Im confused - when to induce flowering with SCROG?

I just finished re-reading half a dozen SCROG tutorials and Im still confused about when to induce flowering. Most of them seem to say to switch to 12/12 light cycle as soon as the plants get up to the bottom of the screen - BUT - then several of them also talk about extending veg time to fill in the screen???

I have a 2x3 tent (7 sq ft) with 4 plants - 2ea Northern Lights and 2ea CBD Critical - all feminized. All are in 5 gal fabric pots, in soil. I just finished installing a new LED light that is much better than my CFL’s I started with.

The screen is set 8" above the dirt.

These plants are all about 5-6 weeks old now. One is quite a bit bigger and more vigorous than the others and one is lagging behind the others, so I have one big, 2 medium and one runt.

The runt is still 2 to 3 inches below the screen, then next one is through maybe 1", then the 3rd one is up to 3" taller and the big brother would be 4 to 5" if I wasnt training it flat along the screen.

I have been doing some FIMing and topping, so all of the plants have 6 to 8 stems. Some of those stems are quite a bit behind the others and still have a long way to go to reach the screen. I am tempted to prune those but not sure. I have been lolly popping for the last 2 weeks off and on, but there is still a ton of stuff below the screen.

Should I trim the slower branches now or wait and see how they do?

Should I start 12/12 now or wait for things to fill in more?

Here are some pics.


Id wait till you have a full screen plus a inch to three of canopy on top lollipop right before you flower or day of i would also suggest introducing the screen at a lower height an younger because anything below the scrog is a waste


That highly depends on the strain (some are more stretchy), but @Growinnblowin is right, I’d wait at least few days to fill the screen from maybe 80%.

Your screen is probably too high. About 20-30 cm between the soil and screen is OK.

With pruning lower branches I’d wait at least two weeks into flower.

Check out my canopy after I switched my Rainforest.


Hi guys,

Sorry, one of those pics was taken before I lowered the screen. Its at 8" or 21cm above the dirt.

Ok, my gut feeling agrees with you guys, so I will leave the lights at 20/4 for a while longer.

I have to ask though - why did virtually every tutorial say to start 12/12 when the plants first touch the screen?

Or are my plants just not trained out to cover enough area?

Thanks for the help!!

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I would guess the tutorials you read were assuming you had spread the canopy to max by the time it tuched the screen. so, you would just keep stretching and weaving your shoots until you had all the squares filled. also some people use two levels of screens so idk that might have had something to do with it.

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I think your right. I went back and looked some of them again and they all have wider canopies than I do when they started.


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Generally I wait until the screen is about 70% full or so, then flip and keep training through stretch.


I’ve grown scrog for years and year’s. Best thing is to figure how much stretch your plant will give you and determine the fill from there. Some strain will finish at 2-3x their initial flower size, so you only want the screen about half full before the flip to 12/12. You’ll still be moving tops around up till close to the fourth week of flower.


my led does not give as much stretch as hps & as some have said some strains stretch more after the flip that’s why cloning & running a strain more than one grow has an advantage I would play it safe & fill the screen before I flipped


Ok, I think Im getting it now. Since this is my first grow, I have no clue how much stretch I will get.

It makes sense to wait. I suppose its better to have too much growth on the top rather than too little. If it gets too thick, I can always trim back the weaker, smaller branches as I go.

Thanks again!!!

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Also multiple plants are gonna fill the screen faster than single plant grows here is both of my single plant grows I’m about 10 days from flipping the larger one


Looks good. Also looks like you timed it right on the first pic. She may still get a bit unruly but that’s about how it should look. 95% full with a little room to work with still, two more weeks or so and they’ll be done stretching and in their flower set position.
If you have good airflow thru the tent, leave the leafs on below the screen until they die off from light deprivation. Imo there’s no point in removing them with the shoots beneath the screen if you have good airflow thru the canopy.

Totally agree, the plant will take the nutrients from the leaves as they die off. Leave em til their dead.


You have a much wider spread on your plants than I do. Mine are mostly compact and dense. I obviously need to do a better job with my LST work!


Darn! I just finished another round of trimming. I have excellent airflow. Oh well, I will leave the rest in place.

My SCROG is starting to fill in. Id say my screen is roughly 50-60% full, so Im getting close to switching to 12/12.

I just read something that suggested that I should wait until the plants are showing “pre-flowers”

Do these look like pre-flowers? I dont want to start 12/12 too soon, but I dont want to wait toooo long either.

Here is what my screen looks like.

I may have done a bone headed move… two of my plants were a good bit shorter than the others and didnt want to stretch and catch up, so I raised the pots 2" so I could start them weaving through the screen with the other two. They are all the same age - all sprouted within about a 4 day period.


Yes those are preflowers. They tell you the plant is sexually mature and ready to flower whenever after that point. A plant will not flower until sexually mature even if you run it 12/12 from seed.

Edit: only the top pic is showing preflowers.


Ah, ok. I thought those spikes in the other pics were the pre-flower. I have lots of those everywhere. Your saying its only the hair looking one in the first pic?

In that case, only one of the 4 is in pre-flower so far. It doesnt have many of those hairs, so I guess Im still too early to switch to 12/12.



Yep. That spike isn’t a preflower. You’ll see those on both side of where a branch meets the main stalk. Preflower always form in the area inbetween the branch and that spike, like in the first pic.

Edit: I just scrolled up and saw you’re running fem seeds lol. You should be good to flower as soon as you start to see the preflower balls form even before you see the hairs from the pistils.
Antidotal evidence shows that if a fem has a tendency or a predisposition to hermie it normally happens about week 4 of flower


Thanks for the help!

You lost me with the hermie stuff though. Thats all new territory for me - gonna have to go Googling now… :slight_smile: