Inputs
Example data table
| Scenario | Sunrise | Sunset | Twilight (min) | Supplement (h) | Blackout (h) | Effective photoperiod (h) |
|---|---|---|---|---|---|---|
| Spring baseline | 06:15 | 18:05 | 20 | 0.00 | 0.00 | 12.83 |
| Extend for long-day crops | 06:10 | 17:50 | 15 | 2.00 | 0.00 | 13.75 |
| Induce short-day flowering | 06:30 | 18:10 | 10 | 0.00 | 2.00 | 10.83 |
Values are rounded. Use your local sunrise and sunset times.
Formula used
The effective photoperiod estimates total daily light exposure hours:
- Daylight is the time between sunrise and sunset.
- Twilight adds early and late low-angle light.
- Supplemental light extends exposure using lamps.
- Blackout/shade reduces exposure to shorten days.
How to use this calculator
- Enter your local sunrise and sunset times for the target date.
- Set twilight minutes to match your light sensitivity goals.
- Add supplemental lighting times if you use grow lights.
- Enter blackout hours if you cover plants to shorten days.
- Select plant response type and adjust the threshold if needed.
- Calculate, then apply changes gradually over several days.
For flowering crops, prioritize consistent dark periods. Even brief nighttime light can delay short-day flowering.
Article
Photoperiod as a production lever
Photoperiod is the daily duration of usable light that plants perceive. In controlled gardens and greenhouses, it is a practical lever for steering flowering, vegetative growth, and uniformity across batches. This calculator converts sunrise, sunset, twilight, supplemental lighting, and blackout into one effective photoperiod value, making planning easier and more consistent.
Interpreting effective hours for crop response
Many ornamentals and herbs respond when daylength crosses a cultivar-specific threshold. Long-day plants generally flower when effective hours meet or exceed the threshold, while short-day plants prefer shorter effective hours and long, uninterrupted nights. Day-neutral plants are less sensitive and usually require attention to temperature, nutrition, and maturity instead.
Twilight, intensity, and why minutes matter
Twilight adds low-angle light that can still influence plant signaling, especially in clear conditions or under reflective surfaces. Because intensity and spectrum vary, treat twilight as an adjustable planning factor. Use conservative values when working with blackout schedules for short-day crops, where even brief nighttime light can reduce flowering reliability.
Supplement lighting and blackout scheduling
Supplemental lighting is most effective when applied consistently at the same time window each day. For long-day targets, extending mornings often reduces heat stress compared with late-night lighting. For short-day induction, blackout should create stable darkness; prioritize covering before stray light events and maintain good ventilation to avoid humidity spikes.
Example dataset for quick validation
Use the following example to verify your setup and confirm that inputs match expected outcomes.
| Date context | Sunrise | Sunset | Twilight | Supplement | Blackout | Effective |
|---|---|---|---|---|---|---|
| Baseline spring day | 06:15 | 18:05 | 20 min × 2 | 0.00 h | 0.00 h | 12.83 h |
| Long-day push | 06:10 | 17:50 | 15 min × 2 | 05:00–07:00 | 0.00 h | 13.75 h |
| Short-day induction | 06:30 | 18:10 | 10 min × 2 | 0.00 h | 2.00 h | 10.83 h |
Operational tip: adjust schedules in small steps (15–30 minutes per day) and record plant response for at least 7–10 days before changing again.
FAQs
1) What does “effective photoperiod” mean?
It is the total daily light exposure time after adding twilight and supplemental lighting and subtracting blackout or shading hours. It helps standardize scheduling across changing natural daylength.
2) Why should I include twilight minutes?
Twilight can influence plant signaling even when the sun is below the horizon. Adding a configurable amount lets you plan conservatively, especially for flowering control where marginal light can matter.
3) My supplemental light crosses midnight. Will it work?
Yes. The calculator supports time windows that pass midnight by treating durations as wrapping across 24 hours. Consistency is still critical for predictable crop responses.
4) How do I choose a threshold value?
Start with supplier or cultivar guidance, then refine based on observed flowering or growth. Many crops fall between 10 and 14 hours, but cultivar differences can be significant.
5) Can blackout replace short-day “night length” control?
Blackout can be effective, but it must be light-tight and consistent. Small light leaks or interrupted darkness can delay short-day flowering, so prioritize reliable covers and stable timing.
6) Does the calculator account for light intensity or spectrum?
No. It estimates duration only. If intensity is low or spectrum is unsuitable, plants may not respond as expected even when hours are correct. Use appropriate fixtures and measure light when possible.
7) How often should I update sunrise and sunset times?
For precise planning, update weekly or whenever daylength changes quickly in your season. In greenhouses with strong supplemental lighting, monthly updates may be sufficient for stable programs.