Timing is everything, and knowing when to start flowering marijuana plants can make or break an indoor harvest. This may seem remedial for many experienced growers, then again, I’ve seen plenty of “pro growers†with burnt plants growing into their lights.
First, let me say that there is no easy answer regarding when to start flowering indoor plants. There are a lot of variables including the strain, your grow room setup and your growing style. That being said, there is a lot I can share on the topic that will better equip you to evaluate your situation and help you decide when to begin flowering.
I should point out that the question of when to flower is usually measured by two metrics time and/or height. Let me explain.
Weeks of Veg = Worthless
Cannabis plants are often measured by their time in vegetative growth. For indoor growing this usually refers to the number of days or weeks that a plant has spent under 18 or 24 hours of light. You will often hear growers refer to plants as being in their 1st, 2nd or 3rd week of veg. This of course is a bit too vague to be of actual use, at least without further information.
Clones
A plant grown from a clone (cutting) is generally regarded as beginning its 1st week of vegetative growth when it has established roots and been transplanted into the growing medium. Most often this takes place when the clone is between 2 and 6 inches tall. While this seems pretty cut and dry, clones of the same strain can develop at drastically different speeds depending on root development. Differences in growth can become even more pronounced depending on the light, nutrients, co2, temperature and a myriad of other factors. After only a few weeks, two previously identical looking clones may look nothing alike.
Seeds
Plants grown from seeds make the measure of age in vegetative weeks a lot more confusing. Some growers refer to a plants 1st week of veg as when it emerges from the soil, some say it is when it produces its first set of true leaves. Still other growers will refer to week 1 as beginning when the seedling is transplanted, which in itself tells us very little. As if this did not confuse the matter, plants grown from seed often grow vertically much slower than clones, at least at first. Later plants from seed often outpace clones once their root systems have become established.
It should be obvious why telling someone to flower plants after some arbitrary number of week of vegetative growth is worthless. Yet, I constantly hear experienced growers coaching others to change the light cycle and begin flowering after 2 or 3 weeks of veg. A three week old seed start may be as little as 4 inches tall, while a clone with 3 weeks of veg could stand over two feet tall.
Plant Height
The second common way to know when a cannabis plant is ready to begin the flowering cycle is the actual height of the plant. This measure has significantly more relevance because the height of a plant is objective and universally understood. Combined with other known quantities like the strain and the size of the grow room, height is a good indicator of when it’s time to flower. A plants height can be measured by stretching a measuring tape from the top of the growing medium to the highest point on the plant. It’s likely that there will be some variation from plant to plant, more on this later. While plant height is useful information, the key is knowing how much room you actually have to work with.
Grow Rooms Size Restricts Maximum Plant Height
The size and style of your grow room will ultimately determine the tallest possible plant height. From this you can deduce the tallest height to begin flowering. Most rooms in a residential home have 8 foot ceilings but many growers use attics, basements, outbuildings, grow tents and I’ve even seen grow rooms built into filing cabinets. What’s important is the distance from the top of the growing medium to the bottom of the lights reflective hood. Many grow rooms are set up so the height of the grow light is adjustable. In this case measure from the lights highest possible setting.
Once you know the distance from the growing medium to the light you need to take into consideration your lighting setup. High Pressure Sodium (HPS) and Metal Halide (MH) grow lights get very hot and can damage the vegetation and even start fires if the plant grows to close to the bulb. It’s important to keep the plants some distance from the light so they don’t burn, but not so far from the light that the lumens are severely diminished. For further explanation on this, read Grow Light Intensity Principles. Light movers and ventilated ducting on your lights can reduce the heat and allow the plants to grow closer to the lights. As a general rule, plants should not be allowed to grow closer than about 12 inches from the bottom of the HID lights reflective hood. For those using fluorescent lighting which produces much less heat, the tops of the plants can grow as close as 4 inches from the bulbs. For more ideas on managing the heat from grow lights read Heat Management in Grow Rooms.
Assuming that you are using one or more 600 or 1000 watt HPS or MH lights, take the distance from the growing medium to the bottom of the reflective hood and subtract 1 foot. This will give you the absolute maximum height of your tallest plant at the end of flowering. This does not necessarily mean you want your plants to reach this maximum, but it does give you a concrete parameter to work within.
Strains
The strains your are growing plays a huge part in how tall to grow your plants before switching to the 12/12 flowering light cycle. Find out as much about the genetics and typical growth characteristics of your strains as possible. Indica and sativa dominant strains often display very different growth patterns and behave differently after the  light cycle is switched to flowering.
Indica Strains
Most pure indica strains slow their vertical growth very quickly once switched to a 12/12 light cycle. It’s not uncommon for an indica strain to add only 25% to 50% to its total height after switching to a 12 hour light cycle. Most of this additional growth will take place in the first week or two of flowering. If an indica plant is switched to flowering when it is 12 inches tall it can be expected to finish flowering at about 18 inches, give or take. That math is pretty consistent, so a 2 foot tall indica will likely finish flowering around 3 feet tall.
Here is the equation for indica strains. The vegged plant height (V) multiplied by 1.5 equals the plant height at harvest (H) or 1.5V = H
Sativa Strains
Pure sativa strains are much more difficult to grow indoors and are becoming increasingly rare in general. I don’t recommend them for novice indoor growers for several reasons. What is important for this topic is that their growth after being switched to a 12/12 light cycle is unpredictable. Pure sativas often continue to produce significant vertical vegetative growth many weeks into the flowering cycle. Sativas often double in size while flowering, and in some cases, will finish at three times the height they were at when the light cycle was switched. A 12 inch plant switched to the 12/12 light cycle could end up as tall as 36 inches, sometimes even taller. This additional vertical growth can quickly become a problem in a normal grow room. I’ve seen growers flower sativas at 2 feet tall only to end up with 6 foot plants at harvest. Most grow rooms just can’t accommodate plants this tall, not to mention some other real problems with tall marijuana plants.
Here is the equation for sativa strains. The vegged plant height (V) multiplied by 2.5 equals the plant height at harvest (H) or 2.5V = H.
Hybrid Strains
Hybrid cannabis strains are crossbreeds of various sativa and indica plants that have been bred selectively to promote specific characteristics. Some hybrids strains grow similar to sativas, other grow more like indicas. Most of the strains sold by seed and clone vendors are some form of hybrid. Find out as much about the strain as possible so you can guess how much vertical growth to expect after the flowering cycle beings. As a general rule, leave enough room for the plants to double in size. Once you are familiar with the strain you can let them grow a bit taller if there is room. Don’t forget, at a certain point, taller is not better.
Here is the equation for hybrid strains. The vegged plant height (V) multiplied by 2 equals the plant height at harvest (H) or 2V = H.
These equations are meant as a general aid and are by no means exact. Actual growth patterns may vary considerably.
Growing Style
Your growing style is a big factor in determining when to begin flowering. You may have heard terms like Sea of Green (SOG), Screen of Green (SCROG), Lollipopping, Super Cropping etc. I will go over these very briefly to point out how growing styles affect when to begin flowering.
Sea of Green (SOG)
Sea of Green is an indoor growing method where the plants are flowered very small and usually produce only one large bud. The plants are packed close together and occupy about half a square foot at full maturity. The plant count higher than other methods so clones are usually preferred. Indica dominant strains work best and should be flowered when they reach between 6-12 inches tall.
Screen of Green (SCROG)
The Screen of Green method utilizes a screen of wire or string mesh stretched horizontally above the plants to create an even plant canopy and help support the flowering tops. When the tops are bent down under the mesh the lateral branches grow up through the net and become tops. This method requires less plants per square foot than the SOG method but still requires clones because males can’t be easily removed. Plants must stay in vegetative growth for several week longer than with the SOG method and usually are allowed to continue vegging for one week after the tops are bent down with the netting. In most cases, the screen is placed between 1 and 2 feet above the growing medium.
Lollipopping
With the Lollipoping method the plants are usually switched to flowering around 1.5 feet for sativas and as much as 3 feet for indicas. The lower ⅓ to ½ of the plants branches are removed before the light cycle switch to direct the plants energy to the tops closest to the light source. This produces plants that resemble lollipops with long bare lower stems and multiple bushy tops.
Super Cropping
The Super Cropping method requires one of the longest veg periods but is designed to produce heavy yields with a minimal number of plants. While the plant is in in the 18/6 veg cycle the main stems are bent down to retard their vertical growth and promote the growth of lower branches. This decreases the overall height and results in bushy wide plants with tons of tops. If done properly vertical height does not become a problem and vegging can continue until the plants fill your horizontal space.
Plant Height Variation
When growing multiple plants you will often find a sizable variation in height of your plant. To maximize light exposure and therefore yield, it is ideal to flower plants of uniform height. Several methods can be used to help create an even canopy of tops before you switch to the flowering cycle.
Topping
By pinching off the tops of your taller plants you will not only create twice as many flowering tops, but you can buy some time for your shorter plants to catch up. This does stress the plants to some degree and I would suggest vegging for 1 week after topping instead of immediately switching to flowering.
Training
A less stressful approach is to train the tops of your taller plants down. This requires more skill and patience, but allows the lower branches to catch up. Several methods including screens (SCROG), plant tie, and stakes can help achieve the even canopy you are looking for.
Rotating Plants
Even with light movers the plants directly under the the lights will tend to grow the tallest and most robust. Rotating your plants each week from the outside edges of the grow area to better lit areas directly under the lights will help to create an even canopy. This only works if your growing method allows you to move the plants so it doesn’t won’t work with the SCROG method.
Plant Health
Half dead plants rarely make a sudden turnaround just because they are switched to the flowering light cycle. The veg cycle creates robust vegetation to support the plants while they flower. Without healthy foliage the plants ability to produce flowers is limited and its resources will be diverted to heal itself and fight off disease. It is much easier to treat nutrient and environmental deficiencies as well as pest problems while the plant is still in veg. A flowering plant that is stressed by poor health is also prone to reaching maturity early. This shortened flowering period gives the plant less time to produce the large trichome rich buds and will greatly affect the yield. Remember, veg usually takes only a few weeks while flowering can take from 7 to 14 weeks. Sometimes it makes sense to cut your losses and start a fresh round of plants rather than trying to flower sickly plants or coax them back to health.
Cliffsnotes
- The number of weeks your plants have vegged is a bad indicator of when they are ready to flower.
- Use the height of the plant as a gauge for determining the height they will finish at.
- Know your strain and how much additional growth it will produce while flowering.
- Know your grow rooms limit for vertical plant height and plan accordingly.
- Choose a growing method that works for your vertical space limitations and don’t over veg for your method.
- Create an even canopy by pruning, training, and rotating your plants.
- Flower only plants that are healthy and happy.
As usual, comments and quick questions are welcome below. If your questions are more involved and require a bit of back and forth, or are not related to this article, please ask them in the forum.
When To Begin Flowering Indoor Cannabis Plants,
Excellent article. I am continuously baffled by some of the practices of so called “experts” in this regard. This article was well done. Thanks.
always great info,please dont stop,this is the best reference i’ve seen,mica.
Hands down, perfect
its.not good advice its common sense to look at all room/GROWS different no grow will b the same and if u argue ur a fool.
I have been growing my plant for 4 months outside now, if I never use fertilizer, will my plant be weak as far as potency ?
The amount of fertilizer you use probably won’t effect the potency, but it probably will effect the overall yield of you harvest.
Best article on this subject I could find. Great info explained clearly! Thanks.
Hey Rambo – ive grown outdoors in South Africa for 6 yrs till i got bust , now i have a flourescent grow indoors and your article about indoor flowering times was fuckin awesome – just what i needed -thank you!
Brett Morgan.
i have still not seen a more accurate example of true blue till u did an article called “when to begin flowering indoor marijuanua plants “………….
Brett Morgan
Hi an goood reading good article I’m looking for acouple of answers since this is my first grow and ima bout to harvest so anyhelp would be usuful u guys can get back at me here or on my gmail, cr.reyes17@gmail.com Iwas wondering since my tops colas will b ready first is it ok to cut only those down first then cut my lower branches when there more mature and the hairs have all changed color an they get more ripen??? Right now Ihave the top colas with about 30 percent color tricomes but when Ii look on my lower branches those are still white and preme.
Glad you liked the article. You will find several others on this page regarding when to harvest and how to harvest. I think your questions will be fully answered in them. Please note, “hairs” are not trichomes and are not a good indicator of bud maturity.
great help
A friend of mine found a container of seeds that he had forgot about. There are hundreds of them. They are from the 1970’s into the 1980’s. Do you think they will still grow? Should I do anything to encourage germination? Someone told me to lightly rub them with sandpaper then soak them in water. What do you suggest?
When do I start flowering stage
5:39 pm on August 14th 2018. If you don’t want to wait until then, try reading the article above. If you still have questions, please take your questions to the forum with as much detail of your situation as possible. http://www.marijuanagrowershq.com/forum/
beautiful tips and pictures
Hello could you recomend best way to prune and grow in a glass house ??? Have plenity of room at the top and a 2 acre space.
Thanks for all of your articles really useful…
A. This is going to require a pretty involved answer and probably will require more detail from you.
B. This has nothing to do with the article “When to Begin Flowering Indoor Cannabis Plants”
Please take questions like this to the forum so others as well as myself can assist.
Thats a load of shit
criticism is welcome, but not without an explanation.
When do u take leave off and were at and how long do it start to get buds on it .my plate started growing last month at end of may but came up in the firat week of june so it up some size now cause it almost another month July. So what do I need to do.I have food sticks for it and I did put putting soil to it.
Hey love the blog have a question what if you left the plant in veg mode to long and the plant is in its second week of flowering and the cola is about to hit the roof but you don’t want to cut the cola off because it is so big and health
This is a pretty common problem. There are no great solution when you have let it get this far out of hand. Your best bet is to try to train the stem down so it forms a kind of S shape. Here are a few articles that may help you.
http://www.marijuanagrowershq.com/category/growing-indoor/
http://www.marijuanagrowershq.com/7-problems-with-tall-marijuana-plants/
Ok so I bought some herb from the medical shop all I smoke is indicas n some hybrid so I kno it has to be either or but I used plain dirt in the beg. It got about 3 inches tall I transplanted it an its about 4 inches now but growing very slowly its ben 5 or 6 weeks now an some of my leaves has turn yellow an the ends crumble wen touched only on a few my soil is far from dry I have no clue if its a male or female an I can’t afford expensive lights I use some sun an some store bought twisty lights lol I’ve put egg shells an lime before no results though an definitly can’t afford a grown room with its materials any very cheap poor mans way to grow indoors n get my baby healthy?
If you need assistance on this level, read as much as you can on the webpage and if your questions are not answered please start a new thread in the forum and ask for help there.
thankyou very much
Okay, this is my first ‘solo’ indoor grow. Got the cute little clones, nurtured them for the past 3.5 weeks. they are indicas and I have a whole lot of room to play with – height wise. My question is twofold re: when to flower. One: they are of mixed size (some taller than others, but about equally as ‘bushy’) and I have only used bamboo stakes in previous grows (with a ‘supervisor’ guiding the process) and now i am on my own and am wondering if i should let them veg for another week or so and 2) should I stake them?
The other ideas you have presented, while incredible, i am sure, I have no experience with them. I learn best by seeing it done and doing it with someone. As i have only ‘staked’ in the past – with my teacher by my side – and i am on my own now, i lean toward that method. Have you ever heard of this before?
Please post you question in the forum. This is a very involved subject and the forum will allow you to post pictures so we will have a better idea of your situation.
I started my first grow recently and have 11 plants about 6 inches tall and been in veg for about a week and I’m using a 3 1/2 ft tall grow box but I was wanting to flower when they’re about a ft tall but how do I know if the stems n branches are strong enough to hold the buds my plants look healthy and I use nutes every other watering I’m just nervous that if I switch to flowering and the stems and branches are to weak my plants won’t produce much bud if any. Does the plant just adapt during flowering and become stronger to hold the buds? Would really appreciate some advice please n thanks.
can I veg outdoor and bring them in door to flower under a light
How did it work out. Veg outside and bring them inside for light
Okay this is going to sound really wierd, but you sound like you know what’s what. So I’ve grown one plant and it was in a really awkward way, I had mid grade weed seeds and I put like 15 in a small pot and put them in my window and watered them for about a week and a half and suddenly I had all these sprouts I chucked a couple but kept like 8 but I only kept one thru veg, also I used 1 light and a mini fan 23w for a while and switched to flo resent light wattage unknown and my box was made in class I laced the inside with tinfoil. At flower stage I put it in a closet same with foil and light. It grew 4ft and was about to bud I belive., I’m so inexperienced I don’t know if it was a female or not but it was pretty and i topped it way to early, like way wy to early, I had to kill it ;( can you help me or just answer this could I do it again?
Hey~ I woke up this morning to 4 super healthy appx. 4 foot from root top robust babies. They have been grown outside and I have no idea about how an outside grow would affect the veg. cycle or be able to predict how much space I am going to need. They are going to come inside into a bathroom with ventilation, I am just wondering if it is too late to pinch them back a bunch since they are so healthy to get a shorter flowered plant. They are super strong and and easy to bend and train at this point.
Aloha, just harvested my first ever indoor crop! I used my 2.5 x 6 ft closet with 2- 200 watt LED Diamond series lights. I grew from random seeds and must say I am very pleased! They must have been Sativa because I started with 10 plants, after 4 weeks of veg they were all 18 in high and bushy, so I switched back to 12/12 cycle and ended up with 6 females 36 inches tall, total time about 2.5 months. I could have stuffed a few more plants in the closet, so next time I know. The buds were tight and stinky The LEDs stayed cool enough with built in fans, that I only needed one fan blowing air out of the closet. The light however is intense and you need sunglasses! Happy growing 🙂
hi! bunch of questions, first timer, OK. do you shine your light on the plant or soil? I have a two week old plant and the leaf on one side is almost curling and the other side is lined up with the other leafs, is my plant still good? how much watts do the light have to be if im growing indoors and how many? thanks
Cece, While you questions are good they don’t really have anything to do with this article. Please as general questions like this in the forum. Thank you.
hi I was wondering what are the best and cheapest lights to use during flowering I plan on growing a indica strain in my closet.
This is a good question for the forum. That way other growers can share their experiences as well. I prefer to flower under HPS 1000’s but Metal Halide works and some have had good results with florescent and LED’s but LED’s will be the most expensive.
My plants are looking quite smarll after 4 weeks what shall I do ?
okay I got about 14 leaves on a six inch 7-inch plant there is some little things developing on the stem between leave base um looks kinda like a football with split tip um can somebody tell me what that might mean female or male it it started inside of a bunch of worm castings in compost good pretty fast um not coming to slow down a little bit
There is a great article on this page about how to tell the sex of your plant. Start there and then post in the forum with questions
I have one plant (hybred-Slightly sativa) indoor under 4 23W CFL’s (400W equivalent) that is about 7 inches tall, If I switch to flowering now will I get a decent yeild? (Im not looking for anything crazy, maybe an ounce)
If you are looking for an ounce I would let the plat get a big larger first and you might consider topping it to make it bush out a bit. I have never tried flowering with CFL’s and while I have heard it can be done with decent results, my experience is all with HPS and MH lights for flowering.
Hello Rambo I have a few questions and seeing how I am a new and current grower I may need a little advice, I’m wondering how to create the horizontal effect of my plant and I want all top buds will it create a bigger yeild if I bend it and it is a clone so I’m kinda hesitant about it
I am an old Grandma growing pot in the bathroom..for me and the old man. This article is the best I have EVER read [I read at least 1 hr a day]. Great job Rambo and spot on.With out all the bullshit. Thanks Carolin
Great post learned a lot from it very usefull
1) when can I start to “top” my plants? I have 3 very indica dominant plants in a closet about 1foot tall and only 30″ available. Also 2 are almost only main stalk and 7 nodes of fan leaves but 1 (biggest) has some side branch development. Should I have worked on training them?I think its possibly due to shock from transplanting since I moved the 2 at the same time and the other later. First time grower, Accept for the one the rabbit got to last summer. Thanks
To skaterfreek12, I currently finished some under 2000 Watts of warm cfls, bloom spectrum, in a custom built parabolic hood as an experiment and they were about 3 ft tall when i started to flower, a sweet tooth and blue dream, and I’m 3 weeks from harvest and Might get an O a piece of some primo nugs if I’m lucky, get an Hid man
i have a plants its tall i need to no what to do
Humbly, I would point out that sexual maturity needs to have reached the stage of showing gender in seed grown plants or hermaphrodites could occur. This can happen even with feminized seed plants.
OK my first grow. I understand about trichome check for harvest. However if u are supposed to stop feeding nutes about 2 weeks before harvest is there a rule of thumb on determining when to stop the nutes? Not sure if I’m explaining clearly. My girl looks exactly like the pic at the top of this page and it was bag seed . Beautiful red and pink hairs.
im into bloom. buds are top heavy. i want to know what to use to make them stronger?
Hu there.
Its my first grow and im 3 weeks into bloom but the tops are too close to the light. I have noticed 1 bud and a few leaves damaged because of it.
Unfotunately i cannot higher the light any more can i put thin plant netting across the top to prevent them from futher damage..?? Help please im worrid about my babies:-(
Wow sure glad I found info to clear up my question. Definitely a pleasure and relief to NOW BE ABLE to understand more about vegetation growth. Thank you and do continue to help the ones like me that truly appreciate you. :).
The cliff notes ending is brilliant. it is nice to have all that information at hand with all of the specific details and options and then at the end it is even more nicer to have the cliff notes that breaks things down in case you were reading this stoned
Finally, I get a straight forward answer of when to flower. I’ve heard so many opinions that boggles the mind but at least now I know what to do. Thanks!
I have been looking up air pump location In Co2 inriched environment and can’t find much. Since roots take in o2 wouldn’t it be a bad idea to position the air pump in the Co2 room. On the other hand by placing it outside it will cause a positive pressure and therefore waste co2 gas. Any insight would be greatly appreciated
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I am new to growing and I’m starting my first plant. I am sick of spending so much money on it and because I do use it for medicinal use for epilepsy, I guess you could say in a way I need it to help control my seizures. I’ve done quite a bit of research on different methods. I am really limited on space and concerned about the smell. I decided to go with an indoor terrarium set up. Out of all the reading I’ve done it should work. Your green screen idea is brilliant As I am limited in height space for the terrarium I am building. Is there any tips you can give me to make this go easier and not such a long drawn out learning process. I do know that the first time will not be a charm but I tend to learn things very quickly. Any info would help. by the way this is the best info I have found so far.
I’m a first time grower; your article was great, but what happens when you don’t know what strain you have. I just started some random seeds I found and I have no idea what I got! I have some that are 3 in. and 2 sets of leaves with more coming. And the others are only 2 in. and 1 set of leaves. They are about 2 wks into veg stage. I am growing them indoor with a reg floresent light in a tent and then in the sun all day in the window. I am using 24 hour light.
Hello, I have a friend that cuts the upper fan leaves to allow more light to shine through to the bottom buds. Is this ok to do or will the plants get stressed ? Thinking on doing that my self.thanks for any help..
i was reading this because at six inch i started the veg cycle and now my plants are dieing i was wounder in anyone out there could help me save my plant ro i will so be out if my meds