A question from a fellow grower:
my confusion is determining when to start the flowering stage. I have a couple in the veg stage now and want to ensure I move to flowering at the right time
It is mostly about size. You don’t want plants outgrowing the grow room or growing into the lights. You start the flowering cycle when they are a little bigger than half the total size you want them to be when finished, as they will grow at least another third their size through the flowering cycle…
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Great advice. Only thing I have to add is; I have seen and been advised of plants that stretch 2-3 times their size at bloom induction. I find this directly proportional to the size and strain of individual plants. As well as the methods being employed in any individual grow. Intensity of light; As mentioned above is key in maintaining a short tight canopy.
So; I like to induce flower stage, when plant is 35-45% of the space between the top of my plants canopy, and the bottom of the lamp/s. 
Hope this helps, Peace/lw
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Another answer could be; Depends on how big you want your plants to get. (Again; I find this directly proportional with how long you keep your plants in a vegetative state); If you have unlimited height in your grow room; You could basically flower anytime you want.
As a general rule: Some growers will tell you that they use X amount of weeks before they flower. i.e. 6 - 8 weeks for Indica; 8-12 weeks for Sativa. 
In some cases; Advanced plant training, topping techniques, etc… may or, need to be employed. Plan ahead
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Sativia’s, generally, will grow much more after switching to 12/12, so that is a factor. Going by height is a great rule-of-thumb, as opposed to some sort of timetable. Cannabis babies can’t read (yet) so they grow as they darn well please.
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Welcome back Jodie!
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Thank you, Sir.
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Question from a fellow grower:
It has always been a mystery to me on how tall ( in inches ) do plants get before they are able to be switched into the bloom stage ?
Or how tall would you recommend that plants be before they are able to be flowered?
For my veg room I am working with a 2x2ft space for the small cutting and seeds, with a 37watt florescent,
My bloom room I am working with a 3x3x5 ft space with a 300watt led grow light
I would say there are two factors to be considered here. One is, is the plant showing pre-flowers? And two, the height of your grow space. The second, naturally, minus the space needed for your lights, and space below them to ensure no liight burn occurs. This gives you your maximum height. Now, you can go 12/12 from seeding, and the plant will flower, but it will still give itself a short veg, prehaps three to four weeks. But, you yeild will be terribly small. Once the pre-flowers show, the plant is old enough to support flowering. In considering what height to go 12/12, you must know the strain you are growing. If you don’t know it by experience, the seed bank you purchase from will have the characteristics listed. If they don’t, I wouldn’t buy from them. Roughly, an Indica, or Indica dominant strain, will stretch less during the intial 21 days of flowering than a Sativa, or Sativa dominant strain. Many of the later will double, or treble thier height in this time. Indica’s much less, maybe 50 to 75% of thier height going in. Lastly, always be prepared to train your flowering Cannabis. LST, Supercropping, SCroGing, all will help you both control your height, and inrease yeild. There is an abundance of info on all on the WEB. Just Google.
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What Jodie stated is quite concise. There is very good information there. However it might depend on how you are measuring the size differences or changes. And an untrained sativa outdoors is going to grow a lot differently than a well trained one indoors.
In general most people will start to flower their plant when it has filled up about half the growing space, as most plants will continue to only grow about another one third its size after flowering has been initiated. So expecting it to double its size, instead of the likely only 1/3 more growth, gives you plenty of extra room, just in case, in most instances.
I have never heard nor seen plants that actually more than double their overall size during flowering, but then it depends on how you are measuring this “double”. What might be double to one person might be only another half to another person, I suppose. Double or triple in pure height in a completely untrained sativa, I can imagine, especially outdoors. I personally have never seen it, but I could see how it could be possible in an instance similar to this.
Just don’t fear doing something wrong. Just do it! Pay attention to what you did, and you will learn. As good as any teacher might be, your own experience is the best teacher. (I was thinking about pure Sativa’s with the doubling comment, but I guess those strains are rather rare nowadays).
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Yeah, even with seeds from a bag of Mexican schwag, you might not be getting pure sativas anymore, as many growers may have been mixing in what ever they can to try and get their crops’ THC higher, and there is no control of cross pollination at all, one farmer might pull all his males, but it does no good when the farmer down the road isn’t pulling any of his males, so the cheap street stuff is totally mixed and a crap shoot if you try and grow out the seeds. But growing anything is better than growing nothing.
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One more thought, you can put them under more intense light as soon as you want to encourage aggressive growth, just acclimate them slowly to the more intense light (keep the light far away and bring it closer slowly) and keep the light at about 16 to 24 hours of light a day to keep them from flowering until they are bout half the size of the room.
Before i started topping my plants they would just about double. This using bag seed. My first two weeks of 12/12 i kept the MH bulb in, then switched to HPS for the final. Kept the plants a foot smaller/shorter, but bud was smaller too, just as strong though. I have 6 vertical feet to grow in. On top of a 5 gal bucket that leaves me just over 4 feet to manage plants and still leave room for my lights. I top now when i have 24 inches of growth, and flower when they grow to be “wide enough” for my preference. I can grow wide, but not tall. Along with topping the top, i do the same with the top three nodes, then hang small fishing weights to droop the top branches opening up the canopy, when im ready to flower the plant is a 3 foot circle. I flower till they stink then test, flower till theyre as hot as i want then flush, they live in the dark for their last week, no water, essentially curing on the plant. All the leaves drop, the ones left are white and trim easy, and the bud’s half dry. Done its a 4 foot plant. Thats how i manage growing in just 6 feet of height. Outside i just letem go, had a 16’ plant last year, a male, but damn… That was a tall plant, came from the same seeds i grew inside… 5 feet up to 16 feet, all from seeds from the same mother plant. The plant will do what you tell it too, when you tell it to, as long as it gets what IT needs.
Advice… Practice “topping” on sucker growth while its still living on the plant. Youll see what to cut, and what happens after you cut and how the plant adjusts. Last cycle topping and carefull cutting turned several end buds from a single 3 inch bud, to two 2.5 inch budS, that seems like more bud, a full half ounce more compared to the plant next to it that i just topped the tip of the plant once in fact. Hope this helped.
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