12/12 from seed to harvest is an outdated method of growing weed, but not a bad cycle for bag seeds and perpetual harvest.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! 12/12 from seed has a big problem. Learn about a better alternative called "Sea of Green" that gives you bigger yields in the same amount of time!
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The 12/12 from seed to harvest method is a bit outdated, but still has its uses.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.
Is 12-12 From Seed Worth It?
Will the “12/12 from seed” cannabis growing technique give you huge yields? In a word, no… but there are benefits to initiating the flowering stage as soon as possible. Giving a 12/12 light schedule from seed germination is a great way to create tiny “bud stick” cannabis plants, which may be perfect for a small stealthy marijuana garden. When modified slightly, the same basic technique can create bigger plants and bigger yields while adding little to no extra time before harvest. And then there is the matter of genetics. Certain strains give better results than others.
The “12 /12 from seed” technique aims to initiate the flowering stage immediately after germination. Learn about cannabis light schedules.
This creates small “bud stick” plants that are typically ready to harvest in 3-4 months. Check out this beautiful example of “12/12 from seed” by Fuel.
If you haven’t heard this term before, “12/12 from seed” means giving your cannabis seedling 12 hours of light and 12 hours of dark each day from the moment of germination. Providing a 12/12 light schedule initiates the flowering stage and causes cannabis plants to start making buds. The idea of this technique is to get seedlings to start flowering as quickly as possible to reduce the time to harvest. Once a plant has started flowering, most strains are ready to harvest within 12 weeks. The sooner you get a plant to start flowering, the sooner you get to harvest.
There’s just one major problem… 12/12 from seed often causes stunted growth and small yields. This results in yields that are often less than 1 ounce per plant, even under strong grow lights.
This seedling took nearly 4 months from seed to harvest under about 150W of fluorescent lights and yielded only 1/2 of an ounce.
I think it looks kind of cool, but 1/2 ounces is not a great harvest after waiting so long. I’ve found you can tweak the light schedule to double or triple your total yield per plant without necessarily increasing time-to-harvest or plant height. More on that below!
12/12 from Seed – What You Need to Know
Something that’s a bit confusing about 12/12 from seed is it seems like seedlings should start flowering immediately. They don’t. I’ve found that photoperiod seedlings won’t start flowering until they’re 3-4 weeks old no matter the light schedule. That means your harvest won’t come any sooner if you initiate 12/12 before seedlings are 3 weeks old. When you give the plant 3-4 weeks of 18+ hours a day, THEN switch to 12/12, you often get better results because plants are quite bigger when buds start forming, without adding much (if any) time to harvest.
Most seedlings won’t make buds until they’re at least 3 weeks old from germination. There’s nothing you could do to get this little seedling to start flowering.
You often get better yields by giving plants 18+ hours of light a day for the first 3-4 weeks. These plants got 18/6 until they reached this size.
I started the 12/12 light schedule right after the above picture. They ended up yielding a lot more than my plants that were given 12/12 from seed in the same setup. Yet they didn’t get much taller or take longer to harvest.
Here are those same plants a month later after they started making buds (plants double or triple in height after the switch to 12/12, especially young plants)
A few weeks of extra light, in the beginning, seems to achieve plants with longer and fatter buds (why was this plant defoliated?)
Plants reward you for giving them a little more time to grow. For example, a solid 4 weeks of 18/6 before 12/12 creates plants that are ready to harvest around the same time as a plant given 12/12 from seed (3-4 months, depending on the strain), but significantly bigger yields.
If you’re worried about plants getting too big, you can gain complete control of plant shape by topping and bending plants to stay flat when they’re young. This increases the amount of bud produced without raising the overall height.
Many people who give 12/12 from seed have chosen this technique to ensure plants stay as small as possible. That’s why it’s common to see these plants in very small pots (like solo cups). Just like with bonsai trees, restricting root space is an effective way to reduce overall plant size. Plants kept in solo cups rarely get bigger than one long bud stick, especially when given 12/12 from seed.
Restricting root size reduces plant size dramatically, especially when combined with 12/12 from seed. Look how big this plant is in relation to the lighter!
12/12 From Clone – When You Want TINY Plants
If you want to grow the absolute smallest plant possible, “12/12 from clone” may be a better choice than 12/12 from seed. A clone is a piece of a bigger plant that was removed and forced to grow roots. Since a clone is technically the same “age” as its mother plant, it’s pretty much a mini mature plant as opposed to a seedling. Because they’re already mature, clones start flowering almost immediately after getting 12/12. That can result in some REALLY small plants at harvest.
Unlike seedlings, clones will start flowering almost immediately after getting 12/12. If you start with small clones in miniature pots, you end up with short buds on sticks at harvest.
These “12/12 from clone” plants are cute and fun! Though not necessarily the most productive when it comes to actual yields.
Auto-Flowering Strains – When Time is of the Essence
Some growers want to use 12/12 from seed to get to harvest as soon as possible, but it’s not actually the best option for a quick harvest. If you’d like to be able to harvest plants even sooner than 3 months, consider giving autoflowering plants a try. Most autoflowering strains are ready to harvest just 2-3 months from germination. That’s less time than almost any photoperiod plant will take, no matter what light schedule you provide. Even better, autoflowering plants on average yield 1-2 ounces each (more if you care for them well), which is better yields than most “12/12 from seed” plants.
The “modern” version of 12/12 from seed is simply using an autoflowering strain. They don’t need special light schedules and are ready to harvest in just 2-3 months from germination.
Autoflowering plants are typically given 18-24 hours of light a day their whole life, which allows them to get bigger in less time than plants under a 12/12 schedule.
Auto-flowering strains are ready in 2-3 months from seed, and yield an average of 1-2 ounces/plant. You can yield even more with low stress training (LST) and great care.
Use “Sea of Green” (SoG) to Maximize Photoperiod Yields
If you want to grow photoperiod strains (or can’t get your hands on autoflowering seeds) but still want to produce big yields as fast as possible, the “Sea of Green” grow style may be a good option. Sea of Green is simply the idea of growing many small plants instead of just a few bigger plants. It works great for photoperiod strains to produce a quick harvest and great yields. It’s kind of halfway between “12/12 from seed” and a typical grow.
For this grow style, growers usually switch to 12/12 when plants are 4-6 weeks old. Since adding a little extra time to the vegetative stage often increases yields, you need to find a balance between getting to harvest as quickly as possible versus harvesting the biggest yields.
For SoG, wait to switch to 12/12 until plants are about this size or a little bigger (note: young plants like this can and will double or triple in height after the switch to 12/12)
SoG in Action – Make sure to fill the entire space under the grow light with plants and you will get a sea of buds!