I also am new to SCROG style of growing, and have done a hell of a lot of reading. I am going to be growing white rhino in mine.
From what I gather, in answer to your question about controling growth, is this:
1) First and formost, you should be either topping, fimming or lst training your girls. This is to encourage side growth, or more correctly, more lateral branching. You want all the branches to be about equal in height.
2) When they are ready to be placed under the screen, leave the lights on 18/6 or 24/7 to keep them in veg while they grow up and through the screen.
3) As the plants grow through the screen, gently tuck them back under the screen, or tie them to the screen, to make them produce more bud sites and to spread them evenly across the screen area. This is basically Low Stress Training on a larger scale.
4) try to direct the growth toward the open areas, to fill the screen completely. I am set up with coated wire mesh that has 2 inch x 4 inch spacing. From what I have been reading you can have 1 or 2 bud sites per opening.
5) When you have a good coverage of the screen ( 2 or 3 weeks in the flower room ) then you switch your lighting to 12/12. This is where the plant will start to stretch upward. You can keep tucking under for about a week or two too fill in any gaps in the screen.
TRIM the undergrowth. The leaves and small branches under your screen will be getting very little light, so trim off anything growing under the screen. You also don't want the immature popcorn buds to steal nutriants from your bud sites
6) water, fertilize, pick, cure and smoke
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