Photoperiod
Grow light schedule
Choose what you are growing and how it is lit. You get the hours on and off, the times to set on a plug-in timer, and a check that the day's light adds up to enough.
Timer settings
On 06:00 6:00 amOff 22:00 10:00 pm
16 h on, 8 h off. Within the 14–16 h suggested for this stage.
Iowa State Extension suggests 14–16 hours a day for seedlings, on a timer.
DLI check at 150 µmol/m²/s
8.6 mol/m²/day
In the target band for seedlings (5–10 mol/m²/day).
- Hours for the band at this PPFD
- 9.3–18.5 h
- PPFD for the band in 16 h
- 87–174 µmol/m²/s
Setting the timer
Pick the plant, then the stage or setting; the suggested hours fill in and you can change them. Enter when the light comes on to get the off time. If the light runs past midnight, the off time is marked "next day". The DLI check multiplies your light's PPFD by the hours. If the total falls short, longer days help only up to the suggested limit; after that the light needs to be brighter or closer.
Short-day and long-day plants
Some plants time their flowering by day length. Poinsettia and chrysanthemum are short-day plants: Missouri Extension says the long days usual under lamps keep them from flowering, and about 10 hours of light a day brings them into bloom. Wisconsin Extension adds that poinsettias need at least 14 hours of uninterrupted darkness each night. Spinach goes the other way: UC IPM says long days make it bolt, running to flower instead of leaf.
Sources
- Iowa State University Extension, How do I set up supplemental lights for my seedlings?: lights on a timer for 14–16 hours a day.
- Beck S. Grow Your Own Microgreens. Utah State University Extension, 2025 (updated 2026): 18 hours on and 6 off, and the covered days.
- Trinklein DH. Lighting Indoor Houseplants. University of Missouri Extension G6515: 16–18 hours with no outdoor light, 12–14 with some window light, about 10 for short-day plants, and the timer.
- University of Wisconsin–Madison Extension, Poinsettias Enhance Holidays & Beyond: at least 14 hours of uninterrupted darkness nightly.
- UC Statewide IPM Program, Bolting: spinach bolts in long days.
- DLI and target bands: Virginia Cooperative Extension SPES-720NP (Stallknecht 2025), Table 3, and Purdue Extension HO-238-W (Torres & Lopez), Table 2.
Questions
Should grow lights stay on 24 hours a day?
None of the sources here suggest it. The longest schedules are 18 hours on and 6 off for microgreens (Utah State) and 16–18 hours for plants with no window light (Missouri). For short-day plants, long days stop flowering.
Does it matter when the light comes on?
Not for the day's total. If the plants also get window light, Missouri Extension suggests running the lamp while the window is bright, and a timer so it switches at the same times every day.
How long do microgreens stay in the dark?
Utah State Extension keeps trays covered for 1–5 days for most crops, longer for basil (4–7) and coriander (7–14), before the lights go on. The microgreens calculator lists the covered days for each crop.