Optimizing nighttime LED lighting to accelerate flowering of long-day plants
According to the American Floral Endowment, growers often face the challenge of promoting early flowering in long-day plants during seasons with short natural daylight hours. A recent study by Dr. Qingwu Meng and Thomas J. Kramer from the University of Delaware, supported by AFE, offers practical insights into how different types and durations of nighttime LED lighting can enhance flowering in crops like snapdragons and petunias. By adjusting lighting strategies, growers can reduce production cycles, save costs and improve plant quality.
Key Findings
The study revealed that the spectrum of light plays a significant role in influencing flowering in long-day plants. Snapdragons and petunias showed distinct responses to warm-white (WW) and red + far-red (R+FR) LEDs. Snapdragons flowered up to 28 percent earlier under R+FR LEDs due to the intermediate far-red (FR) fraction, which is critical for floral promotion in this species. In contrast, WW LEDs were less effective in triggering early flowering in snapdragons. Petunias, on the other hand, responded positively to both light types, although flowering was consistently faster under R+FR LEDs compared to WW LEDs.
The duration of nighttime lighting also significantly impacted flowering time. For both snapdragons and petunias, increasing the lighting duration from four to 16 hours progressively reduced the time to flower. Snapdragons benefited most from longer lighting durations under R+FR LEDs, while in petunias, the gap in flowering times between the two light types narrowed as the lighting duration increased. This suggests that prolonged WW lighting can enhance its efficacy, making it a viable option for petunias.
In addition to flowering time, the study also explored morphological changes induced by different lighting strategies. Snapdragons exhibited reduced plant height when exposed to longer lighting durations under R+FR LEDs, while WW LEDs did not significantly affect their height. Petunias displayed contrasting growth patterns under the two light types: R+FR LEDs encouraged stem elongation, while WW LEDs resulted in more compact growth and increased lateral branching. Additionally, both light types reduced leaf and lateral branch numbers, with R+FR LEDs having a more pronounced effect.
Economic considerations also emerged as an important aspect of the findings. While R+FR LEDs are optimal for achieving rapid flowering in FR-sensitive crops like snapdragons, they are significantly more expensive than WW LEDs. However, by extending the duration of WW LED lighting to 12–16 hours, growers can achieve comparable results for less FR-sensitive crops like petunias. This provides a cost-effective alternative for optimizing flowering without the need for higher-cost specialty lighting.
Key Takeaways
Time to flower (top row A, B and C), main stem length (middle row D, E and F) and leaf number at flowering (bottom row G, H and I) for snapdragon ‘Liberty Classic Yellow’, petunia ‘Easy Wave Burgundy Star,’ and petunia ‘Wave Purple Improved.’ Plants were grown under 8-hour natural short days with or without nighttime lighting from warm-white or red + far-red LEDs, which operated for 0, 4, 8, 12 or 16 hours each night. Data represent averages from two experimental trials and 20 plants per treatment. Letters indicate statistical differences between treatments.
- Match Lighting to Crop Sensitivity:
- Use R+FR LEDs for crops like snapdragons that require an intermediate FR fraction for early flowering.
- For petunias, prolonged WW LED lighting can be sufficient to promote flowering without additional costs.
- Adjust Lighting Duration:
- Increase lighting duration to 12–16 hours for WW LEDs to maximize their efficacy in promoting flowering.
- For R+FR LEDs, shorter durations (four to eight hours) may suffice for certain petunia varieties, reducing energy use.
- Consider Growth Characteristics:
- Choose WW LEDs if compact growth and increased branching are desired.
- Opt for R+FR LEDs to encourage stem elongation where appropriate.
- Energy and Cost Efficiency:
- Nighttime photoperiodic lighting requires minimal energy compared to supplemental lighting. Balancing lighting strategies with crop needs can reduce production costs while maintaining quality.
By tailoring nighttime lighting strategies, growers can take advantage of the specific responses of long-day plants to different LED types and durations. These findings provide a roadmap for optimizing greenhouse operations, ensuring faster flowering and higher productivity throughout the year. Click here to read the full study.
