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Is it time to switch to 12/12?


Well-Known Member
My first month of growing was a disaster. I used the wrong soil and I had the lights too close to my plants. I burnt them to a crisp and they couldnt grow. I ended up ditching every single plant after 30 days and starting over. I'm now on my second grow and on day 39. 1 December makes it day 40. The layout of my grow is as follows: (this is the view from the top)

Its a 240 cm x 120cm tent. Four plants on each side. Okay, the fourth plant on the right is Coriander, so just ignore that.

I have a mix of autoflowers: auto cheese and auto sour diesel haze.
And i have purple haze which is photoperiod.

My problem is, I did sort of try to plant them in a way that would make them all flower at the same time. But I think my calculations went to hell, along with the plants growth. They have grown in such bizarre ways. For example:
- C and D were planted on the same day, but D is 50% bigger than C.
- E was planted a few days later, but it seems more than 5 days "behind".

My problem now is D. It's 45 cm tall. And it's sativa, which means it can apparently triple in size. If it triples in size, it will be 135cm. Which is too big for my grow tent. But if I go into flowering now, then it might be too soon for the rest.

Here's a side view of the tent:

It can comfortably support about 82 cm as max height. The trays I use steal 45cm of my height. Which I could ditch if I need to. But I got them so I could easily let the excess water flow out into a bucket. But it turns out they hardly get full of water. After watering the plants, not too much goes into the tray, and by the time I get back, it seems the lights have somehow evaporated whatever was in the trays anyway. So ditching the trays, or at least remoinv the crates under the trays that hold them up, should give me that extra 45cm. So technically, I have a worst case scenario of 82 + 45 = 127cm. But I dont really want to go that route, as I still want the option of easily draining the trays using gravity.

The other option is to put the lights even higher. That could score me a maximum of 28 cm. But then the lights are very close to the roof of the tent. As in, 10 cm from the roof, give or take. So in this option, I can get a max height of 110cm. If I also remove the trays, I could get a maximum height of 110 + 45 = 155cm.

So my question is: When should I switch to 12/12 ?

My initial feeling is that I should wait til they all average a height of about 30cm. So I can expect a max grow size of about 90cm. So basically another week or so, and then just hope that the Purple Haze's dont go through the roof. But even if I do that, then the other plants havent beein in there that long. Is that okay?


Member of the Month: Dec 2016
Every plant grows differently, even from the same strain, so your calculations has not gone wrong. It was just the x factor of how will they grow, that is impossible to know or calculate beforehand.
You could top the tallest plant, she will spread out after though. So the question is, do you have enough left and right space? From here, it looks like a yes.
Raising the lights is a good idea.

BTW, excellent chart :thumb:made the whole thing a lot easier to understand.


Member of the Month: Dec 2016
If you veg a little, it means plants will not be huge and less end result.
The easy math is
Raise lights = plants will stretch, let them stretch to half the total height you have.
(I would say top and spread out the stems (many different ways) let me know if you want to, and I send you links.).
Switch lights and wait for pistils.


Well-Known Member
Ah. Okay. So then I'm going to wait till they get that height and do my best to move the lights a bit further.

Would it not maybe help to put the shortest plants together and the tallest plants together. And then move the light closer for the tall plants, so as to shorten them? and then move the lights far away for the short plants, so as to try and get them to grow higher? I.e. use the lights to even the heights?


Member of the Month: Dec 2016
Max isn't necessary, but a bit longer yes.
If you cut the top of the tallest, it gives you time for the shortest to grow better.
I can show you how, easily.
All I need is a pic of your tallest top.


Member of the Year: 2018 - Member of the Month: Jan 2018, Nov 2018 - Grow Journal of the Month: Aug 2018 - Plant of the Month: Oct 2018
What in the world is going on here! You want shorter autos? Train them flat (see LST thread in my sig). You want shorter photos? Top top and supercrop. No math involved.

I would definitely group stretchy plants under the same light.
lights have somehow evaporated whatever was in the trays
The plant sucked that water back up would be my guess!


Well-Known Member
It sounds like the consensus is that height isn't that much of an issue. I will investigate LST and Cropping and probably come back with more questions. Will definitely send you a picture of the plant in a bit, @AngryBird. Thanks!

So now, knowing that I can probably get away with height in various ways, does that mean I should try to let them all veg for the recommended period per strain? Purple haze is apparently 65 days veg according to strain profiles on the internet. Does that mean I should let the youngest Purple Haze reach 65 days and then switch the lights? All the while cropping and LST-ing to make sure the plants don't get too big?

I really don't want to cut corners with time. I want them to veg the right amount of time to produce the highest quality bud, even if it means I must wait another 3 weeks, or even 6 weeks before I switch to 12/12.

The autoflowers I also understand can be left as is. They will automatically flower when the time comes, and by that time I will already have switched to 12/12 for the purple haze's, but that shouldnt affect the autoflowers, because they are fine under 12/12 hours, even for veg, right?


Well-Known Member
> lights have somehow evaporated whatever was in the trays
>> The plant sucked that water back up would be my guess!

The pot plants have legs. So they are a few cm from the ground. I can see the watter drip out underneath, and then move to the bottom right of the tray, because I purposefully kept the trays slanted so that the water will move to the bottom right. It cannot be sucked up from this corner. But it also doesn't make sense that it all gets evaporated.

Since switching to 25 liters, and not watering AS much as I did on my first grow, it could also just be that I'm somehow throwing in just enough water for the plant to feed and for a tiny bit to come out the bottom and evaporate. But that also seems unlikely. I know I overwatered my first disastrous grow, but I still expected more of a scenario this time round, where more water collects at the bottom. I even got a mad cool pump to get the water out of the grow room, but now it seems like I may never use it.


Member of the Month: Dec 2016
It all sounds fine.
Could be the wet soil since you have many plants.
You need to start thinking about how to lower humidity for later, when they are flowering.
Do u you have a fan in there? Is air circulating?
When you upload some pictures, I can understand more


Let me start by defining some terms. Topping is cutting off a plant's highest growth to encourage lower growth. Super cropping is bending the stems of your tallest leaders so that they don't outgrow the rest of the canopy.

Personally, I would try to build a cardboard wall between those two sections so I could put the autos on one side and the photos on the other. More light equals more bud. So if you run the Autos 12/12 they won't produce as much for you as if you had run them 18/6 or 20/4.

That said, if splitting the tent in half is not a possibility, then 12/12 will have to work for you. Many growers here end up doing that and have perfectly nice plants! It's not ideal but it certainly happens all the time.

Like Angry Bird said, you will need to lower you hurt your humidity when you get into flour. Most people have exhaust fans to vent the moisture from their tents so that's something you should be looking into at this point, or a dehumidifier if venting isn't a possibility.


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