Why Your Dahlia Blooms "Blow Open": The Role of Light

 Part One in the “The Dahlia Clock” Series

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

A common frustration for any dahlia grower is watching a promising bloom suddenly lose its form. The petals spread too far, the center is exposed, and the flower is instantly "blown." Whether you're aiming for a blue ribbon or just a beautiful bouquet, a blown center means premature failure and a shorter life for the flower.

For years, many of us, myself included, simply chalked this up to bad luck, poor cultivar choice, or an error in our care routine. Yet, the true answer lies not in watering cans or fertilizer schedules, but in photoperiodism.

These premature failures are rarely random; they are often the predictable result of how dahlias interpret light—specifically, the shift in daylength during late summer and fall (1.1).

It is late September, and across gardens in the Pacific Northwest (USA) and beyond, dahlia petals are beginning to litter the paths as plants race to form seed before the first killing frost. With about 13 hours between sunrise and dusk, the dahlias are experiencing 11 hours of darkness—a night that is growing longer by about four minutes each evening.

This subtle measurement of the darkening season is the "dahlia clock" in action.

Most gardeners assume plants respond to the amount of daylight they receive. Dahlias, however, defy this logic.

Like many other ornamentals, they actually read the length of the night. Dahlias are facultative short-day plants, meaning they flower faster when nights are longer, though they will eventually flower even under long days (1.2).

The critical thing is that as summer turns to fall, nights lengthen. Dahlias perceive these changes and adjust their development accordingly—which explains why blooms initiated in mid-July might still look perfect in early September, while those initiated in late August may rush, open poorly, or blow centers quickly (1.3).

As horticulturist Leslie Halleck explained, “What we call short-day plants are actually responding to the length of uninterrupted darkness.” (Lamp’l 2018)

This principle reminds us that the dark period is being read by the plant as critical information, not just a time for rest.

Once I understood the mechanism, the connection was fascinating: the fate of a September bloom is determined weeks earlier—at the moment the bud first begins to form. By the time a grower sees a flower with a blown center, the underlying cause is already set in the past, when that bud was initiated. Dahlias integrate light signals over time, meaning August conditions often dictate September quality (1.4).

This mechanism is known as photoperiodic memory. The plant essentially remembers the light it received, locking in a developmental path. For instance, experiments show that just five days of short-day exposure are enough for a dahlia to lock in a lower ray-floret count. Once that signal is received, the development process carries it forward regardless of future changes in daylength.

Conversely, consistent long days (12 hours or more) are required to secure the development of fully double flowers (1.4).

This is why it's common to see both perfect blooms and blown ones on the very same plant at the same time: each flower bud has its own developmental clock based on the specific light cues it received when it was initiated.

I see this mechanism play out dramatically every September when I host my annual weekend garden tour. In the days leading up to the two-day event, I meticulously deadhead every spent or fading bloom.

On Saturday morning, every plant is in full bloom, and the foliage and stems are sturdy and lush looking. Yet, by Sunday afternoon, the paths are again littered with dropped petals, sagging blooms, and spent buds. In just two days, the effect of the light changes accelerates, demonstrating how quickly the plants respond.

A perennial topic in large dahlia grower groups online is the mystery of buds that stubbornly refuse to open. Growers frequently post photos of these stalled buds, which often look similar to spent flowers that have closed after pollination to form seeds.

The common, though often incorrect, diagnosis is that the gardener simply missed seeing the bloom open and close.

While it's certainly possible to miss an open flower, for many vigilant gardeners who are patiently awaiting their first blooms, that explanation feels like a stretch.

The true answer is more straightforward: those dahlias probably produced blind buds. These are flower structures that, under the plant's current environmental conditions, are structurally incapable of opening (1.8).

It's not just the hours of complete darkness that count; twilight—the dim light at dawn and dusk—also acts as a crucial signal. Research on model plants like Arabidopsis shows that twilight duration can shift a plant's internal clock, a sensitivity dahlias almost certainly share (1.5).

This is particularly relevant in places with long summer days, such as the Pacific Northwest. Here, the lingering dawn and long twilights effectively stretch the apparent daylength.

Even after the sun dips below the horizon, the residual glow can keep dahlias reading “daytime” longer than the clock on the wall suggests.

This extended perceived daylength helps explain why bloom timing and development often differ for growers in various regions.

Therefore, twilight is a key component in the Dahlia Clock, influencing the plant's final decision on whether it is in a “long-day” or “short-day” environment.

Another layer in the Dahlia Clock is energy competition. Flowers, being powerful carbohydrate sinks, aggressively pull sugars and nutrients from the plant.

This energy competition can quickly turn into a resource tug-of-war late in the season. As the photoperiod signals a shift toward tuber formation, the balance tilts, and flowers lose the steady supply of energy needed to hold their form (1.6). This is a primary reason why September blooms are especially prone to blown centers.

I learned this lesson one year while lifting tubers in the rainy Pacific Northwest in November. I found that some plants had poured nearly all their resources into their root systems, leaving the late-season blooms thin and weak.

In reality, the flowers hadn’t “failed” at all; they had simply been sacrificed so the plant could prioritize storing energy underground. This confirmed that what I saw above ground was only half the story.

While some differences are genetic—with certain cultivars naturally holding their form longer—growers can significantly influence outcomes. Recognizing that bloom quality is tied to earlier light cues is the key.

This knowledge allows growers to strategically time practices like disbudding, pinching, and planting to dramatically improve the odds of better, longer-lasting flowers (1.7).

In my own garden, I’ve found that staggering planting dates and adjusting disbudding schedules can significantly spread out peak bloom quality. Instead of risking a single, large September flush that is prone to blowing, this strategy yields a more reliable sequence of strong blooms from August through October.

While those late-season blooms may never quite match the early ones for vase life, my garden remains a riot of color and a haven for pollinators until almost Halloween.

When you see a dahlia bloom fall apart, don’t blame the flower or bad luck. It’s simply the Dahlia Clock at work, responding to night length, twilight, and resource allocation.

Once you learn to read that clock, you gain the power to anticipate bloom quality weeks ahead and strategically adjust your growing practices.

1.1 – Blown centers reflect how dahlias interpret light cues. — 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.

1.2 – Dahlias are facultative short-day plants. — Proietti, S., Scariot, V., De Pascale, S., & Paradiso, R. (2022). Flowering mechanisms and environmental stimuli for flower transition. Plants, 11(3), 432.

1.3 – Bloom initiation timing influences bloom quality. — Konishi, K., & Inaba, K. (1966). Studies on flowering control of dahlia. III. Effects of day-length on initiation and development of flower bud. Journal of the Japanese Society for Horticultural Science, 35(1), 73–79.

1.4 – Flower fate is sealed weeks earlier. — Konishi, K., & Inaba, K. (1964). Studies on flowering control of dahlia. I. On optimum day-length. Journal of the Japanese Society for Horticultural Science, 33(2), 171–180.

1.5 – Twilight alters plant clocks. — Mehta, D., Scandola, S., Kennedy, C., Lummer, C., Gallo, M. C. R., Grubb, L. E., … & Uhrig, R. G. (2024). Twilight length alters growth and flowering time in Arabidopsis via LHY/CCA1. Science Advances, 10(26), eadl3199.

1.6 – Flowers and tubers compete as carbohydrate sinks. — 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.

1.7 – Cultivar and cultural practices influence blown centers. — Okada, M., & Harada, H. (1955). Effects of day-length and photoperiod on ratio of ray-flowers to disk-flowers in dahlia flower heads. Journal of the Japanese Society for Horticultural Science, 23(4), 259–263.

1.8 – Blind buds. — 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.

Blown center — A dahlia bloom whose petals spread too far, revealing the central disk prematurely, often shortening the bloom’s useful life.

Carbohydrate sink — An organ that draws sugars and resources, such as flowers or tubers. Blooms and tubers compete as sinks in dahlias.

Facultative short-day plant — A plant that flowers faster under short days but can eventually flower under long days.

Photoperiod — The length of the light and dark periods in a 24-hour cycle. Dahlias use night length to regulate flowering.

Twilight — The low-light period at dawn and dusk, which acts as an environmental signal influencing plant clocks.

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.