what you see above ground—it’s about how the plant is allocating its energy in response to seasonal signals.

Here is Dahlia ‘Sweet Sanne’ blooming heavily in the author’s garden on September 29. Research points strongly to a trade-off between late-season blooms and optimal tuber development (Author’s photo)

For growers in temperate regions like the Pacific Northwest, recognizing the Dahlia Clock’s underground dynamics is crucial.

• Timing. Planting early enough to allow a long vegetative phase under naturally long days sets the stage for strong tuber development later.
  
  

• Light Management. Supplemental lighting for cuttings or young plants can extend the vegetative phase, but should be withdrawn in time to allow natural short days to initiate tuberization.
  
  

• Soil Temperature. Monitoring soil temperature can also guide decisions: warm soils support vegetative growth, while cooling soils encourage storage.
  
  

• Disbudding. Experienced growers often stagger plantings or manage disbudding schedules to balance bloom quality with long-term tuber health.

2.1 – Short days encourage tuber initiation. — Al-Janabi, M. B. M., & Al-Maathedi, A. F. (2015). Effect of photoperiod, paclobutrazol and pinching on tuber roots and dahlia flowers. Tikrit University Journal for Agricultural Sciences, 15(1), 74–90.

2.2 – Tuberization is a photoperiod-driven reprogramming of selected roots into storage organs that depends on sustained carbohydrate flow from the shoots — Jackson, S. D. (2009). Plant responses to photoperiod. New Phytologist, 181(3), 517–531; Brøndum, J. J., & Heins, R. D. (1993). Modeling temperature and photoperiod effects on growth and development of dahlia. Journal of the American Society for Horticultural Science, 118(1), 36–42.

2.3 – Flowers act as strong carbohydrate sinks that compete with tuber bulking — Yang, Y., Ohno, S., Tanaka, Y., & Doi, M. (2022). Effects of Deflowering and Defoliating on the Postharvest Characteristics of Individual Organs in Cut Dahlias. The Horticulture Journal, 91(4), 551-557.

Adventitious roots — Roots that arise from non-root tissue such as stems or nodes. In dahlias, certain adventitious roots can be reprogrammed into tubers under the right conditions.

Carbohydrate sink — A plant organ, such as flowers or tubers, that draws sugars and other resources from the plant.

Photoperiod — The length of light and dark periods in a 24-hour cycle. Dahlias use photoperiod cues to regulate both flowering and tuber formation.

Tuberization — The biological process by which certain adventitious roots are transformed into underground storage organs in response to short days and temperature cues.

This article was created collaboratively by the author, a dahlia grower and educator, and an AI language model. The author directed the structure, tone, and emphasis; supplied the scientific sources; and oversaw the final text.

The AI assisted primarily with summarizing complex technical material, suggesting phrasing, and linking every substantive scientific statement to the author’s supplied, peer-reviewed sources.

The author carefully reviewed and refined all content to ensure accuracy, clarity, and practical value for readers interested in dahlia science.