How Much Light Does A Weed Plant Need?

Sophia Delphi May 14, 2022 - 8 min read
Fact Checked
Hands Cutting Hemp Plant

We hear so much about hydration that you might think that water is the key to life. Needless to say, water is vitally important. But one thing contributes even more to our existence: light.

Without sunlight, all life would quickly die. It’s true that light plays a huge role in most human metabolic functions, as well as the cycles of the oceans and the effects of gravity. But even more crucial is the fact that plants would be unable to produce the oxygen we breathe, without the sunlight that triggers photosynthesis. 

The importance of light to cannabis plants obviously scores lower on the “crucial” scale, of course. Even so, those who grow their own weed need to understand the effect that light – or the lack of it – has on the success of their grow.

When you plant tomatoes or cucumbers in your backyard, you don’t have to think much about light. As long as you put them somewhere where they’ll get lots of sun, they’ll grow just fine without any intervention on your part.

Marijuana plants are much pickier than that, particularly when you have an indoor grow room. Not only do they require a certain amount of light to thrive, but that amount also changes regularly.

Exactly how much light does a weed plant need? Glad you asked.

The Growth Stages Of Cannabis Plants

Weed plants do best with different amounts of light during each stage of their growth. Before we get into the details, it’s important to first understand each of those growth stages.


You probably know what happens here: a seed sprouts and begins to grow. The simplest way to start a cannabis seed is to place it between wet paper towels, making sure each day that the towels are still saturated with water, and wait for the seed to split. At that point, a sprout (that will eventually turn into the plant’s stem) will pop out. That’s the sign to plant the seed in a pot filled with soil. Soon afterward, you’ll see the plant’s first leaves. (Obviously, you don’t have to worry about the germination stage if you’re starting with clones instead of seeds.)

The typical length of the germination stage: 3-10 days.


A tender seedling begins showing the first signs of becoming a full-fledged plant in this stage. New leaves will have five or seven blades apiece (the very first leaves will have just one blade). Watering isn’t as important as vigilance in this stage; the young plant is still extremely susceptible to mold and disease.

The typical length of the seedling stage: 2-3 weeks.


After the fan leaves emerge with a full complement of blades, the vegetative stage begins and growth occurs rapidly. The plants will require lots of water and nutrients as they grow tall, and they’ll eventually need to be trained or topped. (Males should be removed from the grow as soon as the plants display their sex organs.) The length of this stage varies greatly depending on the plant’s strain.

The typical length of the vegetative stage: 3-8 weeks, but it can last as long as 16 weeks depending on the strain.


This is when your marijuana plant begins producing real marijuana. Pre-flowers (distinguished by their white hairs) appears, followed by the buds. As the flowering stage continues, the plant will stop growing but the buds will grow bigger, eventually showing signs that they’re maturing with the appearance of sticky trichomes. When those white hairs (they’re the flowers’ pistils) turn dark and curl up, it’s time to harvest.

The typical length of the flowering stage: 8-11 weeks but some strains, mostly sativas, will take a little longer.

All of these time frames are approximate. If you extend the vegetative stage, for example, the plants are likely to grow bigger and produce a bigger crop. You can also vegetation short if you started the plants late and you have a short growing season.

That’s right. You’re able to determine when the flowering stage will start. How?

It’s all done with light.

The Amount Of Light A Weed Plant Needs

The reason that light is so important to marijuana plants is that the vast majority are so-called “photoperiod” plants. That means they take their cues to switch between growth stages from the amount of light they receive. You can control those cues, to a large degree, by simply changing the total hours of uninterrupted light (and darkness) that they’re exposed to.

Needless to say, this information all pertains to plants grown indoors, or in a growhouse where you can provide supplemental light (or ensure darkness). As of this writing, outdoor weed growers have no power to control the sun.

Cannabis plants have very different light needs in each stage of their growth. Here’s a brief summary.

  • Germination Stage: The plants should be kept away from light sources as much as possible; they’re most likely to germinate in near-complete darkness.
  • Seedling Stage: The fragile young plants do best with 16 hours of light per day, and eight consecutive hours of darkness.
  • Vegetative Stage: These plants need at least 14 hours of uninterrupted light each day, and they prefer 18 hours. Growers commonly approach this stage in two different ways. Most put their plants on an 18/6 schedule, meaning 18 hours of light followed by…well, you get the picture. Some cultivators, however, provide 24 hours of light per day in the vegetative stage, in order to force their plants to grow faster. Once the plants are exposed to 12 hours of darkness each day, they’ll begin to flower.
  • Flowering Stage: 12 hours of light and 12 hours of darkness per day will put the plants into the flowering stage and keep them there.

You can use those numbers as guidelines, but don’t stray far from them and try to stay to a dependable schedule. The right amount of light (and darkness) at the same time each day will ensure that your plants grow properly and produce the crop you’re hoping for.

What Equipment Do I Need To Control Lighting Conditions?

Well, you’ll definitely need grow lights for an indoor crop, and there are four types of lights used for cannabis grows.

  • High-intensity discharge (HID) lights are the most popular, since they’re most likely to produce large, high-quality yields. There are two types of HID bulbs, metal halide (best for the vegetative stage) and high-pressure sodium (better for the flowering stage). The light fixtures are somewhat expensive, get quite hot and use a lot of electricity, but the bulbs are quite reasonably-priced.
  • LED grow lights are even more expensive but run very cool and are energy-efficient; some let you choose different light spectrums for different growth stages. LEDs also produce excellent crops.
  • Compact fluorescent lamps (CFLs) are much like the fluorescent light fixtures you’re familiar with, and they’re much less expensive to buy and run. They’re much better for the vegetative stage than for flowering plants; you can change to warm white bulbs for flowering, but your yield will suffer compared to one of the first two types of lighting.
  • Light-emitting ceramic (LEC) grow lights are also called ceramic metal halide (CMH) lights. Cannabis plants love the light they emit, but they’re quite expensive and their heat and UV rays aren’t kind to humans, so you have to be careful around them. They’re less common than the other types of lighting fixtures.

If you’re growing in a room where ambient light can get in through the windows, you’ll also need blackout shades in order to completely control the environment.

What If You’re Growing Outdoors?

You have two choices.

As we’ve mentioned, most strains are photoperiod varieties that depend on light conditions to tell them when to begin flowering. They’ll do quite well outdoors because they respond “the right way” when the days get shorter and there’s less light.

However, you can also find “auto-flowering” strains that enter the flowering stage automatically when they’re a few weeks old, regardless of how many hours of light they’re exposed to. Autoflowering plants won’t produce as much weed as photoperiod ones, and their growth cycle is much shorter. But for obvious reasons, they’re much easier to grow in outdoor climates where the amount of sunlight is unpredictable, and creating supplemental light (or blocking sunlight) is next to impossible.

How Much Light Does A Weed Plant Need? FAQ

Q: Does the intensity of the light that the plants receive matter?
A: It can make a big difference. Once you’ve navigated your maiden growing voyage and are ready to commit to the fine details, plants in the vegetative stage need between 15,000 and 70,000 lux, with 40,000 lux the ideal intensity. For the flowering stage, more intense light is better, with 60,000 lux the goal to shoot for. It shouldn’t go higher than 85,000 lux or lower than 35,000.

Q: How many lights will I need?
A: There are all sorts of complicated formulas and equations you can use to figure that out. But generally speaking, 400-600 watts per 10 square feet of grow room will be more than enough light.

Q: Can I grow auto-flowering strains indoors?
A: Absolutely, and they can lessen the “strain” of having to frequently adjust light conditions. They’ll still need light, but you can either go with an 18 hour on/6 hour off schedule or 24 hours of light per day. The plants will start to flower automatically, whichever schedule you choose.