How Long Should Grow Lights Stay On? A Practical Schedule by Plant Type

Use this practical, plant-by-plant guide to set grow light hours (photoperiod) for seedlings, microgreens, herbs, houseplants, and short-day bloomers—plus an easy way to adjust when your light is weak or you’re mixing in

TL;DR

Most house plants do well with 12–16 hours total light per day (natural + grow light). Avoid running lights 24/7.

Seedlings generally prefer 16–18 hours; microgreens often like 14–16 hours (some guides recommend up to 18).

Flower timing depends on whether your plant is a photoperiod bloomer: short-day bloomers like the poinsettia need long unbroken night hours to re-bloom.

If your light is weak, you may need longer “on” hours to hit enough daily light—distance and intensity matter, too.

Grow light timing sounds straightforward—pick a number of hours and set a timer—but plants respond to the combination of (1) light duration at the plant, (2) light intensity at the plant, and (3) a consistent period of darkness.

The right schedule for your light will also vary according to whether you’re growing leafy greens, starting seedlings, keeping houseplants happy, or if you’re trying to trigger flowering.

Safety note: use a high-quality timer and avoid all electrical connections from water (this is especially important around hydroponics, etc., etc.) If your fixture gets hot, leave distance and circulation as per manufacturer instructions.

Quick grow light schedules by plant type (Get started with these)

The schedules below are practical starting points for use in production by home growers and are based on the assumption that you are using a timer and giving your plants a reliable period of night. If they also get light from a window, think of these as the “total hours of light per day” and reduce your grow-light hours accordingly

(extension.umn.edu)

Practical photoperiod (hours on/off) by plant type
Plant type / goal Starting schedule Notes (when to adjust)
Seedlings & vegetable starts (tomatoes, peppers, flowers before transplanting) 16–18 hours ON / 6–8 hours OFF Great for sturdy growth. If your light is weak or far away, you may need more hours (see the DLI section). (extension.umn.edu)
Microgreens (home trays) 14–16 hours ON / 8–10 hours OFF Some extension guidance calls ~15 hours ideal under a lamp; other guidance uses 18/6 for controlled setups. Try 15–16 first and adjust for legginess. (extension.oregonstate.edu)
Leafy greens & herbs (especially hydroponics lettuce/herbs) 12–14 hours ON / 10–12 hours OFF Often enough when intensity is adequate. If growth is slow, prioritize intensity/distance before pushing to very long days. (extension.umn.edu)
Foliage houseplants (pothos, philodendron, many ferns, etc.) 12–14 hours ON / 10–12 hours OFF Longer days can help in winter if light is low, but most plants still need darkness—avoid running lights all day and night. (extension.umn.edu)
Flowering houseplants (general/when you’re not intentionally forcing a short-day bloom) 14–16 hours ON / 8–10 hours OFF A common “boost” for flowering plants, but do not exceed ~16 total light hours for a lot of
houseplants.Short-day bloomers you want to re-flower (poinsettia, holiday cactus, kalanchoe, chrysanthemums).
About 10 hours LIGHT / 14 hours UNINTERRUPTED DARK (for weeks).
Darkness must be uninterrupted—stray room light can delay or stop flowering. (extension.colostate.edu)

Overwintering / maintenance (keeping plants alive, not pushing growth).

8–10 hours ON / 14–16 hours OFF.

Useful if you want slower growth or your plant is semi-dormant. Increase hours in spring if you want active growth.

The simplest way to choose the right number of hours

  1. Identify your plant goal: (a) vegetative growth (leaves), (b) fruiting/flowering, or (c) forcing a seasonal bloom (short-day plants).
  2. Pick a starting photoperiod from the table above and set it on a timer for consistency.
  3. Adjust based on results: if plants stretch (leggy), usually increase intensity/bring the light closer before adding more hours; if leaves bleach or curl upward, shorten hours and/or raise the light.

Your eyes are poor judges of brightness indoors. A spot that looks “bright” to you may be much dimmer than a plant needs—especially in winter. If you can measure light (even roughly), your schedule decisions get easier. (gardeningsolutions.ifas.ufl.edu)

Do grow lights need to be off at night?

They don’t have to match the sun (daytime only), but plants generally do better with a consistent light/d
ark rhythm than with random on/off times.The “night” period can happen at any clock time—many growers run lights overnight to keep temps a bit more stable or avoid peak electricity rates—just keep the schedule consistent.

Also, some plants require darkness to initiate flowering cues. For example, to get a new crop of bright red bracts, poinsettias require long, uninterrupted nights for weeks to trigger blooming; a little light-leak is enough to throw things off. (extension.colostate.edu)

Why “more hours” sometimes helps—and sometimes wastes money

Light duration is only one way to boost how much usable light your plant gets in a day. The complete metric is DLI (Daily Light Integral): total photosynthetically usable light delivered over a 24hr day. DLI goes up when you push intensity (PPFD), add hours, or both. (en.wikipedia.org)

This is the reason weak lights “need” long schedules. One extension example notes that if you are growing sun-loving seedlings under common T8 shop lights, they may need very long runtimes (say, 22hr) to get to an ideal DLI, while stronger horticultural LEDs may reach similar DLI levels in far fewer hours (at lower times during the runtime). (extension.unh.edu)

For many common houseplants, 16 hours of total daily light is about as far as you want to push it; when tempted to “go long”, it’s usually better to improve the intensity of the light you’re supplying (fixture quality, closer placement, reflective walls) vs. simply running super-totally all the time. (extension.umd.edu)

Photoperiod basics (so you don’t accidentally stop them flowering)

  • “short-day” (long-night) plants will bloom only if nights are long enough. Present them light and they bloom later or not at all. (extension.umn.edu)
  • “long-day” plants will bloom only if days are long (nights short)

    Some popular flowering houseplants are referred to as long day in extension guidance. (extension.umn.edu)
  • Day-neutral plants: flowering is less dependent on day length, but influenced by maturity/conditions (though light still affects vigor). (extension.umn.edu)

Practical timer examples you can copy

Example “ON” windows (choose one that matches your target hours)
Target hours Timer ON Timer OFF Works well for
12 hours 8:00 a.m. 8:00 p.m. Low-to-moderate light houseplants; supplementing a bright window
14 hours 7:00 a.m. 9:00 p.m. Most foliage houseplants; leafy greens/herbs when intensity is good
16 hours 6:00 a.m. 10:00 p.m. Seedlings; flowering houseplants (not short-day forcing) (extension.umn.edu)
10 hours light / 14 hours dark 8:00 a.m. 6:00 p.m. Short-day bloom forcing (dark must be uninterrupted) (extension.colostate.edu)

Troubleshooting: signs your lights are on too long (or not long enough)

What you see Most common cause What to try next
Seedlings stretch, fall over, or lean hard toward the lamp Too little light intensity (often distance is too great), sometimes too few hours Lower the light (safely), add reflective surfaces, then consider increasing hours toward 16–18 for seedlings (extension.unh.edu)
Leaves look washed-out, bleached, or develop crisp/brown patches near the top Too much intensity and/or heat; sometimes too many hours Raise the light, shorten the day by 1–2 hours, improve airflow, and confirm the fixture isn’t overheating the canopy
Slow growth, small leaves, weak color Low DLI (not enough total light over the day) Increase intensity first, then extend hours within your plant’s safe range; measure DLI if possible (canr.msu.edu)
A short-day plant won’t bloom (poinsettia, holiday cactus, kalanchoe) Dark period is too short or interrupted by stray light Give ~14 hours of uninterrupted darkness nightly for weeks (no room lights, streetlights, or quick checks) (extension.colostate.edu)

If you want to be more precise: use DLI (a better target than “hours”)

If you’ve ever thought, “I’m already running my grow light 16 hours—why are my plants still struggling?” the missing piece is often intensity.

DLI ties intensity and hours together, so you can stop guessing. (source)

  • In greenhouse production, DLIs below about 10 mol/m²/day can reduce quality and slow young plant production. (source)
  • Vegetable transplant guidelines commonly target higher DLIs for crops like tomato and pepper during propagation (example ranges are published in controlled-environment guidance). (source)
  • Hydroponic lettuce is often discussed in the ~15–20 mol/m²/day range in controlled-environment guidance (varies by cultivar and goals). (source)

Practical takeaway: If you’re already at ~14–16 hours and growth is still weak, don’t automatically add hours—improve intensity (closer placement, better fixture, reflective sides) so your plants get more light per hour.

Common mistakes that throw off your schedule:

  • Counting grow-light hours but forgetting window light (your “total light hours” may be longer than you think). (source)
  • Running lights extra-long to compensate for poor placement (distance matters; intensity drops quickly as the light moves away). (source)
  • Using the same schedule for short-day bloom forcing as for general growth (light leaks can ruin the dark period). (source)
  • Moving from dim to very bright light overnight (plants can
    Ignoring heat: longer runtimes can raise canopy temperature, which stresses the plants and can in some cases look like ‘light burn.’

FAQ

Q: Can I leave grow lights on 24 hours a day?

A: For most home plant situations, no. Many plants benefit from a dark period, and several extension resources recommend limiting lights to about 16 hours total a day for typical houseplant situations. If you’re using weak lights and trying to hit a target DLI, solve it with more intensity/placement help before going 24/7. (extension.umd.edu)

Q: How long should grow lights be on for seedlings?

A: A very common starting point is 16–18 hours on, 6–8 off. It’s also normal to adjust based on your specific fixture—some extension guidance notes that weak shop-light setups may take very long runtimes to reach a good DLI for sun-loving seedlings. (extension.umn.edu)

Q: How long should grow lights be on for houseplants in winter?

A: If you’re supplementing short winter days, many foliage houseplants do good in the area of 12–14 total light hours, while some flowering houseplants may do better a few hours more. Treat these as totals (window light + grow light) and make sure to keep a dark period daily. (extension.umn.edu)

Q: What schedule makes a poinsettia turn red again?

A: Poinsettia is a short-day plant. To trigger reblooming, common extension guidance suggests about 14 uninterrupted dark hours every night for weeks in fall, plus bright light during the daytime. Any stray light during the dark window can trigger later blooming. (extension.colostate.edu)

Q: Do microgreens really need long light hours?

A: Guidance varies based on whether you’re using a sunny window or a lamp. One extension resource suggests just keeping a lamp close during normal awake hours (the ideal is around 15 hours), while another describes an 18/6 light/dark cycle with grow lights on a timer. For most home trays, 14–16 hours under a properly placed light is a great start, then adjust if they get leggy. (extension.oregonstate.edu)

References

  1. University of Minnesota Extension: Lighting for indoor plants and starting seeds — https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants.
  2. University of Maryland Extension: Lighting for Indoor Plants — https://extension.umd.edu/resource/lighting-indoor-plants/.
  3. University of Maryland Extension: Low Light Impacts on Indoor Plants — https://extension.umd.edu/resource/low-light-impacts-indoor-plants/.
  4. University of New Hampshire Extension: Growing Seedlings Under Lights (Fact Sheet) — https://extension.unh.edu/resource/growing-seedlings-under-lights-fact-sheet.
  5. Cornell CALS: Starting Plants Indoors — https://cals.cornell.edu/school-integrative-plant-science/school-sections/horticulture-section/outreach-and-extension/pandemic-vegetable-gardening/pandemic-vegetable-gardening-2021-archive/starting-plants-indoors.
  6. Oregon State University Extension: Microgreens — https://extension.oregonstate.edu/imported-publication/microgreens.
  7. Utah State University Extension: Growing Seedlings Under Lights (Fact Sheet) — https://extension.usu.edu/yardandgarden/research/grow-your-own-microgreens.php.
  8. University of Florida IFAS: Light for Houseplants — https://gardeningsolutions.ifas.ufl.edu/plants/houseplants/light-for-houseplants/.
  9. Colorado State University Extension: Selecting and Planting Poinsettias — https://extension.colostate.edu/topic-areas/yard-garden/poinsettias-7-412/.
  10. Mississippi State University Extension: Selecting and Maintaining Poinsettias — https://extension.msstate.edu/publications/selecting-and-maintaining-poinsettias.
  11. Michigan State University Extension: Low daily light integrals impact young plant quality — https://www.canr.msu.edu/news/how_low_can_you_go_low_daily_light_integrals_impact_young_plant_quality_and.
  12. Purdue University Vegetable Crops Hotline: Managing Daily Light Integral to Improve Vegetable Transplant Quality — __URL_43