We explore whether you can skip the 18/6 light cycle and immediately begin using the 12/12 light cycle after sprouting your marijuana seeds. 12/12 from seed is an intriguing concept and one of the latest trends in the cannabis growing community. If you want to grow short and quick plants, read our 12/12 from seed guide, where we tell how to minimize the cons while enjoying the pros. 12/12 from seed to harvest is an outdated method of growing weed, but not a bad cycle for bag seeds and perpetual harvest. Learn more!
Can you use the 12/12 Light Cycle immediately from Seed?
While continuing our exploration of cannabis light cycles, we now turn our attention to the concept of skipping the vegetative light cycle and going straight to the flowering light cycle.
Is it possible to begin the 12/12 light cycle immediately after sprouting a seed?
A question that comes up from time to time is whether it’s possible to skip the 18/6 vegetative light cycle and go straight to the 12/12 light cycle after sprouting a seed. In theory, the answer should be “yes.” Theoretically speaking, a marijuana plant will still produce buds if subjected to 12 hours of light and 12 hours of darkness.
But theory and real life practice don’t always match up. The only way to truly know is to test the theory with an experiment.
So we have to decided to test this theory by launching a series on flowering cannabis straight from seed. We have never done this before, so we are excited to see what happens!
How to Grow Marijuana Indoors: The Two Light Cycles You Must Use
As we have explained before, Cannabis is a photoperiod plant, with two distinct photoperiods: the vegetative period and the flowering or bloom period.
The 18/6 Vegetative Light Cycle
The vegetative period is the first of the two stages of the cannabis life cycle. During the vegetative stage, the plants grow taller and bigger, but never bloom. Because they never bloom, they remain immature and capable of being cloned. They can also be trained, by being bent, pruned, topped, and even twisted. In this immature state, they remain pliable and flexible.
The purpose of the vegetative period is to allow the plant to grow, and to train the plant before it begins blooming.
The ideal light cycle for the vegetative period is 18 hours of light, six hours of darkness. However, many other light cycles that can be used, such as 20/4 or simply keeping the lights on 24 hours a day, with no darkness.
The 12/12 Bloom Light Cycle
In order to make the plants bloom, they must receive much less light than during the vegetative phase. In fact, cannabis plants must receive at least the same amount of darkness as light in order to produce bud. And if they receive more darkness than light, that is fine too.
Why Skip the vegetative stage?
Why would you want to skip the vegetative stage and immediately begin flowering your seedling? This is a question worth asking.
Without having conducted this experiment, we can hypothesize the advantages and disadvantages of skipping the 18/6 light cycle.
Advantages of skipping or shortening the Vegetative Light Cycle
The perceived advantage would be shorter wait time to harvesting flower. If you take a seedling and put it under a 12/12 light cycle, you are only 60 days away from harvesting actual bud. This is a shorter time period than you would get using autoflowers – strains that are specifically bred to flower without having to even change the light cycle.
There are other perceived advantages, such as more harvests per year.
If you are using the same space for veg and flower, then you will also be able to pull down more harvests per year. A lot more.
Disadvantages to skipping the Vegetative Light Cycle
The main disadvantage will be your yield per plant. A 10-day old seedling is not very big, and even if it triples in size during the 12/12 light cycle, it will still not be that big after 60 days. A smaller plant will always yield less bud than a bigger plant of the same pheno.
The other perceived advantage – more harvests per year – only applies if you are growing in a single room for both vegetative and flowering stages. By simply having more grow spaces — whether rooms or tents – you can get the same number of harvests per year. So this advantage goes away simply by expanding your grow spaces.
There is also the question of whether the quality of bud will be the same without the additional growth that occurs during the vegetative period.
There is only one way to find out.
A video series on using the 12/12 light cycle from seed
We chose to conduct an experiment on this topic. And we documented the entire process. You can read and watch it here:
12/12 From Seed: Doing It Right
Some of the most sacred rules of cannabis growing tell us that in order to grow and successfully harvest photoperiod strains, you need to let the plants grow and gain mass during their vegetative period. Now, what if we told you that some rules are meant to be broken? Thanks to some daring enthusiasts who weren’t afraid to take a risk, we now know that even photoperiod strains can be grown 12/12 from seed – in other words, put straight into the flowering phase the day they germinate. Should you try it? Is it worth it? What about the yields from 12/12 from seed? In the true spirit of experimentation, we’re going to answer these questions and many more in this Herbies article, so read on!
12/12 From Seed: The Method Explained
We know, it’s exciting to find out that there’s a way to stop all the questions about exactly how and when to transition plants grown from photoperiod seeds into the flowering stage. With the 12/12 light cycle from seed, you give your weed plant 12 hours of light and 12 hours of darkness every day without exception, from the day the seed pops until the day you snip off the buds. That’s one problem less – but at what cost does this convenience come?
The Role Of Veg And Flower
Of course, if cannabis plants naturally go through both vegetative and flowering stages, they need these periods for certain reasons. During the vegetative stage, which lasts while photoperiod strains receive more than 12 hours of light per day, cannabis plants actively grow, become thicker and taller, and often reach their maximum possible height, signaling that they’re mature enough to procreate and make buds.
When the schedule is switched to 12 hours of light per day or less, the plant can still stretch a little, especially if it’s a Sativa, but from that moment on, it will direct all its accumulated power to making those sticky and smelly buds we grow cannabis for in the first place.
Apparently, however, you can skip the vegetative phase entirely and set your cannabis to 12/12 from seed. So, what are the consequences of skipping the veg?
Cannabis Light Schedule
What Happens When You Skip The Veg?
If you decide to skip the veg and carry out the 12/12 from seed grow at home, you need to understand that by switching cannabis plants to flowering straight after germination, you deprive them of the time and light energy they would normally use to grow, mature, and prepare themselves for developing buds. With the 12/12 light cycle from seed, your photoperiod cannabis plant will only reach the minimal mass and height that it can use to start procreating. As the result, plants grown on 12/12 from seed become extremely compact, and thus yield less than their counterparts that went through the standard vegetative period.
However, such a bold move proves that there’s still much to learn about cannabis growing. While we wouldn’t have dared to even think of skipping the veg before, we now know that cannabis won’t die, and indeed will still yield buds, if we use 12/12 from the day the seed pops. If you’re looking for proof, there’s lots of it in the special 12/12 from seed Growdiaries section, where growers share their experiences with skipping the vegetative period.
Now let’s move on to the pros and cons of this growing method. Give it a read-through and decide whether 12/12 from seed is worth it for you.
12/12 From Seed to Harvest: When to Choose This Method
Doing 12/12 from seed is mostly a relic of a bygone era, but it’s not a bad way of growing weed and can even offer optimal results in some situations.
The 12/12 from seed to harvest method is a bit outdated, but still has its uses. In this article you can learn everything about yields, pot size and plant size, flowering time — in short, all that you can expect when running 12/12 from seed in your grow.
If you have grown weed before, you probably understand what ‘12/12 cycle from seed’ means. And for the novice growers, the following short paragraph explains all the science.
Cannabis is a short-day plant, meaning that it only starts to flower when days become short enough at the end of summer or the beginning of fall. Indoors, we mimic this by shortening the lighting cycle to 12 hours of light and 12 hours of darkness (12/12) and thus inducing the flowering. Usually, we do this after several weeks of the vegetative growth under 18/6, but sometimes you can reprogram your timer to 12-12 from the day your sprouts emerge. And this is exactly what we mean by 12/12 flowering from seed.
These Black Widows clones were put under 12 and 12 as soon as they rooted. Not exactly a 12-12 from seed scenario, but the idea is the same. Wonderful results btw.
Table of Contents
Growing Weed on 12/12 From Seed: Right Reasons and Wrong Ones
Frankly, this is a notoriously low-yielding method, so there must be other reasons to still choose it, right? Of course, there are.
1. Bagseed Growing
Okay, you’ve bought some buds with seeds in them, liked the smoke, and decided to give those bag seeds a try. Not the most brilliant idea ever, but okay. You don’t know how these seeds will turn out until they’re well into flowering, and this can be in like 1.5-2 months from sprouts (if grown in a regular way). Probably, too much time to spend on a dark horse, right? And here the 12/12-from-seed-no-veg method can easily save you 2-4 weeks or more.
2. Using Regular Seeds
It’s the same as with bagseeds. A certain percentage of regular seeds (probably around 50%) will grow into males, and you want to spot them and discard them as soon as possible. Again, 12/12 lighting from seed saves you several weeks of precious time.
3. Perpetual Harvest with no Extra Veg Room
Some people have just one grow space, but prefer to run a perpetual harvest operation. Obviously, your only grow room will be the flowering room, with lights running at 12/12 continuously. So the only way to add new plants after you’ve harvested some of their predecessors is to give the newcomers 12/12 from seed to harvest.
4. You Don’t Care About Yield, But Value Variety
At one time, I was crazy about mix packs that some breeders sell because they gave me the opportunity to grow (and later enjoy) several different genetics in one go. So I used to put 9 different strains in my 2’x2’ grow tent and run them in a 12/12 cycle from seed. The yield per plant was underwhelming, but more than enough for me. The main thing was that every smoke came out different in terms of effect and flavor.
So, these were the good reasons to choose the 12/12 from seed to harvest method, and here are the bad ones:
- Microgrow/Limited Height. In small grow spaces with a short distance to lights, you’ll probably be better off raising autoflowers in small containers. The alternative is having fewer plants, but training them in veg (topping, LST, ScrOG) to have a flat canopy. Both of these options will guarantee you better yields.
- Weak lights. Some people who use tubular fluorescent lamps (like T5) or something similar (CFLs, LED bulbs) think that lighting systems of this kind call for a SOG grow with many budstick plants in solo cups. It’s probably true, but again autoflowering seeds will give you better results than photoperiod strains grown on 12/12 from seed to harvest. And yet again, simply training those during normal veg would be more effective, too.
- Expectation of a faster turnover. Questions like “How long does 12/12 from seed take?” (which people ask all the time) prove that many of them regard this method as the fastest way to harvest. In fact, it doesn’t really make your plants finish much faster, if at all. More of it below.
There is one more aspect of growing weed 12 12 from seed that we have briefly touched upon when talking about bag seeds and regular seeds. Here it goes.
Some growers try out every bean they lay their hands on in search for a hidden gem. Well, it’s a noble quest, and the 12/12 from seed method allows you to quickly assess the genetics. But suppose you’ve found a phenotype you like. Now what?
You have two options: one is cloning a plant that is already flowering, the other is reverting it back to veg (a procedure known as ‘reveg’).
Taking a cutting from a flowering plant and then rooting it is a fairly difficult procedure compared to cloning in a regular way (when the plant is still in veg). It takes longer and the success rate is lower. (There is an upside, though: the resulting clone turns out into an insanely branchy little monster; hence the name of this procedure – monster cropping.) Another difficulty is that plants grown 12/12-style are basically a ‘bud on a stick’ and often have no side branches to use as cuttings.
What’s the Alternative to Reveg Then?
If you’re searching for a keeper, try vegging every plant for a few weeks, then take a back-up clone from each, and then flower all donor plants to see if there’s a champion among them. It’s easier this way.
The same goes for reveg. Reverting flowering plants back to veg is hardly a beginner-friendly procedure, and can take months (with no guarantee of success). Do you know that some growers in colder climates use this feature to have the harvest in the middle of summer? They make their plants flower indoors for 4 weeks, and then put them outside. And no reveg happens, even though summer days are much longer than nights. Why? Because reveg is a difficult process and requires so much more than simply reverting to 18/6 or even 24/0.
Revegging a plant after harvest is a painfully slow procedure with uncertain prospects.
12/12 From Seed vs Autoflower Growing
In many ways, the 12/12 from seed method is similar to the cultivation of autos, but autos have a much better potential because they can flower on 18, 20 or even 24 hours of light a day. All this extra energy will be transformed into extra bud weight and raised amounts of THC and terpenes. There was a time when autoflowers weren’t high-yielding or potent enough to compete with their photoperiod counterparts, but it’s long gone now. Today, the 12/12 from seed vs auto choice is a no brainer.
And by the same logic, the worst thing you can do is to grow an autoflower on 12/12 from seed. Sometimes people are forced to do this: for example, when there is some free space in the flowering room, but no extra seeds, except autoflowering ones, to fill it. But autos raised in this manner usually stay small and yield little.
12/12 From Seed Instructions and Tips
So, you have a grow room dedicated to the 12/12 from seed to harvest grow. Start by programming your timer so that the lights turn on for 12 hours of light every 24 hours, and then germinate some seeds using your favourite germination method.
This is what you may expect when growing weed in a 12/12 cycle from seed.
What Size Pot for 12/12 from Seed Works Best?
You don’t need a lot of space for your plants’ roots because the vegetative stage will be very short and then the root system basically stops growing. So use solo cups (party cups) or small plastic containers. The pot size can be anywhere from 16 oz (0.5 L) to a gallon (3.78 L). Keep in mind that the smaller the containers, the more frequently you’ll have to water them. More medium also means better buffering for nutrients and less risk of a nutrient burn.
Also different strains react differently to 12 and 12 from seed. Some will take many weeks before they even start flowering, and they will require bigger containers. Growing in 1 litre pots will do fine for most varieties that are worth growing this way.
When Does Flowering Start?
Even on 12/12 from day one, weed plants don’t start flowering right away. Vegetative growth is an important process and will run its natural course. Expect at least 3-4 weeks before the plants are mature enough to show their sex. Of course, when it happens, they will start to flower immediately. As you can see, this is basically the same timeline as for autoflowers.
How Long till Harvest?
Most 12/12 enthusiasts report that their plants finish in 70+ days from sprouts, although some phenos can take up to 3-4 months. Again, autos can do much better than that, and they’ll yield more, too. The reason is that the yields are proportionate to the hours of light a plant receives, and you can give autos from 18 to 24 hours a day.
The Final Size and Yield
It all depends on a lot of factors — strain, pot size, lights etc. It can be as little as an eighth (3.5g) or less from a sickly runt in a solo cup, or up to a half ounce (14g) in a 16oz (0.5L) cup. In the latter case, the skinny one-cola plants will be upwards of 2 feet (60+cm).
This is how a harvested plant can look like if grown 12/12 from seed. Hardly inspiring. I’d even say vaguely humiliating.
Some 12/12 growers prefer even bigger plants in containers of up to a gallon (3.78L): their height can be 3-4 feet (90-120cm) and more, and the yield per plant of 0.5-1oz (14-28g) is considered very good. And it really is, if you grow many such plants SOG-style. There are even reports of plants yielding as much as 3 ounces (85g) each.
SOG (Sea of Green) is the Only Viable Option
Given that the size of the containers is quite small and the plants are lanky, with almost no side branches, we recommend cramming as many of them as possible into your grow space. This way your yield per square meter can prove to be not so bad after all: let’s say 30 half-ounce plants under a 1000W HPS. Hardly record-breaking for a SOG grow, but still.
Would-be 12/12 growers often wonder whether they can train their plants or not. I think the logic here is the same as with autos: any HST technique, like topping, would just lower the yields and (possibly) delay the harvest time. Besides, any training method, including LST, raises the question of spacing: if the plants stand next to each other in tight rows, you shouldn’t try to make them wider and bushier. The single-cola pattern is probably the best.
The Best Strains For 12/12 From Seed To Harvest… and the Worst
As we have said earlier, this method is best reserved for seeds of unknown origin and potential. But if you insist on growing store-bought seeds this way, choose an indica. Indica strains tend to react quite early to the change in light cycle, and if you give them 12/12 from sprouts, the veg will be very short and the plants will remain nice and compact.
As for sativas (like Haze) and sativa-dominant genetics, they have evolved in regions near the equator where days and nights are close to an equal length the whole year. So, they can remain in veg for many weeks and even months before they even show their sex. It kind of defeats the purpose of 12/12. The plants will be neither fast, nor small, and the yields will be disappointing because of too little light hours received in veg.
12/12 From Seed Feeding Schedule
The first thing you should take care of is the development of roots. We recommend using some rooting stimulator when watering your plants in the first two weeks or so.
As for nitrogen-rich fertilizers, it all depends on the pot size. In bigger containers, the soil will probably have enough nutrients to last for the whole of veg. In smaller pots or cups, the plants will need a nitro boost early on. Continue to feed them with veg nutrients till they reach the final height and stop stretching further. This usually means a couple of weeks into flowering.
Starting in the third week from seeds, introduce the bloom fertilizers (with raised P-K levels). First use them along the veg ferts and then without them. 2 weeks before the harvest is time to start the final flush.
The Bottom Line
There are a few special situations where the 12/12 from seed to harvest method can lead to results that are by no means spectacular, but optimal under given conditions. In most other cases, using it would be a mistake. Always check whether you can achieve the same goal with autoflowers, or other growing methods. And if you don’t agree, please share your opinions in the comments.