How much light do cannabis plants need?
A lot. Cannabis is a light-demanding crop, and it wants more as it grows: gentle light as a seedling, bright light across all the leaves while it builds its frame, and stronger light again once it flowers, when the weight is made. The right level also depends on your growing conditions and electricity costs.
In a controlled indoor trial, researchers kept raising the light and the flower weight kept rising with it, up to levels several times brighter than a typical home lamp delivers. That is why indoor growers spend so much on cannabis lighting.
More is not always better, though. Past what the leaves can use, extra light becomes heat and a bigger bill, a lamp too close bleaches the tops, and a very bright room needs more water and air. Our page on high light levels in the growing stage has the detail.
The number that says how much light a plant is getting is PPFD. It measures the light arriving at the leaves: how many usable light particles land on a square meter each second, where the plant is. Watts tell you what the lamp costs to run, not what reaches the canopy. Intensity and hours both matter, but flowering plants still need their uninterrupted night.
Light levels used in trials, alongside general seedling guidance, all measured at the top of the plants:
| Stage | Approximate PPFD (µmol/m²/s) | Usual hours a day |
|---|---|---|
| Seedlings | 100–300 | 18 |
| Rooted cuttings | about 200 | 18 |
| Growing (vegetative) | 600–900 | 18 |
| Flowering | 600–1,000 | 12 |
The vegetative and cutting figures come from a University of Guelph trial on one variety under LED light, where about 900 gave compact plants and about 600 more open ones, and the flowering range from a second Guelph trial. The seedling range has weaker support: it is what growers and the lamp trade commonly advise, and we did not find a published trial that sets one.
To know what your plants are getting, use a PAR (quantum) meter or a phone app calibrated to read PPFD. A lux meter measures brightness as human eyes see it, which is why it misleads for plants. Then adjust for your room: the trial plants were well fed and watered, and yours may want less, especially where it is hot or dry.
How many hours of light a day?
Eighteen hours while the plant is growing, twelve while it flowers. That is the standard indoor schedule, and the drop to twelve is the switch that tells the plant to flower.
Cannabis is a short-day plant: it flowers as the nights lengthen, and indoors you fake that by cutting the lamp’s hours. Keep it on a timer. A plant that gets light during its dark period, even from a gap in the tent, can stall in its flowering; growers also widely blame light leaks for stray male flowers, though that is grower experience rather than a settled finding. The exception is the autoflowering type, which flowers on age rather than day length and can stay on long days throughout. Our home-growing guide covers the timeline.
Cannabis lighting: which lamp should you use?
Three kinds of lamp will grow cannabis.
A fluorescent tube or compact fluorescent is the cheap, cool lamp that is fine for seedlings and cuttings but too dim to flower a plant well.
A high-pressure sodium lamp, usually called HPS, is the orange-glowing bulb that lit indoor cannabis for decades. It is cheap to buy, very bright and very hot, so it needs distance from the plants and a vented room.
An LED fixture is a panel of small diodes. A good one gives the same light as a sodium lamp for far less electricity and heat, which is why it has taken over. The catch is the price up front, and that a cheap panel can be no better than the sodium lamp it replaces.
For a first grow in a small space, a decent LED panel is the easy choice. For what each lamp does to yield and the bill, read our LED vs HPS comparison.
How many grow lights do I need?
As many as it takes to light the whole canopy evenly, which for most home growers is one.
Think in floor area, not plant count. The maker states the patch a fixture lights, and the number to use is the coverage for flowering, which is smaller than for seedlings. A small tent takes one panel; a bigger space needs a bigger fixture or several, because one lamp in the middle leaves the corners dim.
The running cost: take the fixture’s real power draw in watts, divide by 1,000 to get kilowatts, multiply by the hours a day, then by your price per kilowatt-hour. A 300-watt lamp on twelve hours a day uses 3.6 kilowatt-hours; at 25 cents each, that is 90 cents a day. Ignore “equivalent watts” on the box; the draw on the label is what you pay for.
How close should the lamp be?
Start at the hanging height and dimmer setting the maker gives for the stage your plants are in, then check two things: how much light reaches the top of the canopy, with a meter or app if you have one, and how the plants respond. Too low scorches or bleaches the tops; too high “to be safe” starves the plants and makes them stretch.
Watch the plants for a week after a change. Leaves curling at the edges or going pale at the top can indicate too much light or heat; long bare stem reaching upward can indicate too little. Both have other causes too. Raise the lamp as the plants grow, and keep a fan moving air across the tops. The old hand test, feeling for heat at the top of the plants, tells you whether a sodium lamp is cooking the leaves; it says nothing about the light level, and an LED can bleach the tops without feeling warm.
Can you grow cannabis with any light?
Ordinary household lighting is generally not enough for productive growth. A dim window gives a few hours of weak light a day, so windowsill plants stretch and flower poorly; a desk lamp or ceiling light is dimmer still.
The one light that beats a lamp is the sun, in the right place. How much a plant gets outdoors depends on the season, your latitude, the weather and what shades the spot, so pick the patch with the most direct sun and check what shades it in late summer, when the sun sits lower. A private, sunny spot and a long enough summer make the cheapest grow light there is.
The color of the light matters less than beginners fear. Plants use the whole visible range, not only the red and blue of the old purple lamps; white LEDs grow cannabis well. Kevin Frender’s piece on how light spectrum affects cannabis covers what each color does, and Jason Lupoi’s photobiology series explains why a plant reads color.
Outdoors, the season sets the hours
Outside, you do not set a timer; the year does. From spring to midsummer the days lengthen and the plant builds its frame, so plant out once the last frost has passed. After midsummer the days shorten, and that is the signal to flower: roughly late summer at mid-latitudes, harvest in autumn, depending on latitude and variety. A porch light or street lamp on the plants at night can interfere with flowering the way a tent leak does; that is mostly grower experience, but widely reported. Where autumn comes early, an autoflowering type suits best; it does not wait for shorter days.
Does lighting affect terpene production?
It can. In one indoor trial, flower grown under more light carried a higher concentration of terpenes, and in another, the color of the light shifted which terpenes went up. Both effects were modest and came from a handful of varieties, so take them as hints rather than promises. Whether light reaching the lower buds helps them is a separate question, the one behind lamps under the canopy; our subcanopy lighting page goes through the evidence.
Updated November 2026: this article was rewritten by the editors as a plain-language lighting guide for home growers, with the research moved to the linked pages. Originally published 6 February 2019 by Lance Griffin.
Image: Tobias Leznem, Pexels.
