What causes weed plants to flower?
Marijuana flowering is caused by phytochrome, a hormone produced by the plant beginning at germination. Cannabis regulates flowering based on exposure to uninterrupted darkness which causes phytochrome to build to a critical level, which in turns triggers flowering.
How to start flowering in weed plants
In the wild, marijuana is triggered to flower in late summer to fall to coincide with the change in the sun.
Sativa strains which orginate in areas near the equator flower for longer periods of time (up to 10-12 weeks and even more for some strains) than shorter, bushier indica plants that typically come from more mountainous regions.
As an indoor grower you can trigger this reaction in both indica and sativa strains easily by switching your lights to 12-12 to start flowering, which requires 12 hours of light and 12 hours of interrupted darkness.
Some growers such as DJ Short – the creator of the classic Blueberry strain – use an 11-hour on, 13-hour off cycle on equatorial sativa strains. He does this to bring out the pheneotypical expressions in those strains, which in their natural environment flower much longer timeframe then shorter indica-hybrids used by most of us today.
We have tried this ourselves and had mixed results. The AK-47 and Gorilla Glue plants produced great buds – very potent and flavorful compared to previous runs, blanketed with trichomes. As good as we’ve ever gotten them. But in that same tent some Blissful Wizard, a very high THC strain with a strange stumpy indica structure, came out less developed and dense although still very flavorful and blanked white as usual with this cut. A mixed result. But it will save you 8% on your light bill which is nice. I’m still running 11-on and 13-off on subsequent grows for AK and GG#4 so I’ve been converted for a few strains.
Why does flowering require uninterrupted darkness?
Even a few seconds of light can destroy phytochrome and threaten the flowering process. Be careful to block all light leaks in your grow tent or grow room, and avoid entering your room during the dark period.
If you do need to enter during dark, use a green filter on your flashlight or get a green lightbulb specially designed for this purpose. Green filters prevent light from harming your flowering weed plants.
The worst thing a new grower can do is to not fully respect the need for total darkness during the flowering cycle. Even the dim light on a power strip switch or a small crack of light through a door or grow tent zipper could be problematic. Make sure to fully light-proof your grow room to provide the optimal environment to flower your weed plants.
Growers who don’t provide 12 hours of uninterrupted darkness during the entire flowering phase risk having their female plants become hermaphrodites that develop seeds.
When should marijuana plants be forced to flower?
Switching from 18-6 for vegetation (18 hours light and 6 of darkness) to 12-12 can be done at any point of marijuana’s lifespan. Most growers choose to switch to flowering 12-12 when their plants are half the desired final height since plants can grow from 20-50% taller during the first few weeks after the switch. This growth is known as the flowering stretch.
The longer you keep plants in vegetation the larger they will get. Growers using a “sea of green” method will grow many small plants with one large main cola each, these are flowered within a few weeks of veg. Most legal home growers live in states where there are limits to overall plant count, so for many it is preferable to grow just a few larger plants covering a similar area to stay within the legal limit.
There is a diminishing return on indoor marijuana vegetation in terms of plant height, since light penetration can only go so deep into the canopy. Growing an eight foot tall plant indoors won’t necessarily have double the bud yield of a four foot plant of the same width. The light can only penetrate so much, and any buds growing in the shade of the lower canopy will be wispy and of low quality compared to the buds at the top in direct light.
Outdoors however the light intensity and angled movement of the sun allows for better light penetration throughout, which is why outdoor plants yield much greater amounts of weed per plant.
Once your plants are two weeks from the end of flowering you should flush them with lots of plain water (at least 3x water than the size of the containers). Then you should determine when to harvest your plants, and then dry and cure them properly.
Do larger weed plants have more potent buds?
The mature flowers of genetically identical weed plants are equally potent regardless of age, meaning that a 3 week and a 8 week old plant cloned from the same parent would have the same quality buds with identical highs.
With an 8 week plant trained to grow wide and bushy, you’ll have more yield than on a smaller plant with less bud sites.
Marijuana flowering can begin as soon as clones or seeds have established their roots, as early as a week. While possible – and even beneficial if you have incredibly short heights in your grow room or tent – plants made to flower this young will not produce the same total harvest as plants with a bit more veg time.