The Scrog Method
The essential detail of the scrog method is a screen, usually poultry netting, typically suspended between the planting medium and the lamp. The plants grow up to the screen and then are "trained" under the screen, resulting in a flat table of plant growth, a field rather than a forest. Because all the buds are growing at the same height, it is possible to get all the growth within the effective circle of light from the lamp, maximizing production from the space.
It's really that simple.
Well, nothing new under the sun, the method has been used for years. In modern terms, the method for cannabis was first popularized on the internet by the work of pH on the usenet group"ADPC" The method as initially used by pH was designed to tweak production from a large area under fluorescent lights.
Basic flat, fast scrog
The screen method used by pH relied on a long vegetative period for the plants to cover a large area of screen held close to a series of fluorescent tubes. The method I will describe here uses the same sort of growth process that occurs in a sea of green plant, and is very fast. The screen should be set about 8-12" above the planting medium, if possible. There are two purposes for that gap. First, you have to get your hands underneath the screen in order to handle the plant shoots and to remove excess growth shaded out under the screen. Second, there needs to be sufficient space for the plant to branch. Branching is essential to scrog. I prefer a space of about 10" for a 250 watt light, but some growers prefer shorter gaps for smaller lights, as little as 4-6".
Note that the screen does not have to be absolutely flat, and there are good arguments for dishing the screen to match the curvature of the light field. I don't radically dish my screen, but I do tie down the middle of the screen to prevent the screen from being pushed up, which would be counter-productive.
The clones are set under the screen at a density of about 1 plant per sq. ft. Experience in using the method with various types of plants may result in more or fewer plants, but 1 per ft. is a good starting point. Note that plant density is much lower than for plantlet-method sea of green. That means fewer clones to manage and fewer plants to be holding in a bust, a factor in sentencing guidelines.
Why clones, by the way? By the time you find out which plants are male and female from seed, it would be virtually impossible to extract the males from the foliage wound into the screen and fill in the gaps with female shoots, without a real mess on your hands. Seed plants also waste several inches of height before a mature stem section is reached from which branching can begin, whereas clones branch right from the medium. Height control is typically a limiting factor in cabinet growing. With female seeds it may be possible to grow a predictable scrog by raising the screen height, making up for the wasted stem length. Seed plants may react differently to forcing as well. I have no experience growing from seed.
The clones are established and kicked into vegetative growth. Assuming an 8-12" gap, just about the time where the growing tips penetrate a few inches above the screen, say at two weeks, the lights are switched to a 12 hour dark period. Ideally a response similar to the sea of green method kicks in. Instead of stopping and flowering, the plants take off, filling the screen with growth. At a density of 1 plant per ft., it usually works out that the plants stop and "crown off" just as the screen is filled. It's really magic to see it happen. Note that this timing method is not universal. Different plants may require more vegetative growth, or perhaps even less. Start by forcing early, because overgrowth creates an unproductive canopy, more salad than buds.
The timing is so critical. You must be around during this period to guide the growth under the screen, and to make sure all gaps in the screen are filled, one bud site per screen hole with standard poultry netting (2 x 3 inch holes). Excess leaf growth must be removed above the screen, which usually means all the fan leaves are removed. Never mind arguments about whether fan leaves should or should not be left on plants, this is a different animal, and the rule here is, all bud sites must see the lamp in order to develop. At least in a small scrog grow, fan leaves would overwhelm the neighboring buds. Get a good sharp, clean set of pruning scissors and just leave them with the grow. You'll need them every couple of days during this period.
Training really isn't difficult. With a limber plant I usually let the shoots grow vertically above the screen and then pull them under by the stem, re-orienting the stem horizontally under the screen to line up bud sites with screen holes. You don't have to tie anything down, as the upward pressure of the stem will nail the foliage to the screen, but some growers like to tie off stems to the screen during the early phases of screen filling. Here's what one grower, Ultimate, has to say on the subject: "I swear by twist ties and have a huge stock. They can be found just about anywhere. Purchase ties which are most flexible (wire with the smallest diameter) and coated with plastic not paper, as the paper will eventually mold.
"So why twist tie? Two reasons when training for in any screen application.
1. Pre-training. (Exact placement of main stems, growth shoots and branches)
2. Bud-training. (Bending, stem crushing/crimping, and repositioning)
"When initially induced to 12/12, the main tip/tips that hit the netting are immediately trained 90 degrees perpendicular to the netting. This allows for the light to concentrate the most productive part of the plant, forcing the most efficient production the plant can dish out. Branches under the netting are allowed some time to reach the light, but less than half will see light because you're concentrating on efficiency. The most efficient growth will occur where the main stem bends on a 90 degree and beyond, which receives the most light.
"I like to leave the ties long enough for the plant hold the shape desired. Main stem usually around the second week (give or take) , and branches will always vary. Branches coming off the main stem parallel to the netting are spread as far from the main stem as possible making for a even canopy, more bud sites per square, and controlling overall height.
"To a certain extent the buds freeze at a certain point and height/stem length slows. The canopy height is close to being established, but some plants are more vigorous than others and continue stretch beyond the rest of the crop. When bud training the longer colas are controlled by bending and tying down to the screen with twist ties. In extreme cases crushing/crimping is necessary. Moldy buds can be avoided by repositioning buds growing against each other. By using twist ties each bud can be positioned where air flows between each cola allowing efficient light dispersal within the canopy and better air flow.
"Without ties? Yield was lower. A few larger colas had to be tied down shielding smaller buds from direct light, not to mention forcing the light to be raised higher, lowering production (This can be resolved by switching to a more intense bulb). Some branches grew buds with LONG stems between the screen and base of the cola to compete with the large colas. Hybrid vigor in some cases, or plants which tend to "stretch" more than others eventually straighten out the 90 degree angle exposing less area of the most efficient portion on the plant and eventually stretches to a point where more stem was exposed to direct light, above the screen than desired. A view from the bottom (planter to the screen) showed that efficiency could be improved."
Some plants have brittle stems, and are difficult to train. It is possible to bend a stem by crushing it lightly at the bend. So long as the structures in the plant that carry fluids aren't damaged too much, the shoot will heal and be just fine (thanks to Uncle Ben for that trick). It may also be possible to top brittle plants under the screen, so that the future growth will be in several, more slender shoots.
The second pruning step occurs during and after the screen is filled. All growth under the screen must now be clipped off. Shaded growth quickly shrivels and dies, leaving ideal growth mediums for mold. Excess leaves and shoots should be clipped close to the stem, to avoid leaving stumps as mold sites. Robert Clarke recommends pruning away from the stem, but a lot of the standard advice has to be discarded when dealing with the special conditions of a scrog grow. The space under the screen is dark and humid, and you want as little plant material under there as possible. You will haul out buckets of leaves and excess shoots from a scrog grow, but the plants can take it. Clip away.
Subsequent pruning is really limited once the plant sets buds and stops growing. Some plants develop large leaves from the buds themselves, and if the leaves shade out neighboring bud sites, they must be removed. But that's about it. Most of the flowering time in a scrog grow the maintenance level is near zero.
If everything goes well, the extra time required for the plants to reach the screen before the flowering period is lengthened by only about two weeks. No additional time is required to fill the screen, because that time is the same used by the sea of green method to add height. The plants end up just as long, but the growth is directed horizontally. Typically a flat scrog grow ends up resembling a tropical forest canopy, with all the buds in a thick carpet extending 8-10" above the screen. The area underneath the screen contains the tree trunks that support the canopy, like piping connecting the root mat to the canopy.
Does it matter how the canopy is created? Not particularly, in my experience. There does not seem to be a lot of difference between buds that would come from sites lower on side branches from those at the actual tip of the plant. For the most part, a bud is a bud in this method. Note that the buds grown in a scrog field are each a piece of what would be a vertical cola. Each bud grows up vertically 90 degrees from the stem. You are familiar with how a cola is made up of individual bunches of flowers connected to the stem in an overlapping spiral, producing a structure that looks like a single unit. In scrog, each one of those florets matures into a small bud in their own right, typically 4-8" tall, about the size of a cigar.
How much weight? It has been shown that it is possible to reach around 2 oz. per ft. with a suitable plant and enough light density. 400 watt growers have reported up to 2.4 ounces per foot in a flat scrog.
What can go wrong with a flat scrog grow? The worst thing you can do is to allow the plants to grow too long. You would think that excess growth could be cut out or moved to vertical screens, but in practice it's difficult to recover from a badly overgrown screen. Plants that grow into and fill the screen seem to put on better bud weight than overgrown plants that are tied down and whacked back to fit.