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Sprout & Spectrum

Grow Light Hours Per Day, by Plant

What the extension services actually recommend, why the dark period is not optional, and what each extra hour costs you.

By Scooter M. — An enthusiast who's genuinely into this. I read the manuals, compile the published specs, and do the math. No lab coat.

Published · How we pick

A digital plug-in timer connected to a wall socket

Photographs on this site are licensed stock and are illustrative of the fixture type. We do not own the units we write about.

How this page is funded: the buy links here are Amazon Associates links, and we earn a commission on qualifying purchases at no extra cost to you. We have not been paid to feature anything on this page, we accept no free product, and commission plays no part in the ranking. Full disclosure.

We have not tested these units. Everything below comes from the manufacturer’s own published specifications, or from arithmetic we have shown our working for. Where a figure is not published, we say so instead of estimating one. Read the method.

Quick picks

Efficacy is photons out per joule in. Where a manufacturer publishes it we quote theirs; where they publish photon output and wattage but not efficacy, we divide and label it as ours; where they publish neither, the cell says so rather than guessing. Tap a row for the full write-up.

#ProductBest forEfficacyPrice
1
A gooseneck clip-on grow light clamped to a shelf beside a small houseplant
GooingTop Clip Grow Light
The cheapest way to put real light on one plant on one desk, and honest about being exactly that — a 10 W clip lamp with a repeating timer.
Built-in repeating timerNot published
2
A thin aluminum LED grow board above a row of potted houseplants
AC Infinity IONBOARD S22
The highest published efficacy of any fixture here at 2.7 umol/J, on Samsung LM301H diodes, with dimming in real 20% steps rather than a vague knob.
Dim and schedule2.7 umol/J
3
Linked LED strip lights mounted under a wire shelf above trays of seedlings
Barrina T5 4ft Grow Light Strips
The default seed-starting bar for good reasons — 4 ft long, linkable in series, cheap per foot — and the reason we built a coverage calculator, because Barrina publishes no photon data whatsoever.
One timer, eight stripsNot published

The reasoning

Photoperiod is the cheapest lever you have. Adding two hours to a fixture’s run time raises the daily light integral by the same proportion as buying a substantially larger light, and costs a few dollars a season instead of a purchase. It is also the setting most people never touch after installing a timer once.

What the published guidance says

Three extension services give figures for indoor lighting, and they overlap rather than agree exactly — which is worth stating plainly rather than averaging away.

Published photoperiod guidance, quoted from each source
SourceGuidanceApplies to
University of Minnesota Extension16 to 18 hours a daySeedlings
Iowa State University Extension12 to 14 hours generally; 10 hours as a floor, 16 as a ceiling for home growersIndoor plants under supplemental light
University of New Hampshire ExtensionAbout 12 mol/m²/day as a target, which sets the hours once you know your intensitySeedlings under lights

The reconciliation is straightforward: seedlings want the top of the range, established foliage plants want the middle, and every plant wants a genuine dark period.

Practical settings by plant

What we would set a timer to, and why
PlantHoursReasoning
Seedlings and transplants14–16Highest daily-total demand of anything a home grower runs; extension guidance clusters at the top of the range
Leafy herbs and salad greens14–16Growth rate is the whole point; long photoperiods suit them
Microgreens12–16A short crop; the hours mostly decide how fast you harvest
Foliage houseplants10–12Supplementing rather than replacing daylight; more than this is usually wasted
Succulents and cacti12–14High intensity matters more than long hours for these
Winter supplementation of a windowsill6–10You are topping up daylight, not substituting for it; run it around the natural day

The dark period is not optional

Running a light 24 hours a day is a mistake people make in good faith, reasoning that more light means more growth. Plants carry out processes during darkness that do not happen in light, and continuous illumination is a stressor rather than a bonus. It also doubles your electricity cost against a 12-hour schedule for no gain.

Consistency matters too: plants respond to the length and regularity of the photoperiod, so a timer beats remembering. The cheapest mechanical timer works. Some fixtures include one — the GooingTop clip light has a repeating 4, 8 and 12-hour cycle built in.

What an extra hour costs

The price of two more hours a day, for a season

A 100 W fixture, run for an extra 2 hours a day: 100 × 2 ÷ 1000 = 0.2 kWh a day.

Over a five-month winter (about 150 days): 0.2 × 150 = 30 kWh.

At the US residential average of 18.34 cents per kWh: $5.50 for the season.

Those two hours are a 14% to 20% increase in daily light integral depending on your baseline. There is no hardware purchase that buys that much extra light for $5.50.

Price your own schedule in the running-cost calculator, and see how hours convert into daily light integral in PPFD targets by plant type.

When to run them

For plants that get no daylight, the schedule is arbitrary — pick hours that suit you and keep them consistent. Running overnight is worth considering if your utility has time-of-use pricing, and it also keeps a bright fixture out of your eyes during the evening.

For supplementing a window, extend the natural day rather than overlapping it: an hour or two before sunrise and after sunset adds more than running at midday, when the window is already delivering.

Every pick in full

A gooseneck clip-on grow light clamped to a shelf beside a small houseplant
01

GooingTop Clip Grow Light

Clip light

Built-in repeating timer

The cheapest way to put real light on one plant on one desk, and honest about being exactly that — a 10 W clip lamp with a repeating timer.

What GooingTop publishes

Power draw
About 10 W (source)
Spectrum
6000 K, 74 white plus 10 red LEDs, CRI 95 (source)
Dimming
Five brightness levels; repeating 4 / 8 / 12 hour timer (source)

What GooingTop does not publish

  • PPF in umol/s
  • PPFD at any distance
  • Photon efficacy in umol/J
  • The manufacturer publishes no spec sheet at all; the figures above come from the retail listing

Who should skip it

Skip it if the plant is more than about a foot across, or if you want any number at all before you buy — nobody publishes photon data for this fixture.

A thin aluminum LED grow board above a row of potted houseplants
02

AC Infinity IONBOARD S22

LED panel

Dim and schedule

The highest published efficacy of any fixture here at 2.7 umol/J, on Samsung LM301H diodes, with dimming in real 20% steps rather than a vague knob.

What AC Infinity publishes

Power draw
115 W (source)
Efficacy
2.7 umol/J (source)
PPFD
1098 PPFD (no measurement distance published) (source)
Stated coverage
2 x 2 ft (source)
Spectrum
Samsung LM301H, plus 660 nm, 730 nm, 3000 K, 6500 K (source)
Diodes
272 diodes (source)
Dimming
Yes, 0-100% in 20% increments (source)
Dimensions
30.0 x 30.0 cm board, 1.1 cm thick (source)
Rated life
50,000 hours (source)

Our arithmetic

  • Photon output implied by the published wattage and efficacy: about 311 umol/s
    115 W x 2.7 umol/J = 310.5 umol/s. AC Infinity does not print PPF directly, so this is our arithmetic from two figures it does print.

What AC Infinity does not publish

  • PPF in umol/s — only efficacy is given, so photon output has to be derived
  • The measurement distance for the 1098 PPFD figure, without which the number cannot be compared to anything

Who should skip it

Skip it if you want to place it by the numbers. AC Infinity prints 1098 PPFD with no measurement distance attached, which makes that figure impossible to use or compare.

Linked LED strip lights mounted under a wire shelf above trays of seedlings
03

Barrina T5 4ft Grow Light Strips

LED bar

One timer, eight strips

The default seed-starting bar for good reasons — 4 ft long, linkable in series, cheap per foot — and the reason we built a coverage calculator, because Barrina publishes no photon data whatsoever.

What Barrina publishes

Power draw
20 W per fixture; an 8-pack draws 160 W (source)
Spectrum
5000 K (source)
Dimming
No; on/off switch per fixture, linkable up to 8 in series (source)
Dimensions
4 ft strip, 120-degree beam angle, 120 V AC (source)

What Barrina does not publish

  • PPF in umol/s — for any strip in the range
  • PPFD at any stated distance
  • Photon efficacy in umol/J
  • LED count per strip, and rated lifespan

Who should skip it

Skip it if you need to know what you are getting before it arrives. There is no published PPF, no PPFD and no efficacy figure, so nothing about its output can be verified in advance.

Questions people actually ask

+How many hours a day should a grow light be on?
Between about 10 and 16 for nearly everything a home grower keeps. University of Minnesota Extension recommends 16 to 18 hours for seedlings; Iowa State University Extension suggests 12 to 14 generally, with 10 as a floor and 16 as a ceiling. Foliage houseplants sit at the low end, seedlings and greens at the high end.
+Can I leave a grow light on 24 hours a day?
You should not. Plants carry out processes in darkness that do not happen in light, so continuous illumination is a stressor rather than a bonus, and it doubles your electricity cost against a 12-hour schedule for no gain.
+Do I need a timer?
Practically, yes. Plants respond to consistent photoperiods and no human is consistent over a five-month winter. A basic mechanical timer costs very little and removes the variable entirely.
+Is it better to run a light for longer or make it brighter?
For daily light integral the two are interchangeable, and hours are almost always cheaper: two extra hours a day on a 100 W fixture runs about $5.50 over a five-month season. Intensity still matters separately, because some plants need a minimum level rather than just a daily total.

Sources

Read next

Where this sits

This page belongs to Coverage and Sizing Math. Every figure on it is either quoted from a named published source or derived in view — the full method is on our methodology page, and how the site is funded is on the affiliate disclosure.