Minimum hours by light strength
Find your light’s PPFD at the hanging distance (from its chart), then read across to your plant. Times are the minimum to reach the low end of each plant’s range.
| Plant (minimum DLI) | 100 µmol/m²/s | 150 µmol/m²/s | 200 µmol/m²/s | 300 µmol/m²/s | 400 µmol/m²/s |
|---|---|---|---|---|---|
| Low-light houseplants (DLI 3) | 8 h 20 m | 5 h 33 m | 4 h 10 m | 2 h 47 m | 2 h 05 m |
| Medium-light houseplants (DLI 6) | over 16 h | 11 h 07 m | 8 h 20 m | 5 h 33 m | 4 h 10 m |
| African violets and moth orchids (DLI 6) | over 16 h | 11 h 07 m | 8 h 20 m | 5 h 33 m | 4 h 10 m |
| Microgreens (DLI 9) | over 16 h | over 16 h | 12 h 30 m | 8 h 20 m | 6 h 15 m |
| Vegetable and flower seedlings (DLI 10) | over 16 h | over 16 h | 13 h 53 m | 9 h 16 m | 6 h 57 m |
| Succulents and cacti (DLI 12) | over 16 h | over 16 h | over 16 h | 11 h 07 m | 8 h 20 m |
| Herbs and leafy greens (DLI 12) | over 16 h | over 16 h | over 16 h | 11 h 07 m | 8 h 20 m |
| Tomatoes, peppers and other fruiting plants (DLI 18) | over 16 h | over 16 h | over 16 h | over 16 h | 12 h 30 m |
Why not leave the light on all day?
Plants need a dark period. Iowa State University Extension recommends 12–14 hours of light a day for most indoor plants, with a range of 10 to 16 hours. University of Illinois Extension also advises no more than 16 hours. Constant light wastes electricity and can stress many plants.
Tips for a simple routine
- Use a plug-in timer. Plants do better with a regular schedule, and you won’t have to remember.
- Match your daytime. Running the light from, say, 7 a.m. to 9 p.m. gives 14 hours and keeps it off at night.
- Count window light too. A plant near a bright window in summer may need fewer grow-light hours than in winter.
- Raise the light, not the hours, if leaves bleach. Pale or crispy patches near the top mean too much intensity.
Work out the exact hours for your light and plant