When Do Cannabis Plants Start Flowering? What Triggers Bloom

Photoperiod cannabis plants start flowering when they get roughly 12 hours of uninterrupted darkness a day. Indoors, that means switching your lights to 12/12; outdoors, it happens naturally as the nights get longer after midsummer. Every stage of the grow matters, but flowering is the home stretch, when your buds are almost ready to harvest.

In simple terms, the flowering stage is when your weed plants start producing aromatic flowers that, handled right, you'll get to harvest, dry, cure and finally smoke.

That doesn't mean growth stops. At first, your plants keep growing as they start to flower, but at some point that growth stalls and they pour all their energy into building big, resinous flowers. How long it takes depends on a lot of factors, but flowering usually lasts 7 to 9 weeks, though some sativa strains can take a good deal longer.

When does a cannabis plant flower?

A cannabis grow cycle normally wraps up with flowering, and flowering kicks in when the hours of light and darkness the plant gets change. Lots of people think it happens when the plant gets fewer hours of light, but what really matters is how many hours of darkness it gets.

How long does it take weed to start flowering?

To flower properly, cannabis needs about 12 hours of uninterrupted darkness. During the day (while it's getting light), the plant carries out photosynthesis, turning light into energy and releasing oxygen. When night falls, the process shifts and the plant starts producing a hormone called florigen, which is what triggers flowering. Once your plants start getting longer stretches of uninterrupted darkness, they'll produce more florigen and get pushed into flowering.

The three stages of cannabis flowering

• Early flower (weeks 1 to 3): The plant keeps growing and females start developing the famous pre-flowers, along with pistils, the white hairs that mark the start of the buds.
• Mid flower (weeks 4 to 5): This is when the plant stops growing taller and the buds start bulking up.
• Late flower (week 6 on): Trichome density ramps up and the plants get loaded with resin. Now's the time to watch the color of the trichomes to know when to harvest.

The nutrients plants need most at this stage are phosphorus and potassium (PK).

Flowering indoors vs. outdoors

If you've ever grown indoors, you already know that plants start flowering the moment you switch the light schedule to 12 hours on and 12 hours off. The only real difference with outdoor flowering is that outdoors your plants start flowering with the change of season, as the days get much shorter and the nights longer.

Once your female plants make the switch, you'll notice they don't start producing flowers right away. As we said, they keep growing for a couple of weeks, depending on the strain, while they transition into flowering. That's called pre-flowering, or the flowering stretch. Some plants can even double in height indoors, especially if they hadn't grown much in veg.

Keep your seeds' genetics in mind, since each strain has its own flowering timeline. All photoperiod strains follow the same general pattern, but flowering time can vary, especially once you factor in the pre-flowering stretch.

What happens if you interrupt the dark period?

It might not look like it, but cannabis plants are living things that can get stressed, so it's fair to wonder what happens if the dark period gets interrupted while they're flowering. Stress becomes part of the plant's life, and because of that your weed could end up re-vegging: it stops producing the hormones it needs to flower and goes back to vegetative growth.

In extreme cases, the stress can turn your plant into a hermaphrodite, meaning it produces both male and female flowers, and those male flowers end up pollinating the females, leaving you with seedy buds.

What about autoflowers?

Autoflowering seeds are the exception to the rule, since they don't go by how much light or darkness they get but by how long they've been growing.

Autos can flower with fewer hours of darkness, and indoors you can even keep them at 18 hours of light or more if you like. As a general rule, an autoflower starts flowering around 4 to 6 weeks in.

Tips for the cannabis flowering stage

• Flushing: Flush the roots with pH-balanced plain water for the two weeks before harvest.
• Humidity: Keep it at around 30 to 40% at most to avoid pest and mold problems.
• Bud maturity: During those two weeks of flushing, keep a close eye on your plants. That way you'll know when the buds are ready to harvest.
• 48 to 72 hours of darkness: If you want to try boosting potency, some growers leave their plants in total darkness for 48 to 72 hours straight before harvest. Plenty of people claim this trick helps plants produce more trichomes and stronger weed, though the evidence is mostly anecdotal.