Will a streetlight or a neighbour’s flood light stop my plant flowering?
A photoperiod plant counts the uninterrupted night, and it counts in far dimmer light than you would guess. Moonlight is nothing. A light you can read by is not.
By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed
The short answer
- Yes, if it is a photoperiod plant and the light is bright enough to read by. A flood light that lights your lawn well enough to cast shadows is many times brighter than a full moon at the plant. That is a night the plant does not count as night.
- No, if it is an autoflower. Autoflowers flower on age, not on night length. The plant in the same greenhouse that flowered regardless of the neighbour’s lights was an auto; the one that stalled was a photoperiod.
- Moonlight does not do it. A full moon is a fraction of a lux at the ground. A residential side street is around five; a main road around fifteen. A flood light aimed at your yard can be well past that. Scale matters, and the moon is at the bottom of it.
- What it costs is weeks. A photoperiod plant under a lit night keeps growing instead of flowering, or flowers late and loosely, and one already in flower can be pushed back toward growth. Outdoors, in September, weeks are the thing you do not have.
- Cover it at dusk, every night, until the light is gone. A tarp or blackout sheet over the plant or the greenhouse from dark until the light goes off. Growers who did this saw the stalled plant start within a week. Then do the arithmetic on the weeks left before frost.
What the plant is actually measuring
The shorthand is “twelve hours of light.” What the plant measures is the dark. Short-day plants time the length of the uninterrupted night with the phytochrome system in their leaves, and a pulse of light in the middle of that night resets the count: six hours of dark, a light, and six more hours is two short nights, not one long one. That is the mechanism behind every light-leak warning, and it is why a light that is on all night is the worst case — there is no night to count at all.
Cannabis is unusually precise about it. In a controlled study of hemp cultivars, some flowered on a day length of thirteen and three-quarter hours and not on fourteen, a fifteen-minute difference, and a fifteen-minute change in day length delayed flowering by five to thirteen days. The critical night length also varies by where a cultivar’s ancestors grew, so two plants under the same lit night can respond differently. The one thing that does not vary is the direction: a lit night reads as a longer day.
How much light is too much
Honestly: nobody has measured a cannabis threshold. There is no published study that put cannabis under a night light of a known brightness for a known number of minutes and watched for the delay. What exists is the physiology, some numbers from other short-day plants, and the illuminance tables from the light-pollution literature. Put together they say this:
- A full moon is 0.1 to 0.3 lux at the ground under a clear sky. Plants flowered outdoors for the whole history of the species under that, so it is the floor.
- Street lighting is 5 to 15 lux at street level — five on a residential side street, ten in a lit parking lot, fifteen on a main road. That is thirty to a hundred times the moon, and it is the range that the ecological literature records as disrupting photoperiod timing in plants and animals.
- A flood light aimed into a yard is above that. If you can read a page by it at the plant, it is well past any streetlight, and past anything a plant would ever have met at night.
- Colour matters, and the folklore is half right. The night-break response runs on red light through phytochrome. In the chrysanthemum work the interruption stopped flowering once its red-to-far-red ratio was moderate or higher, blue was not needed, and far-red alone did nothing. White LEDs, sodium streetlights and warm flood lights all carry plenty of red. A dim green headlamp for a late check is the safer choice because green excites phytochrome poorly — but green light is not invisible to a plant, so dim and brief are still the operative words.
- Duration is the unknown. The chrysanthemum interruptions were hours long, and the ecological work records effects at less than a lux held all night. Neither says a few seconds of phone torch is safe, and neither says it is harmful. A light on all night is not that question; it is the sure case.
What it does to the plant
- It does not flower, or flowers late. A photoperiod plant that never gets a long enough night stays in growth. Outdoors this looks like a tall, green, healthy plant in September with no buds while its neighbours are finishing.
- A flowering plant can be pulled back toward growth. New green leaf tips out of a bud, single-bladed leaves at the top of a cola, a plant that stretches again in week six — the pattern growers call foxtailing has more than one cause, and a lit night is one of them. Horticultural work on other species records both delayed flowering and renewed vegetative growth under night lighting.
- It can make a female produce pollen. An irregular or interrupted photoperiod is on the published list of stresses that induce anthers in female cannabis. That is the nanner, and seeds in the finished flower are its cost.
- An autoflower shrugs. Its flowering clock is age. It will not be as good as it would be with proper dark — the plant still photosynthesises on a schedule and rests on one — but it flowers.
How to check this yourself
- Stand at the plant after dark with your eyes adjusted for ten minutes. If you can see the plant clearly, it can see the light. If you can read text, the question is answered.
- Put a phone light meter on the leaf, facing the light. The free apps are rough, but they will tell a fraction of a lux from ten, and that is the whole distinction here. Moon: under one. Streetlight: five to fifteen. Flood light on the yard: more.
- Check the timing, not just the brightness. A light that comes on at midnight cuts a September night in half. A motion light that fires for two minutes when a cat walks past is a different and unmeasured case; a light that is on from dusk to dawn is the sure one.
- Check which kind of plant you have. An autoflower on a lit night is not the emergency; a photoperiod is.
- Cover it, then watch the tips. A tarp over the plant or the greenhouse from dusk to whenever the light goes off, every night, without a gap. A photoperiod plant that starts pushing white hairs within a week or two after the covering began has told you the light was the problem.
- Count the weeks left. An outdoor photoperiod that only began flowering in mid September needs its full flowering time from there, eight to ten weeks for most cultivars, which runs into November or later in most of the northern half of the United States. If that is past your first frost, the honest options are an early, light harvest, a covered and heated space, or a lesson for next year and an autoflower this one.
- Talk to the neighbour. Several people in the same thread had the light turned off by asking. It costs less than a tarp.
What would show that we have this wrong
If a known photoperiod plant, under a light bright enough to read by all night, flowers on the same schedule as an uncovered one in the dark, then the night-length rule is not holding for that cultivar and we would want to know its name. And if a covered plant does not begin flowering within two to three weeks of nightly covering in September, the light was not the only thing wrong: check the timer if it is indoors, and the plant’s age and origin if it is out.
Where our device fits, stated narrowly
Croplock is an indoor device, and outdoors under a neighbour’s flood light it has nothing to offer but this page. Inside a tent it holds the two facts this question turns on: it keeps a dated record of when its light channel was switched on and off, and its camera pod’s hourly look during the dark period is logged as a dark scene or a lit one. So “was the tent actually dark last week?” is a lookup, with dates, rather than a memory of a timer you set in July.
Sources
- Song, Y.H., Shim, J.S., Kinmonth-Schultz, H.A. & Imaizumi, T. (2015), Photoperiodic flowering: time measurement mechanisms in leaves, Annual Review of Plant Biology 66:441–464. DOI 10.1146/annurev-arplant-043014-115555 — short-day plants measure the uninterrupted dark period through phytochrome; a night break is a shorter night.
- Zhang, M., Anderson, S.L., Brym, Z.T. & Pearson, B.J. (2021), Photoperiodic Flowering Response of Essential Oil, Grain, and Fiber Hemp (Cannabis sativa L.) Cultivars, Frontiers in Plant Science 12:694153. DOI 10.3389/fpls.2021.694153 — critical photoperiods by cultivar; fifteen-minute sensitivity; five-to-thirteen-day delays.
- Zhang, Q., Chen, X., Guo, H. et al. (2018), Latitudinal Adaptation and Genetic Insights Into the Origins of Cannabis sativa L., Frontiers in Plant Science 9:1876. DOI 10.3389/fpls.2018.01876 — critical photoperiod varies with the latitude of origin.
- Craig, D.S. & Runkle, E.S. (2013), A Moderate to High Red to Far-red Light Ratio from Light-emitting Diodes Controls Flowering of Short-day Plants, Journal of the American Society for Horticultural Science 138(3):167–172. DOI 10.21273/jashs.138.3.167 — night interruption inhibited chrysanthemum flowering at a red:far-red ratio of about 0.66 and above; blue not needed; far-red alone ineffective.
- Gaston, K.J., Bennie, J., Davies, T.W. & Hopkins, J. (2013), The ecological impacts of nighttime light pollution: a mechanistic appraisal, Biological Reviews 88(4):912–927. DOI 10.1111/brv.12036 — the illuminance table (full moon 0.1–0.3 lux; side street 5, parking lot 10, main road 15 lux); short pulses of night light sufficient to disrupt photoperiodism in some species; night lighting producing delayed flowering and enhanced vegetative growth in horticultural work.
- Punja, Z.K. & Holmes, J.E. (2020), Hermaphroditism in Marijuana (Cannabis sativa L.) Inflorescences, Frontiers in Plant Science 11:718. DOI 10.3389/fpls.2020.00718; and Richards, C.D. et al. (2026), Hermaphroditism in Cannabis sativa L.: Impacts, Inducers, and Industry Implications, Plants 15(11):1643. DOI 10.3390/plants15111643 — irregular photoperiod among the stresses that induce anthers in female plants.
- Zhang, T., Maruhnich, S.A. & Folta, K.M. (2011), Green Light Induces Shade Avoidance Symptoms, Plant Physiology 157(3):1528–1536. DOI 10.1104/pp.111.180661 — green light is not inert to a plant.
- The reading-light test, the phone-meter scale, the dusk-to-dawn covering, the “starts within a week or two” expectation and the frost arithmetic are first-party practice and grower knowledge, not citations. No cannabis-specific study measures a night-light brightness or duration threshold; the foxtailing link is drawn from other short-day species and from grower reports, and is labelled as such.
More of these at Answers. The device is at croplock.com.
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