Timing Is Everything: How Temperature and Day Length Affect Dahlia Growth and Tuber Formation

Every autumn, dahlia growers ask the same question : When is the best time to dig up tubers?

While some swear by the "120-day rule"—suggesting that dahlias need at least 120 days of growth to form viable tubers—there’s more to the story beneath the soil. A study from Michigan State University provides fascinating insights into what really happens during those final weeks of the growing season.

Researchers investigated how temperature and photoperiod (day length) affect dahlia growth and development, using controlled growth chambers to isolate the effects of temperature and light.

Here’s what they found—and what it means for your dahlias.

The study explored two key environmental factors: temperature and photoperiod. Both have significant effects on flowering, tuber formation, and overall plant growth.

• Tuber Formation: Cooler temperatures between 59°F and 68°F (15°C to 20°C) are ideal for developing tuberous roots. At higher temperatures (77°F or above), tuber formation decreases significantly, regardless of day length.
  
  

• Flowering: Moderate temperatures of 72°F to 76°F (22.4°C to 24.4°C) promote faster flower development, while higher temperatures increase the number of stem nodes (segments) before flowering and may cause flower buds to abort.

• Short Days for Tuber Growth: A photoperiod of 10–12 hours encourages tuber formation, while longer days (16+ hours) inhibit it.
  
  

• Flowers Under Any Light: Photoperiod had little impact on flowering unless paired with high temperatures, where flower development was reduced or absent.

This seed-grown dahlia produced a large, healthy tuber clump in its first growing season.

The researchers conducted two experiments to determine how temperature and photoperiod interact:

• Temperature and Growth: Plants were exposed to 25 combinations of day and night temperatures (50°F to 86°F or 10°C to 30°C). Key growth indicators—stem length, node count, and flower bud development—were recorded.
  
  

• Temperature and Photoperiod: Plants were subjected to varying day lengths (10–24 hours) at four constant temperatures (59°F, 68°F, 77°F, and 86°F). Researchers measured tuber weight, shoot count and length, and flower development.

The findings can help dahlia growers optimize their plants for different goals, whether they’re aiming for blooms, tubers, or both.

If your primary goal is healthy, viable tubers that will store well:

• Short Days Are Key: Less than 12 hours of light promotes tuber development.
  
  

• Keep It Cool: Aim for temperatures between 59°F and 68°F (15°C to 20°C) during the last weeks of the growing season.

In practical terms, this means waiting until the days shorten naturally in autumn before digging your tubers. Allowing plants to experience these shorter days and cooler nights helps maximize tuber development.

If your focus is on vibrant blooms:

• Moderate Temperatures Are Best: Dahlias flower fastest at 72°F to 76°F (22.4°C to 24.4°C).
  
  

• Day Length Doesn’t Matter (Mostly): Dahlias will flower under various light conditions, provided temperatures aren’t too high.

This explains why dahlias can thrive in a variety of climates, as long as extreme heat is avoided.

The author with a freshly-lifted tuber clump from Dahlia ‘Junkyard Dog’

The “120-day rule” assumes a fixed time-frame for tuber maturity, but this study shows that environmental conditions play a bigger role. 

If your dahlias experience consistent short days and cool temperatures, tuber formation may occur more quickly. Conversely, long days and warm nights could delay or even inhibit tuber growth, regardless of how long the plants have been growing.

Instead of focusing solely on the calendar, watch for natural cues:

• Shorter daylight hours.
  
  

• Cooler nighttime temperatures.
  
  

• The onset of yellowing or browning foliage, signaling that the plant is entering dormancy.

These signs indicate that the tubers are maturing and ready for digging and storage. If your garden experiences freezing ground during winter months, you must always plan on digging your tuber clumps before the deep frost reaches them.

• Timing Is Key: Let your dahlias experience shorter days and cooler temperatures before digging to encourage full tuber formation.
  
  

• Avoid Heat Stress: Protect dahlias from high temperatures late in the season, as this can reduce both flower and tuber development.
  
  

• Monitor Growth Stages: Keep an eye on flower and foliage behavior to gauge when the plants have entered the tuber maturation phase.

For commercial growers and home gardeners alike, understanding how environmental factors influence dahlia growth can help optimize both bloom production and tuber storage.

Whether you’re growing dahlias for a colorful display in your home garden, or focused on producing healthy tubers for resale, tailoring your care to match the plant’s needs will produce better results.

Brøndum, Jens J., and Royal D. Heins. " Modeling temperature and photoperiod effects on growth and development of dahlia. " Journal of the American Society for Horticultural Science 118 (1993): 36-36.

This article was written with the help of AI (ChatGPT) to organize research and summarize key findings from scientific sources. While I carefully reviewed the content to ensure that all statements are backed by specific research papers, I rely on AI’s interpretation of the technical material. This is a collaborative effort—AI helps with analysis and structure, while I make sure everything aligns with the intended discussion.