Duration is the second part of light: how many hours a plant receives each day. Near the equator, daytime lasts about 12 hours all year. At higher latitudes it swings widely, and indoor light often drops a lot in winter because days are shorter and the sun sits lower.
Longer hours can partly make up for weak light. University of Illinois Extension notes that plants in low light benefit from 14 to 16 hours of light, but advises not going beyond 16 hours because plants need a rest period too.
Where windows are not enough, a grow light helps. LED and fluorescent lamps both work, and LEDs run cool and last a long time. Mississippi State University Extension suggests about 12 to 14 hours of light a day with plants 1 to 3 feet from the lamp. A simple plug-in timer keeps the hours steady.
Cool white lamps suit foliage plants; flowering plants do better under specialised grow lights. Whatever you use, keep it close: lamp light, like window light, fades quickly with distance.
Key points
- Equatorial days are about 12 hours all year; temperate winters are much shorter.
- In low light, 14–16 hours can help, but plants need a dark rest.
- Grow lights work best close to the plant and on a steady schedule.
Try this
If you use a lamp for plants, measure how far it is from the leaves and how many hours it runs each day, and write both down.
Check yourself
Read more in the reference
Sources
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