Cannabis Light Periods - What do I need to know about marijuana light cycles? (length of sunlight hours each day)

by Nebula Haze

If you're growing a cannabis plant grown from a random seed ("bagseed"), unless you somehow have an auto-flowering seed, you will need to understand about cannabis life stages and how they are affected by light periods.

If you don't understand light periods, your plant may never start making buds! The light schedule experienced by your plant will actually change its life stage. Learn more...

Cannbis plants have two life stages.

The first stage, "Vegetative" begins when they first sprout, at the beginning of their life.

Most indoor growers give their cannabis plants 18-24 hours of light a day during the vegetative stage. 

Outdoor growers plant their seeds in Spring when the days are naturally longer. In the wild, cannabis seeds naturally germinate in the Spring.

For an indoor grower, when a plant is about half the final size you want it to be, you should change it over to the "Flowering" stage. This is the stage when your plant starts growing buds.

You do this by changing your light so that it only shines for 12 hours a day, and the other 12 hours a day your marijuana plants are kept in TOTAL darkness. 

After 2-3 weeks of the 12-12 light schedule, most cannabis plants will show the first signs of their gender (they either are a female plant which starts growing buds, YAY! or they are a male plant which start growing balls/pollen sacs, NO!).

Boy cannabis plants don't give you any usable amounts of THC, so most growers toss them on sight. These male plants can also impregnate (pollinate) your female plants, which causes your female plants to produce seeds and less buds.

So unless you're planning on breeding, it's important that most growers destroy male plants as soon as you notice them growing grape-like balls where their buds would normally be.

Unfortunately, about 50% of all regular (unfeminized) cannabis seeds are male. Fortunately for small growers, you can purchase feminized (all-female) seeds so you don't have to worry about male plants if you don't want to. Learn more about buying seeds.

When does a cannabis plant start budding?

Marijuana plants have an internal process that allows them to detect how long they receive darkness each night. This is because they are a "photo-period" plant, specifically a "short-day" plant which means these plants start making flowers/buds when days start getting short.

In the wild, as the days get shorter and nights grow longer, a marijuana plant "realizes" that winter is coming and will start budding/flowering. It "knows" it's approaching the end of its life cycle so it frantically starts making buds in time before winter.

When growing marijuana outdoors, a grower doesn't need to do anything to induce flowering because the sun will take care of things on its own. It's just important to make sure that there are no lights shining on your plants during their night period (which will disrupt their dark cyle).

However, when growing weed indoors, a marijuana gardener will have to fool their plants into "thinking" winter is coming to induce flowering and kickstart the creation of buds.

This is done by changing the plant's light schedule to 12-12, where the weed plants gets 12 hours of light a day and 12 hours of total darkness.

You'll get the best results if the start and end time for the light is exactly the same each day, which is why most growers end up getting a timer to flip their lights on and off, like an automatice light switch.

I tend to set my timer to shine line from 8pm-8am. This gives me time to check on my plants at night when the lights first come on, and I can also check them quickly in the morning before I go to work. It also keeps things cooler since the lights go on at night.

But ANY 12 hour period will work, as long as you remain consistent.

Check out my cannabis grow light guide for more info about picking out suitable lights!

Photoperiod dependent strains vs. auto-flowering strains

So all strains of cannabis that respond to light in this way (where the light period effects what stage they're in) are called "Photoperiod dependent" strains.

"Auto-flowering" marijuana strains pretty much ignore how much light they get each day. Generally you don't run into these unless you buy them particularly from a cannabis seed bank.

Learn more about the different between photoperiod & auto-flowering cannabis seeds

Jump to...

The Basics of Growing Weed

10-Step Quick Start Grow Guide

Teach Me About Cannabis Nutrients

7 Steps to Fixing 99% of Growing Problems