Using Artificial Light to Grow Dahlias: LEDs, Timing, & Spectrum

 Part Three in the “The Dahlia Clock” Series

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

Many growers assume dahlias must thrive only outdoors in full sun. However, mastering controlled lighting allows you to unlock year-round growth, providing the power to manipulate bloom timing, improve stem and foliage quality, and even influence tuber development (3.1).

Artificial lighting is not about replacing the sun—it’s about steering the Dahlia Clock to your advantage.

Even with optimal light, a young dahlia will not flower until it has passed the juvenile stage and achieved developmental maturity. Cuttings or seedlings may therefore remain vegetative for longer than expected, simply because they have not yet reached the maturity needed to respond to flowering signals.

Only once they pass this stage can they fully process photoperiod cues, making light timing critical (3.17).

Young dahlias like these seed-grown plants must pass the juvenile stage before they can fully respond to light signals (Author’s photo)

Dahlias are facultative short-day plants—they bloom faster when nights are long, but they will eventually flower even under long days (3.2).

By using night-interruption (NI) lighting—providing dim light for about four hours in the middle of the night—you can trick plants into thinking nights are shorter. This delays flowering and prolongs vegetative growth, leading to sturdier stems (3.3).

Alternatively, day-extension (DE) lighting at dusk achieves a similar effect. Even very low-intensity light—around 10 foot-candles (fc)—is enough to alter flowering (3.4).

Instead of leaving lamps on continuously, many growers use cyclic NI lighting—brief bursts every 20–30 minutes—which can cut energy use by up to 80% with minimal flowering delay (3.16).

Even consumer-level light timers can manage the dual on/off cycles needed for this technique. The idea is simple: instead of relying solely on a constant long day, a grower provides an hour-long light burst in the middle of the night to ensure the dark period is broken.

Stray light from greenhouses, streetlamps, or porch lights can unintentionally interrupt night length, causing dahlias to delay flowering or produce weaker blooms (3.5).

Even leaks as low as 0.5–1 fc through blackout cloth can matter. Use blackout curtains or careful positioning to prevent these accidental signals.

For home growers, the light of a distant porch or streetlight is unlikely to create enough light for dahlias to sense it as “night interruption.” But if someone positioned their dahlia bed directly under a strong artificial light source that remained lit all night, it could present a problem (3.5).

The ratio of red to far-red (R:FR) light profoundly influences plant height and flower form (3.6).

Far-red encourages elongation and, in some cases, can delay flowering or push energy toward vegetative growth (3.7).

A balanced R:FR ratio—similar to late-summer sunlight—supports compact, high-quality plants.

As Leslie Halleck noted, “Plants see light differently than people see light. They don’t need brightness; they need the right spectrum.” This is why the Kelvin choice and R:FR balance matter as much as the hours of light provided (Perrone 2018).

Blue light is crucial for regulating circadian rhythms and overall plant architecture (3.8).

Moderate blue content generally produces the desirable shorter, sturdier stems, while insufficient amounts can lead to spindly growth and reduced photosynthetic efficiency.

Importantly, dahlias generally do not respond to very low-intensity blue light night breaks the way some other ornamentals do (3.9).

Modern LEDs provide a level of control never before possible, allowing for the precise tuning of R:FR and blue:green ratios (3.10).

Full-spectrum LEDs generally provide the most balanced growth, while red + far-red heavy mixes can be used strategically for vegetative expansion.

For growers using consumer bulbs, the Kelvin (K) value provides a quick spectral guide:

• 3,000–3,500 K – red-heavy, favoring some bloom/elongation
  
  

• 5,000–6,500 K – blue-heavy, favoring compact growth
  
  

• A mix of both – recommended for balanced growth of seedlings and cuttings

Photoperiod and spectrum act as signals; intensity is the fuel.

“Plants don’t think about light in terms of brightness,” noted Leslie Halleck in a conversation with Joe Lamp’l. “They quantify light in terms of volume—the total number of photons” (Lamp’l 2018).

This framing makes the Daily Light Integral (DLI)—the total number of photons a plant receives each day—the grower’s best shorthand for plant energy.

DLI governs how much sugar a plant can make through photosynthesis. Higher DLI means faster development, more compact growth, and earlier flowering (3.1).

Bright LED setups don’t just provide energy—they can additionally affect how dahlias perceive daylength.

In contrast, low DLI environments, such as dimly lit basements or persistently cloudy climates, can cause delayed flowering or poor tuber quality. This happens not because the light-signal clock is wrong, but because the plant’s essential energy supply is too low to power rapid development.

For years I have rooted my dahlia cuttings and germinated seeds on a shelving unit in my garage. Each shelf is 24 by 48 inches (61 by 122 cm) and is lit by a pair of four-tube LED shop lights that I source from a big-box store.

These lights are on timers and hang by chains that allow me to adjust how close the lights are to the trays or plants underneath them. I have found a dramatic difference in plant growth by positioning the lights within 2 inches (5 cm) of the plants, or the soil level when germinating seeds. As the plants grow, I continually shorten the chains to raise the fixtures.

This process ensures the plants always receive the highest possible light intensity, maximizing their DLI despite using simple, consumer-grade fixtures.

Bright lights, closely positioned above the plants, send critical signals to these young dahlia seedlings (Author’s photo)

The timing of your light cycles is crucial for managing the plant’s development:

• Build Vegetative Mass: Begin with long days (15–16 h) for the initial phase to maximize vegetative growth.
  
  

• Trigger Flowering: Introduce 10–15 short-day cycles (9 h light / 15 h dark) to strategically initiate flowering earlier than the natural season (3.11).
  
  

• Maximize Stems: Return the plants to long days afterward to maximize stem length and quality.
  
  

• Avoid Continuous Short Days: Prolonged short days reduce stem yield and push the plant toward tuber formation at the expense of quality blooms (3.12).

Extended short-day conditions increase tuber initiation but come at the cost of reduced floral output (3.13).

The ideal strategy is to use artificial lighting to provide long days for vegetative growth and strong stems, followed by controlled short days to program robust tuber formation without severely sacrificing your bloom season.

Choosing the right lamp can save energy and provide the optimal light quality:

• Incandescent bulbs are ineffective, wasting energy and causing undesirable plant elongation (3.14).
  
  

• High-Pressure Sodium (HPS) and Metal Halide (MH) lamps are efficient but generate excessive heat.
  
  

• LEDs offer the best long-term investment: they consume far less power, last longer, and allow for the precise spectrum needed by dahlias (3.15).

Energy Savings Tip: Remember that cyclic NI lighting can reduce energy use by up to 80% with only minimal delay to flowering (3.16).

Regular LED shop lights from a big box store provide ample lighting for early dahlia seedling development (Author’s photo)

The Dahlia Clock isn’t just a metaphor—it’s a powerful framework for understanding every stage of your plants’ lives.

By reading the subtle cues of night length, twilight, temperature, and light intensity, you can anticipate how your dahlias will behave weeks or even months in advance.

For gardeners, that means fewer surprises and more control:

• Recognizing that a blown center in September traces back to light conditions in July.
  
  

• Knowing when to shift from vegetative growth to storage bulking for plumper tubers.
  
  

• Using artificial light only when it serves a clear purpose—whether that’s delaying bloom for show timing or giving young plants a strong, healthy start.

For breeders and advanced growers, it offers a roadmap for precise experimentation:

• Testing different photoperiod schedules to explore genetic variation.
  
  

• Leveraging spectrum tweaks to influence architecture without the use of chemicals.
  
  

• Refining propagation and tuber-lifting schedules based on predictable biological triggers.

The Dahlia Clock series demonstrates that light is both energy and information.

When you work with the plant’s natural signals rather than against them, you can grow sturdier plants, harvest better tubers, and time the production of quality blooms with confidence.

3.1 – Artificial lighting can manipulate bloom timing, stem quality, and tuber development. — Meng, Q., & Runkle, E. S. (2014). Controlling flowering of photoperiodic ornamental crops with LED lamps. HortTechnology, 24(6), 702–711.

3.2 – Dahlias are facultative short-day plants. — Proietti, S., Scariot, V., De Pascale, S., & Paradiso, R. (2022). Flowering mechanisms and environmental stimuli for flower transition: Bases for production scheduling in greenhouse floriculture. Plants, 11(3), 432.

3.3 – NI lighting delays flowering and prolongs vegetative growth. — Lopez, R. G., & Currey, C. (2014). Managing photoperiodic lighting. GrowerTalks, March 2014, 36–40.

3.4 – Very low light levels (~10 fc) are enough to affect photoperiod response. — Lopez, R. G., & Currey, C. (2014). Managing photoperiodic lighting. GrowerTalks, March 2014, 36–40.

3.5 – Stray light can disrupt flowering. — Lanzes, T., Thakur, R., & Choskit, T. (2023). Photoperiodism and vernalization. In Advances and Trends in Agriculture Sciences (pp. 111–123). KD Publications.

3.6 – R:FR ratio influences plant height and flower form. — Paradiso, R., & Proietti, S. (2022). Light-quality manipulation to control plant growth and photomorphogenesis in greenhouse horticulture. Journal of Plant Growth Regulation, 41(2), 742–780.

3.7 – Far-red encourages elongation and can delay flowering. — Sumitomo, K., et al. (2012). Spectral sensitivity of the extension growth of tulips grown with night lighting under a natural photoperiod. Japan Agricultural Research Quarterly: JARQ, 46(1), 95–103.

3.8 – Blue light regulates circadian rhythms and plant architecture. — Su, Q., et al. (2025). Advancing light-mediated technology in plant growth and development: The role of blue light. Horticulturae, 11(7), 795.

3.9 – Dahlias do not respond to low-intensity blue night breaks. — SharathKumar, M., et al. (2024). Several short-day species can flower under blue-extended long days, but this response is not universal. Scientia Horticulturae, 325, 112657.

3.10 – LEDs allow precise spectrum control. — Paradiso, R., & Proietti, S. (2022). Light-quality manipulation to control plant growth and photomorphogenesis in greenhouse horticulture. Journal of Plant Growth Regulation, 41(2), 742–780.

3.11 – 10–15 SD cycles advance harvest by up to two weeks. — Lopez, R. G., & Brown, J. (2025). Secrets for early dahlia cut flower harvests. GrowerTalks.

3.12 – Continuous short days reduce stem yield and promote tuber formation. — Lopez, R. G., & Brown, J. (2025). Secrets for early dahlia cut flower harvests. GrowerTalks.

3.13 – Extended short days increase tuber initiation but reduce floral output. — 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.

3.14 – Incandescent bulbs waste energy and cause elongation. — Lopez, R. G., & Currey, C. (2014). Managing photoperiodic lighting. GrowerTalks, March 2014, 36–40.

3.15 – LEDs are energy-efficient and long-lived. — Meng, Q., & Runkle, E. S. (2014). Controlling flowering of photoperiodic ornamental crops with LED lamps. HortTechnology, 24(6), 702–711.

3.16 – Cyclic NI lighting reduces energy use with minimal delay. — Lopez, R. G., & Currey, C. (2014). Managing photoperiodic lighting. GrowerTalks, March 2014, 36–40.

3.17 – General photoperiod principles (Juvenility). — Jackson, S. D. (2009). Plant responses to photoperiod. New Phytologist, 181(3), 517–531.

Cyclic NI lighting — Energy-saving method using brief light pulses at night to delay flowering.

Daily Light Integral (DLI) — Total photons a plant receives daily; governs sugar production.

Day-extension (DE) lighting — Supplemental evening light extending daylength for growth.

Juvenility — A biological phase in a young plant’s life during which it cannot yet flower, regardless of environmental conditions.

Kelvin rating (K) — Bulb color scale: 3,000–3,500 K red-heavy; 5,000–6,500 K blue-heavy.

R:FR ratio — Balance of red to far-red light influencing elongation, form, and bloom timing.

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.